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The electrochemical production of ferrate ions from iron anodes in alkaline solutions Tuffrey, Nigel Edwin 1984

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THE ELECTROCHEMICAL PRODUCTION OF FERRATE IONS FROM IRON ANODES IN ALKALINE SOLUTIONS By NIGEL EDWIN TUFFREY B.Sc . (Eng. ) , A .R .S .M. Imperial College of Science and Technology (Universi ty of London), 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Meta l lurg ica l Engineering We accept th i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA November 1983 (c) Nigel.Edwin Tuffrey, 1983 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or pu b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of f Y\tlTflLLU R f i l C f l U B^GXUBE9JLN& The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date DE-6 (2/79) i i ABSTRACT An i n v e s t i g a t i o n i n t o t h e e l e c t r o c h e m i c a l p r o d u c t i o n of f e r r a t e i o n s by a n o d i c d i s s o l u t i o n of i r o n anodes i n s t r o n g a l k a l i n e e l e c t r o l y t e s has been c a r r i e d o u t . The s t a b i l i t y o f f e r r a t e i o n s i n a l k a l i n e s o l u t i o n s and the dependence o f the d e c o m p o s i t i o n r e a c t i o n on t e m p e r a t u r e , h y d r o x y l i o n c o n c e n t r a t i o n and f e r r a t e i o n c o n c e n t r a t i o n were d e t e r m i n e d . The s t a b i l i t y o f f e r r a t e i o n s i s f a v o u r e d by low t e m p e r a t u r e s and f e r r a t e i o n c o n c e n t r a t i o n s , and h i g h h y d r o x y l i o n c o n c e n t r a t i o n s . P a r t i c u l a t e f e r r i c h y d r o x i d e i s c a t a l y t i c to the f e r r a t e d e c o m p o s i t i o n . r e a c t i o n . An e l e c t r o c h e m i c a l c e l l was d e s i g n e d t o measure the r a t e o f f e r r a t e p r o d u c t i o n . U s i n g t h i s c e l l and o t h e r t e c h n i q u e s the e f f i c i e n c y of f e r r a t e p r o d u c t i o n as a f u n c t i o n o f the q u a n t i t y o f charge p a s s e d , the s u p e r f i c i a l c u r r e n t d e n s i t y and the h y d r o x y l i o n c o n c e n t r a t i o n wane d e t e r m i n e d . The e f f i c i e n c y o f f e r r a t e f o r m a t i o n i s low and d e c l i n e s g r a d u a l l y t o z e r o as i n c r e a s i n g q u a n t i t i e s o f charge a r e p a s s e d . The e f f i c i e n c y o f f e r r a t e f o r m a t i o n i s i n d e p e n d e n t o f the s u p e r -f i c i a l c u r r e n t d e n s i t y but d e c l i n e s r a p i d l y as the h y d r o x y l i o n c o n c e n t r a t i o n i s r e d u c e d . A l i m i t e d q u a n t i t y of f e r r a t e can be p roduced f rom an a n o d e . There are i n d i c a t i o n s t h a t p e r i o d i c c u r r e n t r e v e r s a l can p r e v e n t the s t e a d y d e c l i n e i i i i n the f e r r a t e p r o d u c t i o n r a t e . The o x i d e or h y d r o x i d e l a y e r wh ich formed on the anode d u r i n g e l e c t r o l y s i s c o u l d not be p o s i t i v e l y i d e n t i f i e d . T h i s l a y e r was x - r .ay amorphous , t h i c k e n e d s l o w l y and an o u t e r l a y e r d i s s o l v e d or s p a l l e d as the l a y e r grew. The anode b e h a v i o u r was examined u s i n g p o t e n t i o d y n a m i c , p o t e n t i o s t a t i c and c o n s t a n t c u r r e n t t e c h n i q u e s . T r a n s i e n t anode b e h a v i o u r was o b s e r v e d d u r i n g the p e r i o d o f f e r r a t e f o r m a t i o n . I t i s p r o p o s e d t h a t the r a t e of f e r r a t e p r o d u c t i o n and the d e c l i n e i n the e f f i c i e n c y of f e r r a t e f o r m a t i o n w i t h i n -c r e a s i n g q u a n t i t i e s of charge passed can be e x p l a i n e d , by : 1) the i n v o l v e m e n t of the o x i d e o r h y d r o x i d e l a y e r on the anode i n the f e r r a t e f o r m a t i o n mechan ism, and 2) changes wh ich o c c u r i n the c h e m i c a l o r p h y s i c a l p r o p e r t i e s o f the anode l a y e r d u r i n g e l e c t r o l y s i s . i v Table of Contents Pa^e Abstract i i Table of Contents iv L i s t of Tables . . . "ix L i s t of FlCJUK'GS ••••••• •••••••••>•»••••••••.•••••''•-••'•••'•••••••'••'••'»•*••'•••••'.•• "»••'•-'•"•••••'•'•••'••• Acknowl edgement x v Chapter 1 INTRODUCTION 1 1.1 General 1 1.2 The Use of Ferrate Ions as an Oxidant 4 1.3 The General Properties of Ferrates 5 1.4 The Chemical Properties of Ferrate Solutions 7 1.4.1 The Chemical K inet ics of Ferrate Decomposition 1° 1.5 The Production of Ferrate Ions 13 1.5.1 L i terature Review I 3 1.5.1.1 Chemical Methods of Producing Ferrates 13 1 .5 .1 .2 The Electrochemical Production of Ferrate Solutions 14 1 .5 .1 .2 .1 Low Current Density Studies I 7 1 . 5 . 1 . 2 . 2 High Current Density Studies 18 1 . 5 . 1 . 2 . 3 General 2 0 1.5.2 The Methods of Production - Conclusion 2 0 1.6 The Electrochemical Generation of Ferrate Ions - An Overall View of the Reactions Involved 21 1.7 The Thermodynamic Properties of Strong A l k a l i So lu t ions . . 2 4 1.8 The Anodic Behaviour of Iron in Strong A lka l ine Solutions 2 ^ 1.9 The Electrochemical Evolution of Oxygen 3 3 1.10 The Purpose of the Present Investigation 3 4 2 THE STABILITY OF FERRATE SOLUTIONS 3 6 2.1 Introduction 3 6 Chapter Page 2.2 Experimental 3 6 2.2.1 Reagents 3 6 2.2.2 Procedure 3 7 2.2 .3 Errors 3 8 2.3 Results and Discussion 3 9 2.3.1 The Effects of the Ferrate Ion Concentrations 3 9 2.3.2 The Effects of Temperature 2 .3 .3 The Effects of Sodium Hydroxide Concentration 2 .3 .4 The Effects of Par t icu la te Fer r i c Hydroxide 5 5 2.4 Summary and Comparison with Previous Work 5 5 2.5 The Addit ive Ef fects of the Individual Variables Af fect ing the Rate of Ferrate Decomposition 45 50 56 3 THE ELECTROCHEMICAL PRODUCTION OF FERRATE IONS •... 5 8 CO 3.1 Experimental Objectives 3.2 Experimental Problems 5 8 3.3 Experimental Methods 3.3.1 Beaker Experiments 3 .3 .2 Continuous Flow Anolyte Experiments 3.4 Experimental 3.4.1 E lec t ro l y te Preparation 3 .4 .2 Electrode Mater ials and Preparation 3 .4 .3 Power Sources and A u x i l i a r y Equipment 6 3 3.4 .4 Mater ials of Construction 6 3 3.4 .5 Electrochemical Ce l l s 6 4 3 .4 .5 .1 Beaker Experiments 6 4 3 . 4 . 5 . 2 Continuous Flow Anolyte Electrochemical Cel l 3.5 Analysis 3.6 Experimental Variables Examined 3.6.1 Beaker Experiments - Type 1 3 .6 .2 Beaker Experiments - Type 2 .... 3 .6 .3 Continuous Flow Anolyte Experiments 60 60 61 61 61 62 65 69 69 70 70 70 v i Chapter Page 3.7 Experimental Errors — 71 3.7.1 Beaker Experiments 71 3 .7 .2 Continuous Flow Anolyte Experiments 72 3.8 Results and Observations 73 3.8 .1 The Effects of Current Density on the E f f i c iency of Ferrate Formation 73 3 . 8 . 1 . 1 Continuous Flow Anolyte Experiments 73 3 . 8 . 1 . 2 Beaker experiments - Type 1 79 3 . 8 . 1 . 3 Beaker Experiments - Type 2 79 3 . 8 . 1 . 4 Summary of the Results of the Experiments Investigating the Effects of Super f i c ia l Current Density on the E f f i c iency of Fer -rate Information 83 3 .8 .2 The Effect of A l k a l i Type and Concentration 85 3 .8 .3 The Effects of Sodium Chloride Additions 90 3 .8 .4 The Effects of Current Reversal 90 3 . 8 . 5 Magnetite Anodes . . . 93 3.9 Scale Morphologies 93 3.9.1 Iron Anodes 9 3 3.9 .2 Magnetite Anodes 1 0 4 3.10 Summary of the Experimental Results of Chapter 3 1 0 6 4 THE ELECTROCHEMICAL BEHAVIOR OF IRON IN CONCENTRATED SODIUM HYDROXIDE SOLUTIONS 1 0 8 4.1 Techniques Used 1 0 8 4.2 The Measurement of the Electrochemical Potent ia l 1 0 8 4.3 The Magnitude of the Measured Components of Anodic i Potent ia l 1 1 0 4.3.1 The Potent ial Drop Due to E lect ro ly te Resistance . . 4 .3 .2 Potent ial Drop Across an Anode Surface Layer . . . . . . 1 1 3 4.3 .3 Anode Potent ial Component Due to the E lec t ro - 1 chemical Reactions ^ 4 VI 1 Chapter Page 4.4 Experimental — ^ 5 4.4.1 Experimental Apparatus 1 1 5 4.4 .2 Luggin Cap i l la ry Tube Probe . . . 1 1 5 4.4 .3 Reference Electrode 1 1 6 118 4.5 Experiments Conducted 4.5.1 Iron Electrodes 1 1 8 4.5 .2 Magnetite Electrodes 1 1 9 1 ?fl 4.6 Results and Analysis 120 4 .6 .1 Potentiodynamic Studies of Iron 4 . 6 . 1 . 1 Summary of the Potentiodynamic Studies of j 126 Iron 127 4 .6 .2 Constant Current Studies of Iron 12g 4 . 6 . 2 . 1 Backside Luggin Cap i l la ry Results 4 . 6 . 2 . 2 Comparison of the Backside and Frontside 130 Luggin Cap i l la ry Probe Results 131 4 . 6 . 3 Potent iostat ic Studies of Iron 131 4 .6 .4 Magnetite Anodes 131 4 . 6 . 4 . 1 Potentiodynamic Studies of Magnetite 4 . 6 . 4 . 2 Potent ios tat ic and Cathodization Studies of 134 Magnetite 5 DISCUSSION 1 3 7 1 37 5.1 Review of Results 141 5.2 The Mechanism of Ferrate Formation 5.2.1 The Role of the Anode Layer in the Ferrate Formation Mechanism ^ 5.3 The Decline of the E f f i c iency of Ferrate Formation 153 with Increased Quantity of Charge Passed 5.4 General Conclusions on the Ferrate Formation Reaction . . . 1 6 0 6 CONCLUSIONS 1 6 2 7 RECOMMENDATIONS FOR FUTURE STUDY 165 v i i i Pagj. REFERENCES 166 APPENDICES A Reactions and E q u i l i b r i a Pertaining to the Potent ial -pH Diagram for the Iron Water System at 25°C 170 A . l ; Substances.Considered 171 A . 2 . 1 . Two Dissolved Species 172 A .2 .2 Two s o l i d Substances 174 A. 2;3 One s o l i d Substance and One Dissolved Substance . . . 175 B The Analysis of Ferrate Containing Solutions 178 B. l Methods Avai lab le . . . . . . . . . "179 B.2 Chemical Analysis . . . . . . . . . 179 B.2.1 The Chromium ( I i i ) - Ferrous Method for the Analysis of So l id Ferrate 180 B.2.1 .1 Analy t ica l Procedure 181 B.3 Spectrophotometric Analysis 181 B.3.1 Ca l ib rat ion 182 B.4 The Electrochemical Analysis of Ferrate Ion Solutions 183 B.5 Total Iron Analysis 184 C Tables of Results 185 i x LIST OF TABLES Table Page 1.1 Previous studies of the electrochemical formation of fer rate ions . . . . . 15 1.2 A c t i v i t y of water for sodium hydroxide solut ions as a function of temperature 25 1.3 Mean molal a c t i v i t y c o e f f i c i e n t s for sodium hydroxide as a function of temperature and. concentration 26 2.1 The e f fec t of temperature on the graphical ly determined fer rate decomposition rate constant [NaOH] =• 14.3 g.mol/1 48 2.2 The e f fec t of hydroxy! ion concentration on the graphica l ly determined fer rate decomposition rate constant @ 323 K 50 3.1 Comparison of the tota l i ron content of continuous flow anolyte experimental samples with, that measured as fer rate ions 77 C. l to The ef fects of fer rate concentration on the fer rate C.4 decomposition rate 186 C.5 to The e f fec ts of temperature on the fer rate decomposition C.9 rate 1 8 7 CIO to The ef fects of sodium hydroxide concentration on the rate C.12 of fer rate decomposition ^ 8 9 C.13 to Continuous flow anolyte experiments (Armco Iron Anodes) C . l 8 C.13 Ferrate formation at 10 KA/m2 1 9 0 C.14 Ferrate formation at TO KA/m2 1 9 0 C.15 Ferrate formation at i9 KA/m2 1 9 1 C. 16 Ferrate formation at 5 KA/m2 1 9 1 C. l7 Ferrate formation at 18 KA/m2 1 9 2 C. 18 Ferrate formation at [NaOH] = 10 g.mol/1 1 9 2 Table x Page C.19 to Beaker Experiments - Type 1 C.22 C.19 Iron removal from Iron anodes at 5 KA/m 193 C.20 Iron removal from iron anodes at 10 KA/m 193 C.21 Iron removal from iron anodes at 60 KA/m 194 C.22 Iron removal from magnetite anodes at 10 KA/m 194 C.23 to Beaker Experiments - Type 2 (Armco Iron Anodes) C.32 C.23 Iron removal at 50 KA/m2 195 C.24 Iron removal at 25 KA/m2 195 C.25 Iron removal at 10 KA/m2 1 9 5 l C.26 Iron removal at 5 KA/m2 196 C.27 Iron removal at [NaOH] = 1 g.mol/1 196 C.28 Iron removal at [NaOH] =..5 g.mol/1 196 C.29 Iron removal at [NaOH] = 10 g.mol/1 1 9 7 1 7 C.30 Iron removal at [K0H] = 10 g.mol/1 197 C.31 The e f fec t of sodium hydroxide concentration on the to ta l 2 i ron removed from iron anodes a f te r 30 K s @ 10 KA/m for various;sodh':um hydroxide concentrations ^ 8 C.32 Iron removal from iron anodes at [NaOH] = 14.3 g.mol/1 with 0.05 M chlor ide addit ions ^ 9 C.33 Iron removal from iron anodes undergoing per iodic current reversal ^ 9 LIST OF FIGURES Figure 1.1 Potent ial -pH Equi l ibr ium diagram fo r the Fe -H 2 0 system at 25°C 1.2 Var iat ion of the mean molal a c t i v i t y c o e f f i c i e n t Y + for NaOH 1.3 The e f fec t of potent ial on the e f f i c i e n c y of fer rate (VI) production 1.4 Amount of charge passed (open c i r c l e s ) and iron dissolved ( s o l i d c i r c l e s ) in 1 h anodic oxidation 1.5 S imp l i f i ed high temperature reaction sequences for i ron passivat ion in 30 wt % KQH. 2.1 The e f fec t of fer rate concentration on the fer rate decomposi-t ion rate [NaOH] = 14.3 g.mol/1 T = 333 K 2-2.2 log [Fe04 ] versus time for various fer rate concentrations . . . 2- % 2.3 l/[FeO^~] 2 versus time for various fer rate concentrations . . . 2.4 The e f fec t of temperature on the fer rate decomposition rate [NaOH] = 14.3 g.mol/1 2.5 to log [FeO^ -] versus time for T= 273, 294, 313, 323, 333 and 2.8 353 K 2.9 Arrhenius plot fo r the fer rate decomposition reaction [NaOH] = 14.3 g.mol/1 2.10 The e f fec t of sodium hydroxide concentration on the fer rate decomposition rate T = 323 K 2.11 to log [Fe0^~] versus time for [NaOH] =. 5, 10 and 12 2.12 g.mol/1 2.13 The sodium hydroxide dependence of the fer rate decomposition rate constant @ T = 323 X x i i Figure Page 3.1 Diagram of the continuous flow anolyte electrochemical c e l l . . . 66 3.2 A schematic diagram of the anolyte flow t r a i n 67 2 3.3 Ferrate production rates @ 10 KA/m versus to ta l anodic charge passed 74 3.4 Ferrate production rate versus to ta l anodic charge passed 2 fo r s u p e r f i c i a l current densi t ies of 5, 9 and 18 KA/m 75 2 3.5 Cumulative to ta l fer rate produced@ TO)KA/m versus to ta l anodic charge passed 78 2 3.6 Iron removed from the anode @ 5 KA/m versus to ta l anodic charge passed 80 2 3.7 Iron removed from the iron and magnetite anodes @ 10 KA/m . . . 81 2 3.8 Iron removed from the anode @ 60 KA/m versus to ta l anodic charge passed . . 82 3.9 Total cumulative iron removed from the anode versus to ta l .<! c anodic charge passed for s u p e r f i c i a l current densi t ies of 5., 10, 25 and 50 KA/m2 84 3.10 The e f f e c t of a l k a l i type and concentration on the cumulative to ta l i ron removed from the anode with quantity of anodic charge passed .... 86 3.11 Cumulative to ta l i ron removed from the anode a f te r 2 30 K s @ 10 KA/m versus the sodium hydroxide concentrat ion. . 88 3.12 Ferrate production rate versus to ta l anodic charge passed. Super f i c ia l current density = 10 KA/m2 [NaOH] = 10 g.mol/1 . . . 89 3.13 The e f fec ts of chlor ide addit ions on the cumulative to ta l 2 i ron removed from the anode @ 10 KA/m with quantity of anodic charge passed — 91 3.14 The e f fec ts of current reversal on the cumulative to ta l iron 2 removed from the anode 0 10 KA/m with, quantity of anodic charge passed 9 2 3.15 S.E.M. X-ray Analyzer spectra for the duplex anode scale a) the upper layer 9 4 b) the lower layer 94 x i i i Figure Page 3.16 Electron micrographs of the anode scales formed with, increas -2 ing quant i t ies of charge passed @ 5 KA/m Cx 100 magnif ica-t ion ) 96 3.17 Electron micrographs of the anode scales formed with 2 increasing quant i t ies of charge passed @ 10 KA/m (x 100 magnif ication) . — 97 3.18 Electron micrographs of the anode scales formed with increas-ing quant i t ies of charge passed @ 60 KA/m (x 100 magnif ication) 98 3.19 High magnif ication electron micrographs of d i f fe ren t oxide (or hydroxide) structures formed on the anode a) anode layer present a f te r 2 K sec @ 10 KA/m2 (x. 4200 . magnif ication) 99 b) same as above (x 21,000 magnif ication) 99 2 c) exposed lower anode layer a f te r 10 K sec @ 10 KA/m (x 4000 magnif ication) 99 3.20 Cross sections of the anode layer a) and b) Anode layer a f t e r 20 K sec @ 10 KA/m2 (x 1000 magnification) ^ 1 c) Anode layer a f te r 30 K sec @ 10 KA/m2 (x ltfOO magnification) ^ 3.21 Electron micrographs of the anode scales formed at various sodium hydroxide concentrations between 1 and 14.3 g.mol/1 (x 100 magnif ication) 1 0 3 3.22 Electron micrographs of a magnetite anode surface a f te r 30 K sec @ 10 KA/m2 a) Anode surface (x 11 magnification) ^ b) Area b (x 440 magnif ication) 1 0 5 c) Area c (x 440 magnif ication) 1 0 5 x i v Figure Page 4.1 Schematic diagram of anode potential measurement for the constant current method 109 4.2 A schematic diagram of the components of potential measured by the reference e lect rode, fo r a working anode 109 4.3 A l ternat ive luggin c a p i l l a r y tube posit ions ... 112 4.4 Potentiodynamic plot fo r an i n i t i a l l y clean iron specimen 121 4.5 Potentiodynamic plot for an iron anode previously anodized for 27.5 K sec @ 10 KA/m2 124 2 4.6 Potent ial versus coulombs of charge/cm of anode. Constant 2 current experiments. Iron anode @ 10 KA/m 127 4.7 Potent ial f luctuat ions observed in backside luggin c a p i l l a r y tube experiments. 'Constant current technique. Iron anode @ 5 KA/m2 129 4.8 Potent iostat ic p lot fo r an iron anode @ + 1.6 V 132 4.9 Potentiodynamic p lot fo r a magnetite specimen 133 4.10 Magnetite anodes - the ef fects of periods of cathodization on the potent ios tat ic anodic current densi t ies 135 5.1 A schematic diagram of the possible re lat ionship between oxygen evolut ion and fer rate formation 145 5.2 A schematic diagram of the possible interact ions between the anode layer and the fer rate formation mechanism 148 a) Direct anode layer oxidat ion 148 b) The oxidation of i ron species d i f fus ing and migrating through the anode layer 148 c) Ferrate formation in pores ^48 ACKNOWLEDGEMENTS I wou ld l i k e to thank my s u p e r v i s o r s , D r . I. H. Warren and Dr . E. P e t e r s f o r t h e i r r e s p e c t i v e c o n t r i b u t i o n s t o the p r o j e c t , and t h e i r a d v i c e and a s s i s t a n c e d u r i n g the c o u r s e of my s t u d i e s . I would a l s o l i k e to thank the s t a f f and s t u d e n t s of the Department f o r t h e i r s u p p o r t t h r o u g h o u t . F i n a n c i a l s u p p o r t f rom Exxon M i n e r a l s L t d . and Dr . E. P e t e r s i s g r a t e f u l l y a c k n o w l e d g e d . 1 C h a p t e r 1 INTRODUCTION 1.1 G e n e r a l 1 - 5 The c u r r e n t i n t e r e s t i n a l k a l i n e l e a c h i n g o p e r a t i o n s w i t h i n h y d r o m e t a l 1 u r g i e a l p r o c e s s e s has prompted s e r i o u s i n -v e s t i g a t i o n as t o the s u i t a b i l i t y o f b o t h , c u r r e n t l y a v a i l -a b l e , a n d , as y e t , i n d u s t r i a l l y unproven o x i d a n t s f o r use i n o x i d a t i v e u n i t o p e r a t i o n s . The p r o p e r t i e s r e q u i r e d o f an o x i d a n t d e p e n d , to a l a r g e e x t e n t , on the t y p e o f m i n e r a l b e i n g l e a c h e d and the l e a c h i n g sys tem b e i n g u s e d . The a p p l i c a b i l i t y o f any s p e c i -f i c o x i d a n t w i l l depend c r i t i c a l l y on both the c h e m i c a l and p h y s i c a l e n v i r o n m e n t o f the l e a c h i n g s y s t e m . The o x i d a n t m u s t , a t t h e d e s i r e d c o n c e n t r a t i o n , e x e r t , t h r o u g h i t s redox c o u p l e , s u f f i c i e n t e l e c t r o c h e m i c a l p o t e n t i a l to o x i d i s e c h e m i c a l s p e c i e s i n the m i n e r a l l a t t i c e , thus d i s r u p t i n g i t and a l l o w i n g meta l i o n d i s s o l u t i o n . Due to the k i n e t i c s of the r e a c t i o n s , o x i d a n t s w i t h o x i d i s i n g p o t e n t i a l c o n -s i d e r a b l y g r e a t e r than t h a t t h e r m o d y n a m i c a l l y r e q u i r e d are o f t e n n e c e s s a r y t o a c h i e v e adequate l e a c h i n g r a t e s . The 2 s t a b i l i t y o f w a t e r , and the l o s s o f o x i d a n t due to u n d e s i r -a b l e s i d e r e a c t i o n s s e t s p r a c t i c a l l i m i t a t i o n s on the o x i d a -t i o n p o t e n t i a l , and c o n s e q u e n t l y the t ype of o x i d a n t wh ich can be used i n any p a r t i c u l a r s y s t e m . The use of o x i d a n t s such a s , h y p o c h l o r i t e s and ozone w h i c h , t h e r m o d y n a m i c a l T y , are a b l e to o x i d i s e w a t e r i s p o s s i b l e i f the r a t e o f w a t e r o x i d a t i o n i s cons i d e r a b1y 1 ess than the r a t e o f t he d e s i r e d o x i d a t i o n r e a c t i o n . In such eases the t e m p e r a t u r e , the p r e s e n c e o f c a t a l y t i c i m p u r i t i e s a n d , o f t e n , the pH of the s o l u t i o n w i l l have a l a r g e e f f e c t on the s t a b i l i t y o f the o x i d a n t . 1-5 O x i d a t i v e l e a c h i n g i s used i n d u s t r i a l l y i n the h y d r o m e t a l 1 u r g i e a l e x t r a c t i o n o f many m e t a l s such as c o p p e r , z i n c , u ran ium and n i c k e l . . O x i d a n t s ' a r e a l s o used to a f f e c t s e p a r a t i o n between d i f f e r e n t m e t a l s such as n i c k e l - e o b a l t , mo lybdenum-copper and rhen ium-molybdenum s y s t e m , t h e y a re a l s o used i n many s o l u t i o n p u r i f i c a t i o n u n i t o p e r a t i o n s . The main o x i d a n t s wh ich have been used i n h y d r o m e t a l -l u r g i c a l l e a c h i n g o p e r a t i o n s a r e o x y g e n , h y p o c h l o r i t e s , h y d r o g e n , p e r o x i d e , c h l o r i n e , c h l o r a t e s , manganese d i o x i d e ( p y r o l u s i t e ) , n i t r o g e n d i o x i d e , ozone and a s s o c i a t e d s p e c i e s of the a b o v e . The use o f gaseous o x i d a n t s g e n e r a l l y r e q u i r e s c l o s e d 3 and s e a l e d r e a c t o r v e s s e l s , o f t e n p r e s s u r i z e d , t o a c h i e v e f a s t e f f i c i e n t o x i d a t i o n . R e l a t i v e l y u n s t a b l e o x i d a n t s g e n e r a l l y r e q u i r e a g i t a t e d t a n k s such t h a t the o x i d a n t i s r a p i d l y b r o u g h t i n t o c o n t a c t w i t h the m i n e r a l or an i n t e r -m e d i a t e o x i d i z i n g s p e c i e s to m i n i m i z e o x i d a n t w a s t e . The o x i d a n t d e c o m p o s i t i o n p r o d u c t s must a l s o be c o n -s i d e r e d . They may be c a t a l y t i c to o x i d a n t d e c o m p o s i t i o n r e a c t i o n s o r , as i n the case o f c h l o r i n e based s p e c i e s , they may c o n s t i t u t e a s e v e r e c o r r o s i o n p r o b l e m . D i f f e r e n t r e q u i r e m e n t s a r e demanded o f o x i d a n t s f o r i n s i t u l e a c h i n g - wh ich i s p r e s e n t l y used i n d u s t r i a l l y f o r u ran ium r e c o v e r y and i s b e i n g c o n s i d e r e d f o r o t h e r t y p e s o f m e t a l s . The o x i d a n t used f o r i n s i t u l e a c h i n g must be s u f f i c i e n t l y s t a b l e t o be a b l e to pass t h r o u g h the f r a c t u r e d orebody to where i t i s r e q u i r e d , r e a c t s u f -f i c i e n t l y r a p i d l y w i t h t h e d e s i r e d s p e c i e s , and h o p e f u l l y have some t o l e r a n c e t o v a r i a t i o n s i n t e m p e r a t u r e and pH. O x i d a t i v e l e a c h i n g i n a l k a l i n e systems i s used i n -d u s t r i a l l y f o r u r a n i u m , v a n a d i u m , molybdenum, ch romium; n o b l e m e t a l s ( c y a n i d e s y s t e m s ) ; c o p p e r , n i c k e l and c o b a l t (ammine s y s t e m s ) . Some of the o x i d a n t s used i n a l k a l i n e sys tems a r e h y p o c h l o r i t e s , hydrogen p e r o x i d e , oxygen and sodium c h l o r a t e . Wi th the e x c e p t i o n o f h y p o c h l o r i t e , the c u r r e n t o x i d a n t s t e n d to r e s u l t i n k i n e t i c a l l y s low and o f t e n 4 i n e f f i c i e n t , meta l i o n d i s s o l u t i o n . The deve lopment of a l t e r n a t i v e s t r o n g , p r e f e r a b l y s o l u b l e , o x i d a n t s w i t h good g e n e r a l s t a b i l i t y , y e t h i g h r e a c t i v i t y w i t h the d e s i r e d meta l c o n t a i n i n g m i n e r a l s would g r e a t l y i n c r e a s e the scope o f f u t u r e a l k a l i n e l e a c h i n g o p e r a t i o n s . In g e n e r a l the c o s t o f the o x i d a n t w i l l be o f g r e a t i m p o r t a n c e , however i t i s the economics o f the o v e r a l l p r o c e s s wh ich w i l l be o f p r ime i m p o r t a n c e . A l t e r n a t i v e o x i d a n t s t o t h o s e c u r r e n t l y a v a i l a b l e may f i n d s p e c i a l i z e d a p p l i c a t i o n i n l e a c h i n g o p e r a t i o n s where the s u i t a b i 1 i t y o f t h e i r p r o p e r t i e s o f f s e t s t h e i r g e n e r a l l y h i g h e r c o s t . 1 .2 The Use o f F e r r a t e Ions as an O x i d a n t The use o f f e r r a t e i o n s ( F e O ^ - ) as an o x i d a n t i n h y d r o -m e t a l l u r g i c a l a l k a l i n e l e a c h i n g systems has n o t , as of y e t , been i n v e s t i g a t e d . I t s use as an o x i d a n t i n waste w a t e r t r e a t m e n t has been i n v e s t i g a t e d by s e v e r a l w o r k e r s . The c h e m i c a l p r o p e r t i e s o f f e r r a t e have a l s o been r e p o r t e d (see s e c t i o n s 1 . 3 and 1 . 4 ) . The p r o p e r t i e s o f f e r r a t e wh ich m e r i t i t s c o n s i d e r a t i o n as an o x i d a n t f o r a l k a l i n e l e a c h i n g sys tems a r e : 5 1) F e r r a t e i s a s t r o n g o x i d a n t . A l t h o u g h i t i s u n s t a b l e w i t h r e s p e c t to w a t e r a t a l l p H ' s , i t i s r e p o r t e d t o have a low d e c o m p o s i t i o n r a t e a t h i g h h y d r o x y l i o n c o n c e n t r a t i o n s and low t e m p e r a t u r e s . 2) The f e r r a t e d e c o m p o s i t i o n p r o d u c t s a r e l o w e r i r o n s p e c i e s which a re i n s o l u b l e i n a l k a l i n e s o l u t i o n s . In g e n e r a l , t h e s e s h o u l d not cause majo r c o r r o s i o n or s o l u t i o n contami n a t i o n p r o b l e m s . 3) The f e r r a t e i o n (FeO^ ) i s h i g h l y s o l u b l e . To be s e r i o u s l y c o n s i d e r e d as, a p o s s i b l e o x i d a n t f e r r a t e wou ld have to be p roduced as e f f i c i e n t l y and c h e a p l y as p o s s i b l e . The d e t e r m i n a t i o n o f an e f f i c i e n t , economic method o f p r o d u c i n g f e r r a t e i o n s was the s t a r t i n g p o i n t of t h i s i n v e s t i g a t i o n . 1 . 3 The G e n e r a l P r o p e r t i e s o f F e r r a t e s The f e r r a t e i o n ( F e O ^ j ) i s the d i v a l e n t oxyanilon: -o f i r o n i n i t s h i g h l y o x i d i z e d h e x a v a l e n t s t a t e . S t a b l e c r y s t a l l i n e f e r r a t e s of B a , S r , C a , Ag and K 7 8 can r e a d i l y be p r e p a r e d . ' C r y s t a l l i n e p o t a s s i u m f e r r a t e has a t e t r a h e d r a l s t r u c t u r e i s o t y p i c a l o f p o t a s s i u m permanganate . 6 S o l u t i o n s of f e r r a t e a r e s t r o n g l y c o l o u r e d . F e r r a t e i o n s i n p o t a s s i u m and sodium h y d r o x i d e a re a v i v i d p u r p l e c o l o u r ,v'&fld;:-v e x h i b i t , peak a b s o r b a n c e o f monochromat ic g 1 i g h t a t about 516 nm. The f e r r a t e i o n i s a s t r o n g o x i d a n t ; the l o w e r v a l e n c e i r o n s p e c i e s b e i n g the reduced s p e c i e s i n t h e redox e q u i l i -b r i u m . F e O 2 " + 5 H + + 3e~ = F e ( 0 H ) 3 + H 2 0 - 1 . 1 E° = 2 . 1 0 6 - 0.098.4 pH + 0 . 0 1 9 7 l 0 g [ F e O 2 - ] E° = 0 . 7 3 V 0 pH 1 4 . 2 9 8 K . [ F e O 2 - ] = l > g . m o l / l The s t a n d a r d e l e c t r o c h e m i c a l p o t e n t i a l f o r the f e r r a t e -9 10 f e r r i c h y d r o x i d e e q u i l i b r i u m of e q u a t i o n 1 .1 ' can be compared to the h y p o c h l o r i t e e l e c t r o c h e m i c a l e q u i l i b r i u m o f . . , o 1 0 e q u a t i o n 1 . 2 . . CIO" + 2 H + + 2e~ = CI." + H 2 0 - 1 . 2 E° = 1 .715 - 0 .0591 pH + 0 . 0295 l o g E° = 0 . 8 9 V @ pH 14 . 298 K . [CI 0~] : Ed.."] = 1 T h e r m o d y n a m i c a l l y , f e r r a t e i o n s a r e not as s t r o n g an o x i d a n t i n a l k a l i n e s o l u t i o n s as h y p o c h l o r i t e , however t h e y 7 are s t i l l u n s t a b l e w i t h r e s p e c t t o w a t e r a t a l l p H ' s . The thermodynamic s t a b i l i t y o f f e r r a t e can be seen on the E h . - p H Diagram of F i g u r e 1 - 1 . The d i a g r a m i s based on t h a t of P o u r b a i x , ^ however the f r e e energy of f o r m a t i o n of 9 the f e r r a t e i o n used i s t h a t o f Wood. P o u r b a i x acknowledges ?-h i s v a l u e o f the f r e e energy o f f o r m a t i o n o f FeO^ o f - 1 1 1 2 -K C a l / g . m o l FeO^ i s an e s t i m a t e . Wood's c a l c u l a t e d thermodynamic p r o p e r t i e s of the f e r r a t e i o n i n s o l u t i o n a r e : A 'H° = - 1 1 5 + 1 ;K C a l / g . m o l A S° = 9 + 4 e . u . A G° = - 7 7 + 2 K C a l / g . m o l A f u l l l i s t o f the e q u a t i o n s used to c o n s t r u c t F i g u r e 1 .1 a re g i v e n i n Append i x A. 1 .4 . - " The Chemica l P r o p e r t i e s o f F e r r a t e S o l u t i o n s The f e r r a t e d e c o m p o s i t i o n r e a c t i o n w i t h w a t e r e . g . E q u a t i o n 1 . 3 , has been f o u n d , under c e r t a i n c o n d i t i o n s , to be r e l a t i v e l y s l o w . 2 F e O 2 " + 5 H 2 0 = 2 F e ( 0 H ) 3 + 1 . 5 0 2 + 4 OH" - 1 . 3 8 I t has been r e p o r t e d 1 1 t h a t a 2 M N a 2 F e 0 4 s o l u t i o n i n 50% NaOH decomposed 5% i n 4 days a t room t e m p e r a t u r e . The s t a b i l i t y o f F e r r a t e i o n s i s r e p o r t e d to be f a v o u r e d by : 8 12 13 1) H igh h y d r o x y l i o n c o n c e n t r a t i o n . 2) Low Tempera tu res ( < 5 0 G C ) . 8 , 1 2 ' ] 3 3) Low F e r r a t e i o n c o n c e n t r a t i o n . 1 ^ 4) The absence of low oxygen o v e r v o l t a g e d e c o m p o s i t i o n 1 2 c a t a l y s t s . The d e c o m p o s i t i o n of f e r r a t e i s enhanced b y : 1) The p r e s e n c e of f e r r i c h y d r o x i d e . 1 2 ' 1 ^ 2) The p r e s e n c e of t r a n s i t i o n meta l i o n s i n s o l u t i o n 1 5 e s p e c i a l l y n i c k e l and c o b a l t . 7 ?- 8 : - 8 3) The p r e s e n c e of NaCl , CO^ , OCT . 4) The p r e s e n c e o f ammonia and many t y p e s of o r g a n i c c o. m p 6 u: n ;d s 1 1 ' 1 ^ ' 1 8 ( t h e d e c o m p o s i t i o n of f e r r a t e i o n s i s i n c r e a s e d by d i r e c t r e a c t i o n w i t h such s p e c i e s ) . I t i s g e n e r a l l y a g r e e d the p r o d u c t o f the f e r r a t e decom-1 9 p o s i t i o n r e a c t i o n i s f e r r i c h y d r o x i d e . C h r i s t i a n e t a l . found t h a t i n e l e c t r o c h e m i c a l r e d u c t i o n of f e r r a t e on a Pt c a t h o d e i n 6 N -K0H i the f i r s t s t e p was the r e d u c t i o n of F e O 2 - to F e ( I I I ) a t + 0 . 3 V v e r s u s Hg/HgO \ 6 N KOH r e f e r e n c e e l e c t r o d e . However C a r r has o b s e r v e d f e r r o u s i o n s as an i n t e r m e d i a t e i n the f o r m a t i o n o f f e r r i c h y d r o x i d e . 10 1 . 4 . 1 The Chemica l K i n e t i c s of F e r r a t e D e c o m p o s i t i o n 20 G o f f and Murmann d e t e r m i n e d by s p e c t r o p h o t o m e t r i c - 3 2 -t e c h m q u e s t h a t the d e c o m p o s i t i o n of 10 M FeO^ i n 0 . 0 5 M phosphate b u f f e r s o l u t i o n a t 25°C had e i t h e r mixed f i r s t and second o r d e r ( e q u a t i o n T.-4) or 3/2 o r d e r c h e m i c a l k i n e t i c s ( e q u a t i o n 1 .5 ) w i t h r e s p e c t to the f e r r a t e i o n c o n c e n t r a t i o n . - d [ F e 0 2 " ] ? ? ? - 4 = k.t [ F e O p + . k 2 [FeOj T - 1 . 4 dt 2 _ , 2 _ 3/ 2 - d [FeOj ] = k 3 [ F e 0 j ~ ] - 1 . 5 dt where k-j , k,,, and k^ a r e d i f f e r e n t d e c o m p o s i t i o n r a t e c o n -s t a n t s . Below pH 9 . 6 . k-j / i s f i r s t o r d e r w i t h r e s p e c t to [H + ] and t h e r e i s no dependence on [0H^] f o r pH 9 . 6 - 1 4 . At pH 7 . 2 : k ] = 6 . 0 + 0 . 5 (x ! 1 0 " 3 ) s e c " 1 k 2 = 163 + 10 g . r n o l " 1 s e c - 1 T r a c e r s t u d i e s showed t h a t a t h i g h pH 's a l a r g e p o r t i o n of the 0 2 fo rmed d u r i n g w a t e r d e c o m p o s i t i o n came f rom the f e r r a t e i o n s . 11 1 8 C a r r d e t e r m i n e d f e r r a t e d e c o m p o s i t i o n had mixed f i r s t and second o r d e r r e a c t i o n k i n e t i c s w i t h r e s p e c t to f e r r a t e i o n c o n c e n t r a t i o n f o r pH 's up t o 1 1 . k^  was found to t e n d to z e r o as pH r o s e (>5) and k 2 was found to be second o r d e r w i t h r e s p e c t to the hydrogen i o n c o n c e n t r a t i o n . S e l e c t e d r e s u l t s f o r k-j and k 2 a t 25°C a r e : - l - 1 - 1 pH = 7 k T = 0 . 3 0 9 sec k 2 = 668 g .mol sec pH = 9 . 3 k 1 = 2 x 1 0 " 6 s e c - 1 k ? = 0 . 1 9 g . m o l " 1 s e c " 1 C a r r a l s o o b s e r v e d t h a t o n l y 60 - 70% o f the e x p e c t e d oxygen of d e c o m p o s i t i o n was d e t e c t a b l e . A l s o f e r r o u s i o n s were d e t e c t e d as an i n t e r m e d i a t e i n the f o r m a t i o n o f the f e r r i c h y d r o x i d e d e c o m p o s i t i o n p r o d u c t . C a r r s p e c u l a t e s t h a t the s t r o n g pH dependence o f t h e r a t e c o n s t a n t s k-j and k 2 i s due to the i n v o l v e m e n t o f p r o t o n a t e d f e r r a t e i o n s <in the decom-p o s i t i o n mechan ism. I t was s u g g e s t e d k-j has t h r e e compon-e n t s . k l ^ e O 2 - ] = [H 2 Fe0 4 ] + k H ^ - [H FeO"] + k F e Q 2 - [FeO 2"] -1 k l = k H 2 F e 0 4 aH 2FE0 4 + kH Fe0 4 aHFeO~ + k F e 0 ^ a F e 0 4 " -1 a = The fe r ra te f rac t ion in a par t i cu la r form. The d e c l i n e o f k-j a t pH >5 i s p o s t u l a t e d t o be due t o the f a c t t h a t o n l y p r o t o n a t e d f e r r a t e s p e c i e s can t a k e p a r t i n the d e c o m p o s i t i o n r e a c t i o n . The second o r d e r te rm k 2 and 2 - + i t s second o r d e r dependence on both [FeO^ ] and [H ] i s i n t e r p r e t e d i n terms of a F e 2 0 7 s p e c i e s ( e q u a t i o n 1 . 8 ) . 2 F e O 2 " + 2 H + ^ F e 2 0 2 " + ^ 0 - 1 . 8 I f t h e . a c t i v i t y ; of .waterTs,assumed'uni :ty. the e q u i l i b r i u m c o n s t a n t i s [ F e . O ? " ] K = M - 1 . 9 [ F e O 2 " ] 2 [ H + ] 2 2 _ ? - 2 + 2 [ F e ? 0 7 ] [FeO^ T [H ] = ^ - J — - 1 . 1 0 4 K Thus t h e s e c o n d c o r d e r ' t e r m . ' i s i d e s c r i b e d by e q u a t i o n 1 . 1 1 . k 2 [ F e O 2 " ] 2 [ H + ] 2 = ^ [ F e 2 0 2 " ] - 1 . 1 1 - 3 - 1 k 2 = F e r r a t e d e c o m p o s i t i o n r a t e c o n s t a n t g .mol s K = Thermodynamic e q u i l i b r i u m c o n s t a n t g .mo l . T o u s e k 2 1 d e t e r m i n e d t h a t f o r 30 x 1 0 ~ 3 M F e O 2 " i n 10 -50 wt % NaOH a t 30°C the f e r r a t e d e c o m p o s i t i o n r e a c t i o n had f i r s t o r d e r c h e m i c a l ' k i n e t i c s . F e r r a t e d e c o m p o s i t i o n was found to d e c l i n e r a p i d l y as the sodium h y d r o x i d e c o n c e n t r a -t i o n was i n c r e a s e d . The a c t i v a t i o n energy o f the f e r r a t e d e c o m p o s i t i o n r e a c t i o n i n 19 g . m o l / 1 sodium h y d r o x i d e was d e t e r m i n e d to be 98 'KjAg. 'mol . 1 . 5 The P r o d u c t i o n of F e r r a t e Ions 13 1 . 5 . 1 L i t e r a t u r e Review 7 8 11 A r e v i e w o f the l i t e r a t u r e ' ' r e v e a l e d t h a t s o l i d c r y s t a l l i n e f e r r a t e s or s o l u t i o n s c o n t a i n i n g f e r r a t e i o n s c o u l d be p roduced by e i t h e r c h e m i c a l o r e l e c t r o c h e m i c a l o x i d a t i o n o f l o w e r v a l e n c y i r o n s p e c i e s to f e r r a t e ( V I ) . 1 . 5 . 1 . 1 Chemica l Methods o f P r o d u c i n g F e r r a t e s g M i l l e r i n v e s t i g a t e d the v a r i o u s c h e m i c a l methods of p r o d u c i n g f e r r a t e s o l u t i o n s and c r y s t a l l i n e p r o d u c t s . Of the many p r e v i o u s l y p r o p o s e d methods o f p r o d u c i n g f e r -r a t e s , M i l l e r c o n c l u d e d o n l y the use o f c h l o r i n e to o x i d i s e a f e r r i c h y d r o x i d e s l u r r y i n s t r o n g a l k a l i , s o l u t i o n , as 14 r e f i n e d by S c h r e y e r , was m o d e r a t e l y s u c c e s s f u l i n p r o d u c -8 i n g f e r r a t e e x p e r i m e n t a l l y . M i l l e r found t h a t h i g h f e r -r a t e p r o d u c t i o n and c h l o r i n e e f f i c i e n c y a r e f a v o u r e d by h i g h sodiurn h y d r o x i d e and f e r r i c h y d r o x i d e c o n c e n t r a t i o n , the l i m i t b e i n g the a b i l i t y t o s t i r the s l u r r y . H igh t e m p e r a t u r e s a l s o f a v o u r e d the f o r m a t i o n of f e r r a t e , however above 50°C d e c o m p o s i t i o n o f the f e r r a t e formed became a p r o b l e m . At 30°C, 10 M NaOH, 400 g/1 F e ( N 0 3 ) 3 > . 9 H 2 0 au 2 -0.2 M FeO^ s o l u t i o n a t 8 7 . 7 % c h l o r i n e e f f i c i e n c y was p r o -duced a f t e r 3 h o u r s . To produce a s t a b l e f e r r a t e s o l u t i o n t h i s s l u r r y r e q u i r e d f i l t e r i n g . 8 9 M i l l e r and Kent Murmann p r o p o s e d the use o f h y p o c h l o r i t e 14 as an o x i d a n t to produce s t r o n g f e r r a t e s o l u t i o n s . M i l l e r found 0 . 1 M f e r r a t e can e a s i l y be p r e p a r e d by a d d i n g n m i l l i l i t r e s o f c o m m e r c i a l l i q u i d h y p o c h l o r i t e b l e a c h •(= 5 . 2 5 wt % ( 0 . 7 M) NaQCl) to n/2 grams o f s o l i d NaOH. To t h i s hot s o l u t i o n n/25 grams o f F e ( N Q g ) 3 9 H 2 Q , d i s -s o l v e d i n the minimum amount o f w a t e r , i s a d d e d . The m i s t u r e i s s t i r r e d f o r t h i r t y seconds and then r a p i d l y c o o l e d to room t e m p e r a t u r e . U n r e a c t e d f e r r i c h y d r o x i d e i s f i l t e r e d o u t . To produce h y p o c h l o r i t e f r e e p o t a s s i u m f e r r a t e c r y s t a l s , the f e r r a t e i s p r e c i p i t a t e d out as bar ium f e r r a t e . The s o l i d b a r i u m f e r r a t e i s then d i s s o l v e d i n s t r o n g p o t a s s i u m h y d r o x i d e . The s o l u t i o n i s c o o l e d and s o l i d p o t a s s i u m f e r r a t e c r y s t a l -l i z e s o u t . I t can be c o n c l u d e d t h e s e methods o f p r e p a r a t i o n wou ld not be v e r y p r a c t i c a l f o r c o m m e r c i a l p r o d u c t i o n because of the n e c e s s i t y of f i l t e r i n g f e r r i c g e l s f rom h i g h v i s c o s i t y , h i g h l y c a u s t i c s o l u t i o n s . 1 . 5 . 1 . 2 The E l e c t r o c h e m i c a l P r o d u c t i o n of f e r r a t e S o l u t i o n s The e l e c t r o c h e m i e a l g e n e r a t i o n of f e r r a t e i o n s by the a n o d i c d i s s o l u t i o n o f i r o n anodes i n s t r o n g a l k a l i n e s o l u t i o n s has been examined i n t e r m i t t e n t l y , d u r i n g the l a s t 8 11 2 1 - 2 6 hundred y e a r s , by a number o f r e s e a r c h e r s . ' * The r e s u l t s o f the v a r i o u s s t u d i e s a r e l i s t e d i n t a b l e 1 , 1 . 15 T a b l e 1.1 P r e v i o u s S t u d i e s o f t h e E l e c t r o c h e m i c a l F o r m a t i o n o f F e r r a t e I o n s . Researcher S o l u t i o n g.mol/1 Temperature (°C) C u r r e n t D e n s i t y A/m 2 Anode-Cathode S e p a r a t o r 2_ [FeO^ ] Genera ted C u r r e n t E f f i c i e n c y Comments R e f e r -ence P i c k NaOH > 14 0 10 No Ve ry l i t t l e 2 -FeO^ formed Cont inuous g e n e r a t i o n 22 70 10 No = 100% C u r r e n t E f f i c i e n c y f a l l s w i t h t i m e . O c c a s i o n a l c u r r e n t r e v e r s a l used. Grube and Gme1i n NaOH 14.3 40-80 33-266 Yes 0.12 M. 45-847. F e r r a t e p r o d u c t i o n i n c r e a s e d w i t h t empe ra tu re 23 60 333 A/m 2 „ w i t h 500 A/nT AC super imposed Yes 160% i n c r e a s e o v e r DC a l o n e Ken t -Murmann NaOH 6-20 -10 t o 25 H igh Yes up t o 2 M 2-45% Cont inuous g e n e r a t i o n 11 NaOH 20 0 H igh Yes 45% Con t i nuou s g e n e r a t i o n Ken t -Murmann NaOH o r KOH 8-20 40-50 60-150 ( l o 3 ) C a t i o n Exchange Membrane H igh c u r r e n t e f f i c i e n c y Cont inuous g e n e r a t i o n 24 V e n k a t a d r i Bauer Wagner KOH 10.5 25-40 P o t e n t i o s t a t i c C o n t r o l 0 .55 -0 .65 V v s . Hg/HgO/10.5 M KOH REF E l e c t r o d e 7 = 5-200 A/nT No < 61% > 0.7 V ( v s . Hg/HgO) oxygen e v o l u t i o n . O x i d e forms on anode. F e r r a t e g e n e r a t i o n c on t i nuou s f o r f i r s t 10-15 hou r s . 25 Tousek NaOH 14.3 30 5-200 Yes 60-70% C u r r e n t e f f i c i e n c y f a l l s w i t h t i m e < 10 A/m2 a s o l u b l e I r on ( I I I ) s p e c i e s produced > 10 A/m 2 FeO^" produced 21 M i l l e r NaOH 0-20 10-90 > 10 ,000 . No < 1 0 " 3 m < 6% C u r r e n t e f f i c i e n c y f a l l s r a p i d l y w i t h t i m e . Ox ide forms on t he anode. H igh t e m p e r a t u r e , and [OH"] beat f o r f e r r a t e f o r m a -t i o n 8 D i r e c t c o m p a r i s o n of the r e s u l t s i s not p o s s i b l e because the s t u d i e s were c a r r i e d out under d i f f e r e n t e x p e r i m e n t a l c o n -d i t i o n s . A l l the s t u d i e s agree t h a t h i g h l y a l k a l i n e s o l u t i o n s ( > 1 0 M ) a r e r e q u i r e d f o r ' e f f i c i e n t ' f e r r a t e g e n e r a t i o n . H a b e r 2 6 and M i l l e r 8 found t h a t NaOH was b e t t e r than KOH f o r 24 25 f e r r a t e p r o d u c t i o n . O ther i n v e s t i g a t o r s ' s u g g e s t t h a t a t equa l m o l a r i t y both NaOH and KQH a r e s i m i l a r l y e f f e c t i v e . The t e m p e r a t u r e s s u g g e s t e d f o r optimum e f f i c i e n c y 11 22 of f e r r a t e f o r m a t i o n range f rom -10/ to 70°C. I t i s 8 12 known ' t h a t f e r r a t e s o l u t i o n s decompose r a p i d l y above 50°C t h u s , p r o b a b l y ( e s p e c i a l l y when v e r y low c u r r e n t p -I _ p -3 p r d e n s i t i t e s a r e used ~ ' ) , the optimum t e m p e r a t u r e f o r f e r r a t e g e n e r a t i o n w i l l be a compromise between the r a t e o f f e r r a t e f o r m a t i o n and the r a t e o f f e r r a t e d e c o m p o s i t i o n . The i n v e s t i g a t i o n s r e v i e w e d i n t a b l e 1 .1 can be d i v i d e d i n t o two d i s t i n c t g r o u p s : 9 oo 1) Low C u r r e n t D e n s i t y S t u d i e s (< 500 A/m ) . Pick, Grube 23 25 21 and G m e l i n , V e n k a t a d r i and Tousek . 2) High C u r r e n t D e n s i t y S t u d i e s {> 1 0 , 0 0 0 A/m 2 ) 1 1 2 3 8 Kent -Murmann * and M i l l e r . 1 . 5 . 1 . 2 . 1 Low C u r r e n t D e n s i t y S t u d i e s 22 2 P i c k found t h a t a t a c u r r e n t d e n s i t y o f 10 A/m , a t 0°C, t h e e f f i c i e n c y o f f e r r a t e f o r m a t i o n was l o w . A t 70°C the e f f i c i e n c y of f e r r a t e f o r m a t i o n was v i r t u a l l y 100% but o c c a s i o n a l c u r r e n t r e v e r s a l was r e q u i r e d t o keep c u r r e n t e f f i c i e n c i e s h i g h . 23 Grube and Gmel in found t h a t s u p e r - i m p o s i t i o n o f a 2 2 500 A/m' a l t e r n a t i n g c u r r e n t on a 333 A/m base c u r r e n t a t 30-60°C caused up t o a 160% i n c r e a s e i n the amount o f f e r -r a t e formed ove r t h a t formed by d i r e c t c u r r e n t a l o n e and the p r o d u c t i o n of f e r r a t e was: x o n t i n u o u s . 21 Tousek s u g g e s t s t h a t a t 30°C and c u r r e n t d e n s i t i e s 2 l e s s t h a n 20 A/ni the d i s s o l u t i o n of i r o n i s up t o 80% e f f i c i e n t w i t h most of the i r o n e n t e r i n g s o l u t i o n as an 2 -i r o n ( I I I ) s o l u b l e s p e c i e s (Fe20^ ) . A t h i g h e r c u r r e n t 2 d e n s i t i e s (up to - 200 A/m ) the s o l u b l e i r o n s p e c i e s i s p r i m a r i l y f e r r a t e i o n s wh ich forms a t 50 to 60% c u r r e n t e f f i c i e n c y . The r a t e o f t o t a l i r o n removed f rom the a n o d e , and a l s o , f e r r a t e i o n p roduced f a l l s w i t h t i m e . The mechanism Tousek p o s t u l a t e s to e x p l a i n t h i s i s : 1) The i n i t i a l p a s s i v e l a y e r which forms i s decomposed s l o w l y g i v i n g o n l y a s o l u b l e i r o n ( I I I ) s p e c i e s . 18 2) A s e c o n d a r y porous l a y e r forms on top o f the p r e v i o u s and the i r o n ( I I I ) s p e c i e s e n t e r i n g the pores o f the s e c o n d a r y l a y e r ( p r o d u c e d by d i s s o l u t i o n of the p r i m a r y l a y e r ) a re f u r t h e r o x i d i z e d to f e r r a t e i o n s . 25 V e n k a t a d r i ' s work i s i n agreement wi th , the e f f i c i e n -c i e s o f f e r r a t e p r o d u c t i o n found by T o u s e k , however no i r o n ( I I I ) s p e c i e s were o b s e r v e d and the e x p e r i m e n t s were c a r r i e d out i n 1 0 . 5 M KOH. 1 . 5 . 1 . 2 . 2 High C u r r e n t D e n s i t y S t u d i e s o M i l l e r s t u d i e d the e f f e c t o f f i v e v a r i a b l e s on the e l e c t r i c a l e f f i c i e n c y o f f e r r a t e f o r m a t i o n a t h i g h c u r -r e n t d e n s i t i e s . The v a r i a b l e s s t u d i e d w e r e : 1) Time of e l e c t r o l y s i s , 2) C u r r e n t ( D e n s i t y ) , 3) Tempera -t u r e , 4) Sodium h y d r o x i d e c o n c e n t r a t i o n and 5) Anode s u r f a c e a r e a . M i l l e r does not q u a n t i f y the s u p e r f i c i a l c u r r e n t d e n s i t y used but i t i s p r o b a b l y g r e a t e r than 1 0 , 0 0 0 A/m . Over the whole range o f v a r i a b l e s s t u d i e d the c u r r e n t e f f i c i e n c y was l e s s than 6% and f e l l r a p i d l y w i t h t i m e . The b u l k of the c u r r e n t r e s u l t e d i n oxygen e v o l u t i o n but a s m a l l amount (a few p e r c e n t or l e s s ) went i n o x i d e f o r m a t i o n on the anode . M i l l e r c o n c l u d e d i t was the f o r m a t i o n of the 19 anode o x i d e wh ich r e s u l t e d i n the d e c l i n e i n the r a t e of f e r r a t e p r o d u c t i o n . T y p i c a l l y f e r r a t e f o r m a t i o n c e a s e d a f t e r about 150 m i n u t e s . The amount of f e r r a t e p roduced was p r o p o r t i o n a l t o : the anode a r e a and i n d e p e n d a n t of the c u r r e n t d e n s i t y . The r a t e o f f e r r a t e f o r m a t i o n i n c r e a s e d r a p i d l y w i t h i n c r e a s i n g t e m p e r a t u r e r e a c h i n g .5% c u r r e n t e f f i c i e n c y a t 85°C, however above 50°C f e r r a t e d e c o m p o s i t i o n became a majo r p r o b l e m . 1 1 2 4 The r e s u l t s of Kent -Murmann ' a r e c o n t r a d i c t o r y . The i n i t i a l r e p o r t ( R e f . 11) s u g g e s t s t h a t a t h i g h ( u n -s p e c i f i e d ) c u r r e n t d e n s i t i t e s and a t t e m p e r a t u r e s l e s s than 0°C f e r r a t e i o n s c o u l d be g e n e r a t e d a t 3 0 - 4 5 % c u r r e n t e f -f i c i e n c i e s a p p a r e n t l y c o n t i n u o u s l y to produce 2 1 Na2Fe0^ 24 s o l u t i o n s . P r i v a t e c o m m u n i c a t i o n s w i t h P r o f e s s o r K e n t -Murmann r e s u l t e d i n a d i f f e r e n t s e t of e x p e r i m e n t a l c o n d i -t i o n s b e i n g p r o p o s e d . The c u r r e n t d e n s i t i e s s u g g e s t e d were 6 0 - 1 50 KA/m 2 j : : the : , suggested ; ] t e m p e r a t u r e was 4 0 - 5 0 ° C . H igh e l e c t r i c a l e f f i c i e n c i e s were s t i l l c l a i m e d , however t h i s i s i n d i r e c t c o n f l i c t w i t h the p r e v i o u s w o r k 1 1 where i t was r e p o r t e d t h a t e l e c t r i c a l e f f i c i e n c y f e l l to l e s s than 5% above 25°C. C o n t i n u o u s f e r r a t e p r o d u c t i o n was a l s o r e p o r t e d . I t was s u g g e s t e d t h a t the p r e s e n c e o f Cl " i o n s i n c r e a s e s the e f f i c i e n c y of f e r r a t e p r o d u c t i o n and HglOg) h e l p s s t a b i l i z e f e r r a t e i o n s i n s o l u t i o n . 1 . 5 . 1 . 2 . 3 G e n e r a l In most cases a d i a p h r a g m 1 1 ' 2 1 ' 2 3 or c a t i o n 24 exchange membrane was used to s e p a r a t e the anode f r o m / t h e ca thode and t h u s p r e v e n t the f e r r a t e i o n s p roduced a t the anode f rom b e i n g reduced a t the c a t h o d e . 24 Kent -Murmann found t h a t pure i r o n and p l a i n low c a r b o n s t e e l s were the b e s t anode m a t e r i a l s , e a s t i r o n was found m e d i o c r e and a l l o y s c o n t a i n i n g n i c k e l were v e r y poor o anode m a t e r i a l s . M i l l e r has c o n c l u d e d a l l t y p e s o f i r o n were s u i t a b l e f o r use as anodes w i t h c a s t i r o n b e i n g the most e f f i c i e n t and wrought i r o n the l e a s t . 1 . 5 . 2 The Methods of P r o d u c t i o n - C o n c l u s i o n Both the c h e m i c a l and e l e c t r o c h e m i c a l methods o f p r o d u c i n g f e r r a t e i o n s have not been d e v e l o p e d to a s t a g e where they c o u l d be c o n s i d e r e d i n d u s t r i a l l y f e a s i b l e . The c h e m i c a l methods appear t o t a l l y i m p r a c t i c a l f o r l a r g e s c a l e p r o d u c t i o n . The e l e c t r o c h e m i c a l methods have not been i n -v e s t i g a t e d s u f f i c i e n t l y to a l l o w a f u l l e v a l u a t i o n of i t s f e a s i b i l i t y . The e l e c t r i c a l e f f i c i e n c y of f e r r a t e g e n e r a -t i o n i s r e p o r t e d to be h i g h a t both v e r y low and v e r y h i g h c u r r e n t d e n s i t i e s , however no i n f o r m a t i o n i s a v a i l a b l e at the i n t e r m e d i a t e c u r r e n t d e n s i t i e s (1 ,000 - 1 0 , 0 0 0 K A/m') -the range o f c u r r e n t d e n s i t i e s most a p p l i c a b l e i n d u s t r i a l l y f o r p l a n a r e l e c t r o d e s . 21 The mechanism of f e r r a t e f o r m a t i o n f rom an i r o n anode i n s t r o n g a l k a l i n e s o l u t i o n s has not been s a t i s f a c t o r i l y e s t a b l i s h e d . The f o r m a t i o n and e f f e c t s o f o x i d e s on the anode have not been t h o r o u g h l y i n v e s t i g a t e d . 1 .6 The E l e c t r o c h e m i c a l G e n e r a t i o n of F e r r a t e Ions -An O v e r a l l View of the R e a c t i o n s I n v o l v e d The mechanism of f e r r a t e f o r m a t i o n d u r i n g the e l c t r o -c h e m i c a l d i s s o l u t i o n of i r o n anodes has not been c o n c l u s i v e l y i d e n t i f i e d . In f a c t i t has not been d e t e r m i n e d t h a t . f e r r a t e 21 i o n s a r e formed e l e c t r o c h e m i e a l l y . Tousek has p roposed t h a t i r o n e n t e r s s o l u t i o n as a l o w e r v a l e n c y i r o n s p e c i e s and i s then f u r t h e r o x i d i z e d to f e r r a t e p o s s i b l y by a c h e m i c a l o x i d a t i o n r e a c t i o n . N o t w i t h s t a n d i n g the a b o v e , the anode c u r r e n t ( the f l o w o f e l e c t r o n s f rom t h e s o l u t i o n , a c r o s s t h e s o l u t i o n - a n o d e i n t e r f a c e t o t h e anode) c a n , i n g e n e r a l te rms , be t h o u g h t o f as p r o p o r t i o n a t i n g i t s e l f to the f o l l o w i n g o v e r a l l r e a c t i o n s : 1) F e ( m ) +• 8 OH" = F e 0 2 - ( a g ) + 4 H.,0 + 6 e" - 1 . 1 2 2 (a ) F e ^ m ) + . 2 x OH" = F e O x ^ $ j + x HgO '+2-x e" - 1 .Ti3a (b) F e ( m ) + y OH" = F e ( 0 H ) y ( $ ) + y e~ - l . V 3 b 3) 4 OH" = Q 2 + 2 H 2 0 + 4 e" - 1 . 1 4 m = meta l s - s o l i d aq = aqueous 22 The anode f i l m may be i n the fo rm of an o x i d e ( e q u a -t i o n 1 . 1 3 a ) o r a h y d r a t e d i r o n o x i d e ( e q u a t i o n 1 . 1 3 b ) . The f e r r a t e i o n may form by d i r e c t o x i d a t i o n of the o x i d e o r h y d r a t e d o x i d e l a y e r on the anode ( e q u a t i o n 1 . 1 5 a and 1 . 1 5 b ) . F e 0 x ( S ) + ( 8 _ 2 x ) 0 H " = F e 0 4~(aq) + ( 4 _ x ^ H 2° + ( 6 _ 2 x ) e " - 1 -1 5a F e ( 0 H ) y ( s ) + (8-y) OH" = F e 0 2 " ( a q ) + 4 H20 + (6-y) e" - 1 .15b The r e a c t i o n o c c u r r i n g a t the c a t h o d e i s the e v o l u t i o n of hydrogen ( e q u a t i o n 1 . 1 6 ) . 2H 2 0 = H 2 + 20H" + 2 e" - 1 . 1 6 The t o t a l c u r r e n t w i l l be the sum of the c u r r e n t s due to the i n d i v i d u a l r e a c t i o n s . ^ o t a l ^ e O 2 " + I F e Q + I Q - 1 .1.7 x 2 The c u r r e n t e f f i c i e n c y f o r f e r r a t e f o r m a t i o n i s t h u s : I F 0 2 " . 100 CURRENT EFFICIENCY = 4 . ; •' - 1 . 1 8 ! T o t a l The q u a n t i t y of f e r r a t e produce ( g . m o l s ) w i l l b e : o FeO/, F e O 2 " = — ± — - 1 - 1 9 4 n F 23 I p e Q 2 - = F e r r a t e f o r m a t i o n c u r r e n t (Amps) t = Time ( s e c o n d s ) n = Number o f e l e c t r o n s ( o v e r a l l n = 6) F = Faraday c o n s t a n t . A l t e r n a t i v e l y e q u a t i o n s 1 . 1 7 to 1 .19 can be w r i t t e n i n p terms o f the c u r r e n t d e n s i t y i (A/m ) . 24 1. 7! The Thermodynamic P r o p e r t i e s o f S t r o n g A l k a l i  S o l u t i ons In v e r y s t r o n g a l k a l i s o l u t i o n s (up to 19 g . m o l / 1 NaOH) the s h e e r q u a n t i t y o f sod ium h y d r o x i d e i n s o l u t i o n i s s u f -f i c i e n t to reduce the a c t i v i t y of the wate r i n the sys tem s i g n i f i c a n t l y f rom u n i t y . The c o n c e n t r a t i o n s o f sodium h y d r o x i d e c o n s i d e r e d are so g r e a t t h a t a c o n s i d e r a b l e q u a n t i t y o f t h a t p r e s e n t i s i n the form of a s s o c i a t e d m o l e c u l e s ; a l s o the mean m o l a l a c t i v i t y c o e f f i c i e n t of the s o l u t e e x h i b i t s l a r g e ' [ d e v i a t i o n s f rom i d e a l i t y . 2 7 Macdona ld and McKubre have c a l c u l a t e d the e f f e c t of t e m p e r a t u r e ( 0 - 1 2 0 ° C ) and c o n c e n t r a t i o n ( 1 - 9 . 9 g . m o l / k g ) on the thermodynamic p r o p e r t i e s o f sodium h y d r o x i d e s o l u t i o n s . The a c t i v i t y o f w a t e r was c a l c u l a t e d f rom the vapour p r e s -s u r e o f w a t e r o v e r pure c a u s t i c s o l u t i o n s u s i n g e q u a t i o n 1 . 2 0 . £ _ = {1+;-[(;T-174)(a + bm + cm2 + d/m) - 0.03170] ml -1 .20 *o a = - 8 . 6 7 1 5 x 1 0 " 5 b = 3 . 3 6 8 x 1 0 " 5 c = - 1 . 354 x 1 0 " 6 d = 7 . 8 8 x 1 0 " 5 T = Temperature (°C) m = M o l a l i t y ( g . m o l . k g " 1 ) The e q u a t i o n i s v a l i d f o r t e m p e r a t u r e s between 20 and 100°C and f o r m < 1 2 . 5 g .mol k g " 1 . I f t h e vapour p r e s s u r e i s low e n o u g h , t h e f u g a c i t y c o r r e c t i o n s can be i g n o r e d and t h e a c t i v i t y o f the w a t e r i n s o l u t i o n i s g i v e n by R a o u l t ' s Law a s : a w = p/p 0 - 1 . 2 1 The t r e n d s i n the a c t i v i t y o f w a t e r o v e r t h e range 2 7 o f v a r i a b l e s c o n s i d e r e d by M a c d o n a l d i s g i v e n i n t a b l e 1 . 2 . - 1 * E s t i m a t e s o f t h e a c t i v i t y o f w a t e r a t 1 6 . 6 7 g .mol kg u s i n g e q u a t i o n 1 . 2 0 g i v e : a , , = 0 . 2 0 7 @ 20°C w a w = 0 . 2 5 8 @ 50°C T a b l e 1 .2 A c t i v i t y o f Water f o r Sodium H y d r o x i d e S o l u t i o n s  as a F u n c t i o n o f T e m p e r a t u r e m(mol kg" 1 ) Tern aw ?erature (°C) 0 20 40 60 80 100 120 1.0 0.964 0.965 0.965 0.966 0.966 0.966 0.967 2.0 0.932 0.932 0.933 0.933 0.934 0.934 0.935 3.0 0.890 0.892 0.894 0.895 0.897 0.899 0.900 4.0 0.841 0.845 0.849 0.852 0.856 0.860 0.863 5.0 0.786 0.793 0.799 0.805 0.812 0.818 0.824 6.0 0.727 0.736 0.746 0.755 0.765 0.774 0.784 7.0 0.664 0.677 0.690 0.703 0.716 0.729 0.743 8.0 0.599 0.616 0.633 0.650 0.668 0.684 0.70o 9.0 0.534 0.555 0.575 0.596 0.617 0.638 0.659 9.9 0.476 0.500 0.524 0.549 0.573 0.597 0.621 Source: Macdonald and McKubre *16.67 g.mol/kg NaOH corresponds to the 14.3 g.mol/1 NaOH pr imar i l y used in t h i s inves t iga t ion . See chapters 3 and 4. 26 The mean m o l a l a c t i v i t y c o e f f i c i e n t s f o r sodium h y d r o x i d e c o n c e n t r a t i o n s up to 9 . 9 g . m o l / k g were c a l c u l a t e d by Macdona ld u s i n g t h e computer programme ACTIV which i s l i s t e d i n r e f e r e n c e 2 7 . The mean mola l ' a c t i v i t y c o e f f i c i e n t s were c a l c u l a t e d f rom the o s m o t i c c o e f f i c i e n t ' s o f the medium u s i n g 1) the Debye - Hucke l e q u a t i o n f o r <T m o l a l [NaOH] , 2) n u m e r i c a l i n t e g r a t i o n o f the G ibbs - Duhem e q u a t i o n f o r >1 m o l a l [NaOH]. The o s m o t i c c o e f f i c i e n t s were computed f rom e x p e r i m e n t a l vapour p r e s s u r e v e r s u s c o n c e n t r a t i o n d a t a . The r e s u l t s a r e g i v e n i n t a b l e 1 . 3 and f i g u r e 1 . 2 . E s t i m a t e s o f t h e mean m o l a l a c t i v i t y c o e f f i c i e n t s f o r more c o n c e n t r a t e d sodium h y d r o x i d e s o l u t i o n s can be T a b l e 1 . 3 Mean M o l a l A c t i v i t y C o e f f i c i e n t s f o r Sodium H y d r o -x i d e as a F u n c t i o n o f Temperatu re and C o n c e n t r a t i o n m(mol k g - 1 ) log n Temperature (°C) 0 20 40 60 80 100 120 1.0 -0 .175 -0.187 -0.199 -0 .213 -0.227 -0.241 -0.256 2.0 -0 .194 -0.207 -0.220 -0.234 -0 .248 -0 .263 -0.278 3.0 -0.150 -0 .168 -0 .188 -0.207 -0 .228 -0.249 -0.270 4.0 -0 .079 -0 .107 -0.134 -0.162 -0.191 -0 .220 -0.249 5.0 0.005 -0.032 -0.070 -0.107 -0 .145 -0 .184 -0.222 6.0 0.099 0.051 0.003 -0/046 -0.094 -0 .143 -0.191 7.0 0.200 0.140 0.080 0.020 -0.040 -0.099 -0.158 8.0 0.307 0.233 0.161 0.089 0.018 -0 .053 -0.122 9.0 0.419 0.331 0.244 0.160 0.077 -0 .005 -0.085 9.9 0.523 0.421 0.321 0.225 0.130 0.038 -0.052 S o u r c e : Macdona ld and McKubre 27 28 27 o b t a i n e d by e x t r o p o l a t i n g the r e s u l t s of Macdona ld and McKubre u s i n g the M e i s s n e r - T e s t e r g e n e r a l c h a r t s f o r a c t i v i t y c o -e f f i c i e n t d e t e r m i n a t i o n g i v e n i n r e f e r e n c e 2 8 . The mean m o l a l a c t i v i t y c o e f f i c i e n t f o r 1 6 . 6 7 g . m o l / k g NaQH^was; d e t e r m i ned to b e : Y + = 6 . 6 @ 20°C Y + = 3 . 2 @ 50°C 2 7 Macdona ld and McKubre a l s o s u g g e s t e d t h e t h e o r e t i c a l pH o f the s o l u t i o n goes t h r o u g h a maximum at about 5 g .mol k g - 1 NaOH and d e c l i n e s w i t h i n c r e a s i n g t e m p e r a t u r e . The c a l c u l a t i o n s o f the thermodynamic p r o p e r t i e s o f v e r y c o n c e n t r a t e d sodium h y d r o x i d e s o l u t i o n s : a r e v e r y i n -a c c u r a t e ; however they i n d i c a t e t h a t d e v i a t i o n s f rom i d e a l i t y are l a r g e , and thus a s i g n i f i c a n t f a c t o r i n the c h e m i c a l and e l e c t r o c h e m i c a l r e a c t i o n t h e r m o d y n a m i c s . 1 .S The A n o d i c B e h a v i o u r o f I ron i n S t r o n g A l k a l i n e  S o l u t i o n s P r e v i o u s work on the a n o d i c b e h a v i o u r of i r o n i n s t r o n g a l k a l i n e s o l u t i o n s has p r i m a r i l y been r e l a t e d t o p a s s i v a t i o n s t u d i e s and a l k a l i n e b a t t e r y s t u d i e s . 25 The work o f V e n k a t a d r i e t a l . (see pageiT5) i s the o n l y s t u d y r e l a t i n g f e r r a t e f o r m a t i o n to the a n o d i c p o t e n t i a l . The r e s u l t s shown i n f i g u r e 1 . 3 summarize the i n i t i a l coulom b i c e f f i c i e n c y o f f e r r a t e f o r m a t i o n as a f u n c t i o n of anode p o t e n t i a l . The s u p e r f i c i a l c u r r e n t d e n s i t i e s i n c r e a s e w i t h 2 i n c r e a s i n g anode p o t e n t i a l t o v a l u e s around 200 A/m . At h i g h anode p o t e n t i a l s t h e major e f f i c i e n c y l o s s e s a r e due 29 to oxygen e v o l u t i o n and o x i d e f o r m a t i o n . Sato and Kudo found t h a t i n a n e u t r a l b u f f e r e d s o l u t i o n i r o n d i s s o l u t i o n f rom a p a s s i v e f i l m began about 0 . 5 V above t h e oxygen e v o l u t i o n p o t e n t i a l ( see f i g u r e 1 . 4 ) . The i r o n e n t e r e d s o l u t i o n p resumab ly as an i r o n ( I I I ) s p e c i e s ; the amount i n c r e a s e d r a p i d l y w i t h i n c r e a s i n g anode p o t e n t i a l . C o n s i d e r a b l e d i s a g r e e m e n t e x i s t s i n the l i t e r a t u r e as to the t y p e and c o m p o s i t i o n o f the o x i d e s (or h y d r o x i d e s ) wh ich fo rm d u r i n g the a n o d i c p a s s i v a t i o n of i r o n . P a s s i v a -t i o n f i l m s have been i d e n t i f i e d as a s i n g l e 1 a y e r o f F e 3 0 4 , 2 9 - 3 1 Y F e 2 0 3 2 9 ' 3 2 " 3 3 o r F e O O H . 3 4 ' 3 5 A d u p l e x l a y e r c o n s i s t i n g o f an o u t e r 1 a y e r o f Y f ^ O - j and an i n n e r l a y e r of Fe^O^ has been p r o p o s e d by V e t t e r and o b s e r v e d by s e v e r a l r e s e a r c h e r s . 3 ^ ' 3 9 S e v e r a l a u t h o r s have r e p o r t e d a d e f e c t s t r u c t u r e a c r o s s t h e o x i de,wi th the o u t e r s u r f a c e d e f i c i e n t i n i r o n 38 40 38 i o n s . ' Nagayama and Cohen, f o r a b u f f e r e d s o l u t i o n a t pH 8 . 4 , s u g g e s t e d the o u t e r d e f e c t s t r u c t u r e was 7 0 5 6 0 U J 0 5 0 Ll_ u_ L J 4 0 o 1 3 0 o g 2 0 o 10 2 5 E C 3 0 E C 35* C - x - x - 40° C / / \ ' ' hi/ \ V 7< \ ^ / / \ \ 30 .55 . 6 0 .65 .70 POTENTIAL (volts vs. Ho/HoO/l0.5M KOH) F ig . 1.3 The e f f e c t of potent ial on the e f f i c i e n c y of fe r ra te (VI) production. 25 Source: Venkatadri et a l . 12 rV 8 E \ -o E O 4 0 possivity ^ 0± evolution , J w, -I 0 I .E. V(sce) 8 I 4 £ rf E i f F ig . 1.4 Amount of charge passed (open c i r c l e s ) and i ron dissolved ( s o l i d c i r c l e s ) in 1 h anodic ox idat ion . Source: Sato and Kudo 2 9 6 + F e x . ^ e2-2x " ^ " ^ 3 " ^ ^ 1 S 3 c a * 1 0 n vacancy and a t 40 oxygen e v o l u t i o n p o t e n t i a l s x - 0 . 3 . Chen and Cahan s u g -g e s t e d 5 -10% of the charge f o r f i l m r e d u c t i o n i s a s s o c i a t e d w i t h the r e d u c t i o n of h i g h e r v a l e n c y i r o n s p e c i e s . 4 1 4 2 Macdona ld and McKubre ' have c o n d u c t e d an e x t e n -s i v e s t u d y o i f . t h e b e h a v i o u r of i r o n i n s t r o n g a l k a l i n e e l e c t r o l y t e s a t a n o d i c p o t e n t i a l s below t h a t of oxygen e v o l u t i o n . The a n o d i c p r o c e s s e s were examined by c y c l i c v o l t a m e t e r y ( s t a t i o n a r y , r o t a t i n g d i s c and r o t a t i n g r i n g d i s c e l e c t r o d e s ) , c o u l o m e t r y and a . c . .iimpedance s t u d i e s . The e f f e c t s o f h y d r o x y l i o n c o n c e n t r a t i o n up to 9 . 1 g . m o l s / 1 and t e m p e r a t u r e s between - 2 0 and 120°C were e x a m i n e d . From i n t e r p r e t a t i o n of RRDE s t u d i e s the a u t h o r s p roposed h i g h and 1ow t e m p e r a t u r e r e a c t i o n sequences f o r a n o d i c o x i d a t i o n of i r o n . The p r o p o s e d h i g h t e m p e r a t u r e ( > 5 0 ° C ) mechanism i s g i v e n i n f i g u r e 1 . 5 . S e v e r a l p a r a l l e l r e a c t i o n p a t h s were p r o p o s e d and d i s s o l v e d i r o n ( I I ) and i r o n ( I I I ) i n t e r -m e d i a t e s p e c i e s were o b s e r v e d . The i r o n i s o x i d i z e d f i r s t to Fe(0H. ) 2 v i a a d i s s o l v e d i r o n ( I I ) s p e c i e s (H F e 0 2 ) ; w i t h i n c r e a s i n g p o t e n t i a l m e t a l l i c i r o n and t h e F e ( 0 H ) 2 p r e s e n t a r e o x i d i z e d to m a g n e t i t e . A t : h i g h e r p o t e n t i a l s the F e ( 0 H ) 2 p r e s e n t i s o x i d i z e d to F e 2 0 3 f o l l o w e d b y - o x i d a t i o n of the m a g n e t i t e p r e s e n t , v i a a d i s s o l v e d i r o n ( I I I ) s p e c i e s ( s u g g e s t e d F e ( 0 H ) 4 ) to F e 2 0 3 - At more' p o s i t i v e 32 Oxidation Fe 01 02(b) 02(a) Fe(OH). Precipitation HFeO-T F e 3 0 4 0 3 04 Fe(OH), Precipitation t F e 2 0 3 Kiah Potential FeOOH .Reduction . Fe R1 Fe(OH), N o Pe2k Precipitation H F e 0 2 ~ R2 R3 F e 2 0 3 FeOOH 1 . 5 S i m p l i f i e d h i g h t e m p e r a t u r e r e a c t i o n sequences f o r i r o n p a s s i v a t i o n i n 30 wt % K0H. 42 S o u r c e : McKubre 33 p o t e n t i a l s FeOOH f o r m a t i o n o c c u r s . The a u t h o r s c a l c u l a t e d t h e o x i d e t h i c k n e s s e s formed were between 1 and 3 um and d e c l i n e above 40°C due t o b u l k f i l m d i s s o l u t i o n . They c a l c u l a t e d t h a t a t 5 0 Q C and 75°C, 2 . 5 5 and 5 .98% r e s p e c t i v e l y of the charge passed r e s u l t e d i n meta l d i s s o l u t i o n . 1 .9 The E l e c t r o c h e m i c a l E v o l u t i o n of Oxygen The T a f e l s l o p e of a r e a c t i o n i s a measure of t h e dependence o f the e l e c t r o c h e m i c a l r e a c t i o n r a t e on the p o t e n t i a l and i s thus an i n d i c a t i o n of the o v e r a l l r e a c t i o n k i n e t i c s . The T a f e l s l o p e f o r oxygen e v o l u t i o n on i r o n i n 44 45 1 M a l k a l i i s r e p o r t e d as 0 . 0 7 V. Young c o n c l u d e s t h a t , g e n e r a l l y , oxygen e v o l u t i o n r a t h e r i n e x a c t l y obeys the T a f e l e q u a t i o n and the r e s u l t s a r e o f t e n , u n r e p r o d u c i b l e . He a t t r i b u t e s t h e s e f a c t s to the s t r o n g dependence o f the r e a c t i o n on the s u r f a c e c o n d i t i o n o f the a n o d e . A g e n e r a l mechanism of oxygen e v o l u t i o n on m e t a l s has 36 4 5 - 4 6 not been d e v e l o p e d . ' The d i r e c t c h e m i c a l p a r t i c i p a -t i o n o f t h e s u r f a c e o x i d e , i n the mechanism has been re -p o r t e d , ^ 6 » 4 6 however the p a r t i c i p a t i o n o f a h i g h e r v a l e n c e 45 meta l o x i d e i s g e n e r a l l y d i s c o u n t e d . A commonly p roposed m e c h a n i s m , i n w h i c h the e l e c t r o c h e m i c a l s t e p ( .equat ion 1 . 2 2 b ] i s r a t e d e t e r m i n i n g , OH" (OH) ads + e - 1 . 2 2 a (OH) ads + OH (0) ads + H 2 0 + e - 1 . 2 2 b 2 (0) ads - 1 . 2 2 c The o v e r a l l r e a c t i o n b e i n g : 4 OH - 0 2 + 2 H 2 0 + 4 e - 1 . 2 3 At v e r y h i g h c u r r e n t d e n s i t i e s r e a c t i o n 1 . 2 2 c c o u l d become the r a t e d e t e r m i n i n g s t e p . In the d i r e c t sense the r e a c t i o n wou ld t h e n be i n d e p e n d e n t o f p o t e n t i a l and d e v i a t i o n s f rom 47 a T a f e l r e l a t i o n s h i p may r e s u l t . B o c k r i s and Azzam have o b s e r v e d d e v i a t i o n s f rom t h e T a f e l r e l a t i o n s h i p f o r hydrogen e v o l u t i o n on c e r t a i n m e t a l s Ce.g. N i ) a t v e r y h i g h c u r r e n t 48 d e n s i t i e s . R u e t s c h i and De lahay have s u g g e s t e d t h a t the bond energy Me-OH c o n t r o l s the oxygen e v o l u t i o n o v e r p o t e n t i a l . 1 .10 Purpose o f the P r e s e n t I n v e s t i g a t i o n The i n v e s t i g a t i o n has as i t s g o a l s two main o b j e c t s : 1) The d e t e r m i n a t i o n o f the e f f i c i e n c y o f f e r r a t e f o r m a -t i o n by e l e c t r o c h e m i c a l means , and 2) a s t u d y o f the s t a b i l i t y o f f e r r a t e i o n s f o r t h e range o f e x p e r i m e n t a l c o n d i t i o n s c o n s i d e r e d f o r e l e c t r o c h e m i c a l f e r r a t e f o r m a t i o n . From t h e s e e x p e r i m e n t a l r e s u l t s i t was hoped t h a t the optimum c o n d i t i o n s f o r the e l e c t r o c h e m i c a l p r o d u c t i o n o f f e r r a t e i o n s c o u l d be d e t e r m i n e d . W i t h i n the l i m i t s p o s s i b l e f o r an e x p e r i m e n t a l s t u d y , i t was a l s o hoped t h a t a mechanism f o r f e r r a t e f o r m a t i o n c o u l d be d e v e l o p e d f rom t h e r e l a t i o n s h i p found i n the measurements . C h a p t e r 2 THE STABILITY OF FERRATE SOLUTIONS 2 . 1 I n t r o d u c t i on The d e t e r m i n a t i o n of the s t a b i l i t y o f f e r r a t e s o l u t i o n s as a f u n c t i o n of the sys tem v a r i a b l e s i s e s s e n t i a l f o r a s t u d y o f the e l e c t r o c h e m i c a l g e n e r a t i o n of f e r r a t e i o n s . To c a l c u l a t e the t r u e e f f i c i e n c y of f e r r a t e f o r m a t i o n f rom measurements o f f e r r a t e c o n c e n t r a t i o n the amount o f f e r r a t e d e c o m p o s i t i o n wh ich has o c c u r r e d must be d e t e r m i n e d . The d e t e r m i n a t i o n o f the dependence o f f e r r a t e s t a b i l i t y on e x p e r i m e n t a l c o n d i t i o n s w i l l a l s o be u s e f u l f o r the o p t i m i z a t i o n o f f e r r a t e y i e l d s e s p e c i a l l y i f the e x p e r i m e n t c o n d i -t i o n s r e q u i r e d f o r m a x i m i z i n g f e r r a t e g e n e r a t i o n and m i n i m i z i n g f e r r a t e d e c o m p o s i t i o n a r e i n c o n f l i c t . The e x p e r i m e n t a l 1 2 v a r i a b l e s wh ich have been r e p o r t e d to be o f i m p o r t a n c e to the d e c o m p o s i t i o n o f f e r r a t e s o l u t i o n s a r e : 1) t e m p e r a t u r e , 2) h y d r o x y l i o n c o n c e n t r a t i o n , 3) f e r r a t e i o n c o n c e n t r a t i o n and 4) s o l u t i o n i m p u r i t i e s , i n c l u d i n g s u s p e n s i o n s . 2 . 2 E x p e r i m e n t a l 2 . 2 . 1 Reagents Sodium h y d r o x i d e s o l u t i o n s were p r e p a r e d by d i s -s o l v i n g the r e q u i r e d q u a n t i t y of a n a l y t i c a l r e a g e n t grade sodium h y d r o x i d e i n a known volume o f d i s t i l l e d w a t e r . F e r -r a t e s o l u t i o n s were p r e p a r e d by a s t a n d a r d p r o c e d u r e . F r e s h l y abraded Armco I ron Anodes were e l e c t r o l y z e d a t 10 KAym 2 - i n 1 4 . 3 g . m o l / l NaOH a t 50°C f o r up to 10 x 1 0 3 s e c o n d s . A p l a t i n i u m c a t h o d e was u s e d ; s e p a r a t e d f rom the a n o l y t e by a porous p o l y p r o p y l e n e membrane ( C e l a n e s e R e s e a r c h C o . , Summit , N . J . ) . The s o l u t i o n s were s t o r e d o v e r n i g h t a t 0°C p r i o r to e x p e r i m e n t a t i o n . 2 . 2 . 2 P r o c e d u r e The s t a n d a r d f e r r a t e s o l u t i o n s were d i l u t e d to the d e s i r e d sodium h y d r o x i d e c o n c e n t r a t i o n and samples were p l a c e d i n c l e a n 50 ml g l a s s b o t t l e s . The g l a s s sample b o t -t l e s were then p l a c e d i n a w a t e r bath ( P r e c i s i o n S c i e n t i f i c C o . ) . The t e m p e r a t u r e was r e g u l a t e d to + 0 . 1 ° C . On r e a c h i n g the bath t e m p e r a t u r e an a l i q u o t of the sample was removed and s p e c t r o p h o t o m e t r i c a l l y a n a l y z e d f o r f e r r a t e c o n c e n t r a -t i o n u s i n g a Bausch and Lomb ( S p e c t r o n i c 88) S p e c t r o p h o t o -meter a t a w a v e l e n g t h o f 515 nhi. The a n a l y s e s was s t a n d a r d -i z e d u s i n g s t a n d a r d s o l u t i o n s p r e p a r e d f rom c h e m i c a l l y a n a l y z e d s o l i d p o t a s s i u m f e r r a t e . The a n a l y s i s and s t a n d a r d -i z a t i o n p r o c e d u r e s a r e d e t a i l e d i n Append ix B. A l i q u o t s f rom each sample were t a k e n a t r e g u l a r i n t e r v a l s and a n a l y z e d f o r f e r r a t e c o n c e n t r a t i o n . 38 2 . 2 . 3 E r r o r s The t h r e e main s o u r c e s o f e x p e r i m e n t a l e r r o r a r e : 1) The c a l i b r a t i o n o f the s p e c t r o p h o m e t e r . I t was c a l c u -l a t e d (see Append ix B) t h a t the e r r o r a s s o c i a t e d w i t h the s p e c t r o p h o m e t e r c a l i b r a t i o n i s +^  2%. 2) The a c c u r a c y o f r e a d i n g the s p e c t r o p h o t o m e t e r w i t h i n the range 0 . 4 - 0 . 7 a b s o r b a n c e u n i t s ; wh ich i s 3) The e r r o r s due t o the a c c u r a c y o f c o n t r o l o f the e x p e r i m e n t a l v a r i a b l e s . The v o l u m e t r i c e r r o r s of p i p e t t e s and v o l u m e t r i c f l a s k s of about 0 . 1 % and the d i l u t i o n e r r o r s a r e i n s i g n i f i c a n t . Q u a n t i f i c a t i o n of e r r o r s r e s u l t i n g f rom the c o n t r o l o f e x p e r i -menta l v a r i a b l e s i s not p o s s i b l e . The v a r i a t i o n i n measurement e r r o r s o v e r the e x p e r i m e n t a l range o f f e r r a t e c o n c e n t r a t i o n s were c a l c u l a t e d a s : @ 3 x 10 g .mol FeOj /I E + 7 x 10 - 5 g .mol/1. (2 .3%) E + 3 x 10 - 5 g .mol/1. (3%) @ 5 x 1 0 " 4 g .mol F e 0 2 " / 1 E + 2 x 10 - 5 g . m o l / I (4%) @ 1 x 1 0 - 4 g .mol F e O 2 " / ! . + 1 . 2 x 1 0 - 5 g . m o l / 1 (12%) 39 2 . 3 R e s u l t s and D i s c u s s i o n 2 . 3 . 1 The E f f e c t s o f F e r r a t e Ion C o n c e n t r a t i o n s The e f f e c t s o f the i n i t i a l f e r r a t e c o n c e n t r a t i o n on the r a t e o f d e c o m p o s i t i o n o f f e r r a t e s o l u t i o n s at333K and i n 1 4 . 3 g . m o l s / 1 NaOH are g i v e n i n t a b l e s C . l to C .4 and f i g u r e 2 . 1 . Four d i f f e r e n t f e r r a t e c o n c e n t r a t i o n s r a n g i n g f rom 1 .797 x 1 0 ~ 3 to 0 . 4 6 1 x 1 0 " 3 g . m o l / 1 were u s e d . The a p p a r e n t d e c l i n e i n d e c o m p o s i t i o n r a t e w i t h t i m e s u g g e s t s t h a t the r a t e o f d e c o m p o s i t i o n o f f e r r a t e i o n s i s dependent on the f e r r a t e c o n c e n t r a t i o n . The o r d e r o f the f e r r a t e d e c o m p o s i t i o n r e a c t i o n k i n e t i c s , w i t h r e s p e c t to the f e r r a t e i o n c o n c e n t r a t i o n , was d e t e r m i n e d g r a p h i c a l l y . For f i r s t o r d e r r e a c t i o n s k i n e t i c s : 2 -dCFeO, 3 2 _ .. = " k D C F e 0 4 1 - 2 - 1 dt k D = F i r s t o r d e r d e c o m p o s i t i o n r e a c t i o n r a t e c o n s t a n t o " [ F e 0 4 _ ] = F e r r a t e c o n c e n t r a t i o n ( g . m o l s / 1 ) . R e a r r a n g i n g and i n t e g r a t i n g between the l i m i t s t = 0 and t = t g i v e s : 6 7 8 Time, seconds 10 (xlO 3) II F i g . 2 .1 The e f f e c t of f e r r a t e c o n c e n t r a t i o n on the f e r r a t e d e c o m p o s i t i o n r a t e . [NaOH] = 1 4 . 3 g .mol/1 T = 333K. , 2 -l Q 9 e [ F e 0 4 ] t = t " l Q 9 e [ F e ° P t = 0 = " V - 2 . 2 Thus a p l o t o f l o g ^Q [ F e O ^ - ] v e r s u s t i m e s h o u l d g i v e a s t r a i g h t l i n e o f s l o p e - k g / 2 . 3 0 3 i f the k i n e t i c s of the f e r r a t e d e c o m p o s i t i o n r e a c t i o n a r e f i r s t o r d e r w i t h r e s p e c t to t h e f e r r a t e c o n c e n t r a t i o n . For 3/2 o r d e r f e r r a t e d e c o m p o s i t i o n r e a c t i o n k i n e t i c s d [ F e C - p dt = - k [ F e p j - ] u 3 / 2 4 3/2 - 2 . 3 R e a r r a n g i n g and i n t e g r a t i n g between the l i m i t s t=0 and t=t g i v e s : FeO' 2 -t=t FeO, t=0 = ~ k n t u 3 / 2 - 2 . 4 Thus i f the k i n e t i c s of the f e r r a t e d e c o m p o s i t i o n r e a c t i o n 3 a re ^ o r d e r w i t h r e s p e c t o f f e r r a t e i o n c o n c e n t r a t i o n a graph o f l / [ F e 0 4 ] 2 v e r s u s t i m e s h o u l d g i v e a s t r a i g h t l i n e o f s l o p e k. J 3/2 /2 H i g h e r o r d e r r e a c t i o n k i n e t i c s were c a l c u l a t e d s i m i l a r way. The e x p e r i m e n t a l r e s u l t s were examined to a t h i r d o r d e r d e c o m p o s i t i o n r e a c t i o n k i n e t i c s . i n a f o r up P l o t s f o r f i r s t o r d e r and ^ o r d e r r e a c t i o n k i n e t i c s r e l a t i o n s h i p s u s i n g the e x p e r i m e n t a l d a t a a re g i v e n i n f i g u r e s 2 . 2 and 2 . 3 . The s i m p l e r e a c t i o n o r d e r r e l a t i o n s h i p s wh ich p r o v i d e 3 the b e s t f i t a r e the f i r s t and j o r d e r r e a c t i o n k i n e t i c s . In both c a s e s the i n i t i a l f e r r a t e d e c o m p o s i t i o n d a t a p o i n t s do not f i t w i t h i n measurement e x p e r i m e n t a l e r r o r . T h i s c o u l d be due to the r e a c t i o n k i n e t i c s h a v i n g mixed o r d e r k i n e t i c s or due t o some v a r i a t i o n i n t h e e x p e r i m e n t a l c o n -d i t i o n s . The i n i t i a l decay was not a l w a y s o b s e r v e d i n l a t e r e x p e r i m e n t s . The p r e s e n c e o f s m a l l q u a n t i t i e s of r e d u c i b l e i m p u r i t i e s c o u l d produce t h e s e d e v i a t i o n s f rom l i n e a r i t y . The s i o p e s v t f f t h e d e c o m p o s i t i o n c u r v e s show a s l i g h t i n c r e a s e as the i n i t i a l f e r r a t e c o n c e n t r a t i o n i s d e c r e a s e d . The s.Hopes s h o u l d be c o n s t a n t i f the c o n c e n t r a t i o n o f f e r r a t e i s the o n l y v a r i a b l e of i m p o r t a n c e t o the d e c o m p o s i t i o n r e a c t i o n . The f e r r a t e d e c o m p o s i t i o n p r o d u c t ( f e r r i c h y d r o -x i d e ) i s c a t a l y t i c to f e r r a t e d e c o m p o s i t i o n and t h u s a d e c r e a s e i n the f e r r a t e d e c o m p o s i t i o n r a t e w i t h d e c r e a s i n g i n i t i a l f e r r a t e c o n c e n t r a t i o n (which i s o p p o s i t e to t h a t o b s e r v e d ) c o u l d be e x p e c t e d . A l t h o u g h t h e f e r r a t e d e c o m p o s i t i o n r e a c t i o n d a t a can 3 be a p p r o x i m a t e d e q u a l l y w e l l to f i r s t o r d e r o r ~- o r d e r 43 f g . m o l s / 1 . o 1 8 0 ( x l O - 3 ) V 1 - 3 4 • 0 - 9 2 A 0 - 4 6 8 10 Time, seconds (x lO 3 ) F i g . 2.2 l o g [FeO^V ] v e r s u s t i m e f o r v a r i o u s f e r r a t e c o n c e n t r a t i o n s . ' < 44 n o 7>o { F e ° 4 lint. 9.™>ls/l. -o 1-80 ( xlO"3) V 1-34 - -• 0-9 2 A 0 - 4 6 -0 2 4 6 8 10 12 Time, seconds (x I0 3 ) 2 - i-F i g . 2 . 3 1 / [ F e 0 4 ] 2 v e r s u s t i m e f o r v a r i o u s f e r r a t e c o n c e n t r a t i o n s . r e a c t i o n k i n e t i c s , f o r the i n v e s t i g a t i o n o f the o t h e r e x p e r i m e n t a l v a r i a b l e s the r e a c t i o n i s c o n s i d e r e d t o have f i r s t o r d e r k i n e t i c s w i t h r e s p e c t to the f e r r a t e i o n c o n -c e n t r a t i o n . 2 . 3 . 2 The E f f e c t s o f Temperature The r a t e s c o f f e r r a t e d e c o m p o s i t i o n f o r t e m p e r a t u r e s f rom 273 to 353 K i n 1 4 . 3 g . m o l / 1 NaOH a^e g i v e n i n t a b l e s C . l , C . 5 to C .9 and f i g u r e s 2 . 4 to 2 . 8 . The e f f e c t o f the _ 3 i n i t i a l f e r r a t e c o n c e n t r a t i o n s r a n g i n g f rom 1 . 3 x 10 to _ 3 3 . 0 x 10 g . m o l / 1 i s c o n s i d e r e d n e g l i g i b l e . The r a t e o f f e r r a t e d e c o m p o s i t i o n i n c r e a s e s r a p i d l y w i t h i n c r e a s i n g t e m p e r a t u r e . Over the range o f t e m p e r a t u r e s c o n s i d e r e d the f e r r a t e d e c o m p o s i t i o n r a t e i n c r e a s e s by about a f a c t o r of ten f o r e v e r y 25°C . . i n c r e a s e ' ' i n t e m p e r a t u r e . The f e r r a t e d e c o m p o s i t i o n r a t e c o n s t a n t s f o r the v a r i o u s t e m p e r a t u r e s as d e t e r m i n e d g r a p h i c a l l y a r e g i v e n i n t a b l e 2 . 1 . The r a t e c o n s t a n t k n can be w r i t t e n a s : D • A exp - 2 . 5 RT A C o n s t a n t (s~ ) A c t i v a t i o n energy ( J/g . imol ) 46 F i g . 2 . 4 The e f f e c t o f t e m p e r a t u r e on t h e f e r r a t e d e c o m p o s i t i o n r a t e . [NaOH] = 1 4 . 3 g . m o l / 1 . - 2 - 2 - 2 - 4 O o °.-2-8 47 - 3 2 r — i 1 — 1 1 T = 2 7 3 K . iu slope = - 3 - 3 6 x I0 - 7 ( r ' ) - \ « - --1 1 1 I V - 2 - 2 - 2 - 4 • = 2 9 4 K . V=3I3K. slope= - 4 0 3 x l O " 6 (s -') 4 8 12 16 20 T i m e , seconds (x lO 5 } F i g . 2 . 5 10 Ti F i g . 2 . 6 20 30 ime , seconds ( x l O ) - 2 - 6 O ^ - 3 - 0 o o T • = 3 2 3 K . V = 3 3 3 K . s lope= -2 -70 x I0- 5 (s-') 1 - 3 - 2 - 3 - 4 - 3 - 6 - 2 - 4 - 2 6 -2 -8 O i f -3-0 o o slope = - 5 - 4 2 x I0" 5{s"' - 3 2 - 3 - 4 - 3 - 6 I ' - 1 — 1 T=353K. • ^ s l o p e = -3 -97 x I0"4 - -1 1 1 4 8 12 T ime , seconds ( x l O 3 ) F i g . 2 . 7 0 4 0-8 12 1-6 2-0 Time, seconds (xlO 3 ) F i g . 2 . 8 F i g . ; . 2 . 5 to l o g [ F e O 2 - ] v e r s u s t ime f o r T = 2 7 3 , 2 9 4 , 31 3 , F i g . 2 . 8 323, 333 and 353 K. 48 T a b l e 2 . 1 The E f f e c t of Temperatu re on the G r a p h i c a l l y D e t e r m i n e d F e r r a t e D e c o m p o s i t i o n Rate C o n s t a n t [NaOH] = 1 4 . 3 g . m o l / 1 Temperature kD 1/T log k n  3 e D (K) Is" 1 ) (K" 1 ) ( x l O - 3 ) 273 7.74 x 1 0 ' 7 3.6590 -14.07 294 9.28 x 1 0 " 6 3.3979 -11.59 313 4.10 x 1 0 " 5 3.1918 -10.10 323 6.22 x 1 0 " 5 3.0931 - 9.69 333 1.25 x 1 0 " 4 3.0003 - 8.99 353 7.82 x 1 0 - 4 2.8305 - 7.15 An Ar rhen iusv p l o t o f l o g e k D v e r s u s 1/T i s g i v e n i n f i g u r e 2 . 9 . The s l o p e o f the graph i s - 7 . 9 4 5 x 1 0 3 (K) , t h u s f rom e q u a t i o n 2 . 5 : E a = 66 K J / g . m o l FeO?" A = 3 . 7 x 1 0 6 s " 1 I t i s d i f f i c u l t to a s s e s s a c c u r a t e l y the e r r o r s o f g r a p h i c a l l y d e t e r m i n e d q u a n t i t i e s . The c a l c u l a t e d maximum range o f the g r a p h i c a l s l o p e s due t o measurement e r r o r s l e a d s to an a c t i v a -t i o n energy e r r o r o f about + 1%. The e r r o r s due t o the c o n t r o l o f e x p e r i m e n t a l v a r i a b l e s , wh ich cannot be q u a n t i f i e d , a r e -- 8 -I i T : 1 - 1 0 — \ s l 0 p e = - 7 89 x I0 3 (K). 1 o» o _J - 1 2 -/ - 1 4 i I 1 I -2 8 3 0 3 2 3 4 3 - 6 3 8 l/T. K"' (xlO-3) F i g . 2 . 9 A r r h e n i u s p l o t f o r the f e r r a t e d e c o m p o s i t i o n r e a c t i o n . [NaOH] = 1 4 . 3 ; g . m o l / 1 . 50 p r o b a b l y s i g n i f i c a n t and thus the p r o b a b l e e r r o r o f the d e t e r -mined a c t i v a t i o n energy w i l l most l i k e l y be c o n s i d e r a b l y g r e a t e r than + 1 %. 2 . 3 . 3 The E f f e c t s o f Sodium H y d r o x i d e C o n c e n t r a t i o n The r a t e o f f e r r a t e d e c o m p o s i t i o n f o r sodium h y d r o x i d e c o n c e n t r a t i o n s ffr^.m 5 t o 1 4 . 3 g . m o l / 1 a t 323 K a r e g i v e n i n t a b l e s C I O to C .12 and f i g u r e s 2 . 1 0 to 2 . 1 3 . The s t a b i l i t y o f f e r r a t e i o n s o l u t i o n s i n c r e a s e s r a p i d l y as the sodium h y d r o x i d e c o n c e n t r a t i o n i s i n c r e a s e d . T a b l e 2 . 2 and f i g u r e 2 . 1 3 i n d i c a t e t h a t , f o r the range o f sodium h y d r o x -i d e , c o n c e n t r a t i o n s c o n s i d e r e d , the d e c o m p o s i t i o n r a t e c o n -s t a n t can be a p p r o x i m a t e d to a l i n e a r f u n c t i o n of the sodium h y d r o x i d e c o n c e n t r a t i o n . T a b l e 2 . 2 The E f f e c t s o f H y d r o x y ! Ion C o n c e n t r a t i o n on the  G r a p h i c a l l y D e t e r m i n e d F e r r a t e D e c o m p o s i t i o n Rate C o n s t a n t @ 323 K [NaOH] k D g . m o l s / 1 s " 1 5 4 . 0 7 6 x 1 0 " 4 10 2 . 1 5 1 x 1 0 " 4 12 1 . 2 4 8 x 1 0 " 4 1 4 . 3 6 . 2 1 8 x TO" 5 51 i 1 r [ N a O H ] . g . m o l s / 1 . 2 4 6 8 10 ' 12 Time, seconds (x lO 3 ) F i g . 2 . 1 0 The the e f f e c t o f sodium h y d r o x i d e c o n c e n t r a t i o n on f e r r a t e decomposi t i o n . r a t e . T = 323 K. 52 o O -2-8 ! -3-0 •3-2 -3-4 1 1 1 1 1 r +, (NaOH) . g.mols/1. A 1 2 O 10 ^ i s s s l o p e = - 5 - 4 2 x l O _ s ( s - ' ) " s lope = - 9 - 3 4 x l O " 5 ($-') 1 1 1 \ 1 1 ^ 6 8 10 12 14 Time, seconds (xlO ) F i g . 2 . 1 2 F i g . 2 . 1 1 to l o g [ F e O 2 / ] v e r s u s t ime f o r [NaOH] = 5 , 10 F i g . 2 . 1 2 a n d 1 2 ' g . m o l / 1 . 53 4 6 8 10 12 14 16 | N G O H | . g.mols/l. F i g . 2 . 1 3 The sodium h y d r o x i d e dependence o f the f e r r a t e d e c o m p o s i t i o n - r a t e c o n s t a n t @ T = 323 K. An e m p i r i c a l l i n e a r e q u a t i o n t o f i t the e x p e r i m e n t a l d a t a i s : k D(NaOH) = 5 , 9 6 x 1 0 " 4 " 3 , 8 9 5 x 1 0 " 5 C N a 0 H ] " 2 - 6 5< [NaOH] c< l 4. 3 g .mol/1 k D(NaOH) = Sodium h y d r o x i d e dependent r a t e c o n s t a n t ( s " 1 ) G> 323 K E q u a t i o n 2 . 6 i s not v a l i d a t h i g h e r sodium h y d r o x i d e c o n -c e n t r a t i o n s . The p a r a m e t e r of i m p o r t a n c e t o the f e r r a t e decom-p o s i t i o n r e a c t i o n r a t e c o n s t a n t w i l l be the a c t i v i t y of the w a t e r ( w i t h wh ich the f e r r a t e r e a c t s ) and c o n s e q u e n t l y the h y d r o x y l i o n a c t i v i t y . E q u a t i o n 2 . 6 c o u l d be w r i t t e n i n terms of the a c t i v i t y or c o n c e n t r a t i o n of w a t e r i n the s y s t e m . The c a l c u l a t i o n s f rom the work o f Macdona ld and 2 7 McKubre (see page/25) s u g g e s t t h e r e i s a g r a d u a l d e c l i n e i n the a c t i v i t y of w a t e r o v e r the range o f e x p e r i m e n t a l sodium h y d r o x i d e c o n c e n t r a t i o n s examined i n t h i s s e c t i o n . The i n a c c u r a c i e s o f the c a l c u l a t i o n s cdos not a l l o w the f e r r a t e d e c o m p o s i t i o n r a t e t o be d e t e r m i n e d , a c c u r a t e l y , as a f u n c t i o n of the wate r a c t i v i t y , however the r e s u l t s do s u g g e s t a f i r s t o r d e r dependence on the w a t e r a c t i v i t y or c o n c e n t r a t i o n . 55 2 . 3 . 4 The E f f e c t s o f P a r t i c u l a t e F e r r i c H y d r o x i d e No q u a n t i t a t i v e r e s u l t s as to the e f f e c t s of f e r r i c h y d r o x i d e s u s p e n s i o n s on the d e c o m p o s i t i o n r a t e o f f e r r a t e s o l u t i o n s were o b t a i n e d . I t was however o b s e r v e d t h a t the p r e s e n c e o f f e r r i c h y d r o x i d e s u s p e n s i o n s d r a m a t i c a l l y i n -c r e a s e d the r a t e o f f e r r a t e d e c o m p o s i t i o n . The two prob lems t h a t p r e v e n t e d q u a n t i f i c a t i o n of the r e s u l t s w e r e : 1) f e r r i c h y d r o x i d e s u s p e n s i o n s i n t e r f e r e w i t h the s p e c t r o -p h o t o m e t r y d e t e r m i n a t i o n o f the f e r r a t e i o n c o n c e n t r a t i o n , 2) a t t e m p e r a t u r e s g r e a t e r t h a n room t e m p e r a t u r e the c a t a -l y t i c e f f e c t o f f e r r i c h y d r o x i d e s u s p e n s i o n s i s so l a r g e t h a t the f e r r a t e d e c o m p o s i t i o n i s too r a p i d t o be measured a c c u r a t e l y . 2 . 4 Summary and Compar ison w i t h P r e v i o u s Work 1 o The work o f Tousek (see page'12) i s the o n l y p r e v i o u s s t u d y o f f e r r a t e d e c o m p o s i t i o n wh ich was c a r r i e d out ove r a comparab le range o f e x p e r i m e n t a l v a r i a b l e s . A summary o f the f i n d i n g s o f t h i s i n v e s t i g a t i o n , and a c o m p a r i s o n w i t h the f i n d i n g s of Tousek aine g i v e n b e l o w . 1) The f e r r a t e d e c o m p o s i t i o n r e a c t i o n can be a p p r o x i m a t e d 3 to e i t h e r f i r s t o r d e r o f o r d e r r e a c t i o n k i n e t i c s w i t h r e s p e c t to the f e r r a t e i o n c o n c e n t r a t i o n . Tousek d e t e r m i n e d the f e r r a t e d e c o m p o s i t i o n r e a c t i o n k i n e t i c s were f i r s t o r d e r w i t h r e s p e c t to the f e r r a t e i o n . 56 c o n c e n t r a t i o n . 2) The a c t i v a t i o n energy of the f e r r a t e d e c o m p o s i t i o n r e a c t i o n was determined-to be 66 K J / g . m o l . T h i s r e s u l t i s 31% s m a l l e r than t h e a c t i v a t i o n energy found by Tousek . The e x p e r i m e n t a l a c c u r a c y of T o u s e k ' s work i s not known. 3) The r a t e o f f e r r a t e d e c o m p o s i t i o n d e c r e a s e s a p p r o x i -m a t e l y l i n e a r l y w i t h i n c r e a s i n g / sodium h y d r o x i d e c o n c e n t r a t i o n f o r sodium h y d r o x i d e c o n c e n t r a t i o n s between 5 and 14 g . m o l / 1 . T h i s s u g g e s t s a f i r s t o r d e r dependence on w a t e r c o n c e n t r a t i o n or a c t i v i t y . T o u s e k ' s r e s u l t s were not q u a n t i f i e d but the g e n e r a l shape o f the d e c o m p o s i t i o n c u r v e and the observed' sodium h y d r o x i d e c o n c e n t r a t i o n dependence was not i n c o n -s i s t e n t w i t h the r e s u l t s o f t h i s i n v e s t i g a t i o n . 4) F e r r i c h y d r o x i d e s u s p e n s i o n s were found to g r e a t l y i n c r e a s e the r a t e o f d e c o m p o s i t i o n of f e r r a t e s o l u -t i o n s . T h i s f i n d i n g i s i n agreement w i t h the work of Tousek . 2 . 5 The A d d i t i v e E f f e c t s of the I n d i v i d u a l V a r i a b l e s  A f f e c t i n g the Rate of F e r r a t e D e c o m p o s i t i o n I t i s i m p r o b a b l e t h a t the e f f e c t s o f the i n d i v i d u a l sys tem v a r i a b l e s on the r a t e o f f e r r a t e d e c o m p o s i t i o n a r e d i r e c t l y a d d i t i v e . The c o r r e l a t i o n between the f e r r a t e d e c o m p o s i t i o n r a t e and the sodium h y d r o x i d e c o n c e n t r a t i o n i s s u s p e c t and the v a l i d i t y o f u s i n g t h i s e q u a t i o n i n c o n -j u n c t i o n w i t h v a r i a t i o n s of o t h e r p a r a m e t e r s i s d e b a t a b l e . However , n o t w i t h s t a n d i n g t h i s , rough e s t i m a t e s o f the f e r -r a t e d e c o m p o s i t i o n r a t e s can be made u s i n g . t h e ; combined e q u a t i o n s f o r the e f f e c t s of f e r r a t e c o n c e n t r a t i o n , t e m p e r a -t u r e and sodium h y d r o x i d e concentration, ( e q u a t i o n s 2 . 1 , 2 . 5 and 2 . 6 ) -go vehvi.n e q u a t i o n 2 . 7 : 2- - o d[FeO* ] in r - 4 - 5 • 1 4 = - 9 . 5 x 10 . 15.96 x 10 - 3.895 x 10 [NaOH] I d t - t i n x n* 3 exp R T [FeO 2 - ] - 2 . 7 The e f f e c t o f f e r r i c h y d r o x i d e s u s p e n s i o n s i s not i n -c l u d e d i n e q u a t i o n 2 . 7 . The q u a n t i f i c a t i o n o f the e f f e c t o f f e r r i c h y d r o x i d e would be v e r y d i f f i c u l t as i t i s p r o b -a b l e the fo rm of the f e r r i c h y d r o x i d e ( e . g . the e f f e c t i v e s u r f a c e a r e a per mol) w i l l depend on the sodium h y d r o x i d e c o n c e n t r a t i o n and p o s s i b l y the a c t i v i t y of the w a t e r i n the s y s t e m . 58 C h a p t e r 3 THE ELECTROCHEMICAL PRODUCTION OF FERRATE IONS 3 .1 E x p e r i m e n t a l O b j e c t i v e s The o b j e c t i v e o f the f i r s t p a r t o f the i n v e s t i g a t i o n o f the e l e c t r o c h e m i c a l p r o d u c t i o n of f e r r a t e i o n s was to q u a n t i f y the e f f e c t s o f s u p e r f i c i a l c u r r e n t d e n s i t y , h y d r o x y l i o n c o n -c e n t r a t i o n and the t o t a l q u a n t i t y o f e l e c t r i c a l charge passed on the e f f i c i e n c y of f e r r a t e p r o d u c t i o n . The d e t e r m i n a t i o n of the e f f e c t s of the above e x p e r i m e n t a l v a r i a b l e s on the p h y s i c a l and c h e m i c a l p r o p e r t i e s o f t h e anode s u r f a c e was a l s o an aim o f t h i s p r e l i m i n a r y i n v e s t i g a t i o n . 3 . 2 E x p e r i m e n t a l P rob lems A c c u r a t e c o n t r o l o f the e x p e r i m e n t a l c o n d i t i o n s i s e s s e n t i a l t o o b t a i n q u a n t i t a t i v e measurements o f the e f f i c i -ency of f e r r a t e p r o d u c t i o n . The use of h i g h l y a l k a l i n e s o l u t i o n s and h i g h c u r r e n t d e n s i t i e s c r e a t e major p rob lems f o r the d e s i g n of e x p e r i m e n t a l a p p a r a t u s wh ich can both w i t h -s t a n d the h i g h l y c a u s t i c e n v i r o n m e n t and e f f e c t i v e l y c o n t r o l the e x p e r i m e n t a l c o n d i t i o n s . The e f f e c t of ' t e m p e r a t u r e on the r a t e o f e l e c t r o c h e m i c a l 59 o f o r m a t i o n o f f e r r a t e has been r e p o r t e d as l a r g e , and thus a c c u r a t e t e m p e r a t u r e c o n t r o l w i t h i n the e x p e r i m e n t a l a p p a r a t u s i s e s s e n t i a l . The use of h i g h c u r r e n t d e n s i t i e s makes the c o n t r o l o f t e m p e r a t u r e w i t h i n the e x p e r i m e n t a l a p p a r a t u s d i f -f i c u l t . The p rob lems of e x c e s s i v e heat g e n e r a t i o n due to e l e c t r i c a l r e s i s t a n c e s w i t h i n the e l e c t r o c h e m i c a l c e l l a r e m u l t i p l i e d by the o t h e r e x p e r i m e n t a l r e q u i r e m e n t s and c o n -s t r a i n t s . S u f f i c i e n t f e r r a t e must be p roduced t o be measured and thus t h e minimum s i z e o f anode wh ich can be used w i l l be l i m i t e d . The e v o l u t i o n of gases a t the e l e c t r o d e s and the r e q u i r e m e n t of a d iaphragm to p r e v e n t the r e d u c t i o n , a t the c a t h o d e , o f the f e r r a t e p roduced a t t h e a n o d e , w i l l both r e -s u l t i n i n c r e a s e d ohmic r e s i s t a n c e s and c o n s e q u e n t l y i n c r e a s e d heat g e n e r a t i o n w i t h i n the e l e c t r o c h e m i c a l c e l l . The a b i l i t y to remove the e x c e s s t h e r m a l energy l i b e r a t e d i n the e l e c t r o c h e m i c a l c e l l i s reduced by the m a t e r i a l c o n -s t r a i n t s p l a c e d on the d e s i g n o f e x p e r i m e n t a l a p p a r a t u s . The use of hot c a u s t i c e l e c t r o l y t e s e f f e c t i v e l y l i m i t s the m a t e r i a l s o f c o n s t r u c t i o n to p l a s t i c s w h i c h , i n c o m p a r i s o n to o t h e r m a t e r i a l s , have poor heat t r a n s f e r p r o p e r t i e s . The p rob lems of a c c u r a t e t e m p e r a t u r e c o n t r o l were not s a t i s f a c t o r i l y s o l v e d i n t h i s i n v e s t i g a t i o n and t h u s , f o r the methods u s e d , s a t i s f a c t o r y t e m p e r a t u r e c o n t r o l was a c h i e v e d a t the expense o f o t h e r r e q u i r e m e n t s d e s i r e d o f the e l e c t r o -chemi c a l e e l 1 . 3 . 3 E x p e r i m e n t a l Methods Two e x p e r i m e n t a l methods were used to s t u d y the e f f i c i -ency o f f e r r a t e f o r m a t i o n . 3 . 3 . 1 Beaker E x p e r i m e n t s 2 The use of s m a l l anodes ( 0 . 5 to 2 cm ) a l l o w e d a d e -quate t e m p e r a t u r e c o n t r o l o v e r the whole c u r r e n t d e n s i t y range d e s i r e d t o be s t u d i e d . Due t o t h e s m a l l anode s i z e and the l e n g t h o f t ime o f e l e c t r o l y s i s the f e r r a t e s o l u t i o n s p roduced were v e r y d i l u t e and p a r t i a l l y decomposed and thus d i r e c t measurement of the f e r r a t e c o n c e n t r a t i o n was not p o s -s i b l e . I n s t e a d the t o t a l i r o n wh ich had l e f t the a n o d e , p r e s e n t : 1) i n s o l u t i o n , 2) as p r e c i p i t a t e s and 3) reduced on the c a t h o d e , was m e a s u r e d . Two t y p e s o f b e a k e r e x p e r i m e n t s were c o n d u c t e d . Type 1 - I n d i v i d u a l spec imens were e l e c t r o l y z e d i n b e a k e r s f o r p r e d e t e r m i n e d tifm.es and t h e t o t a l i r o n produced was m e a s u r e d . The r e s u l t s o f a s e r i e s o f such e x p e r i m e n t s f o r d i f f e r e n t q u a n t i t i e s o f c h a r g e passed were combined to s t u d y the c u r r e n t e f -f i c i e n c y o f f e r r a t e f o r m a t i o n as a f u n c t i o n o f the charge p a s s e d . Type 2 - A s i n g l e anode spec imen was e l e c t r o l y z e d a t the r e q u i r e d c u r r e n t d e n s i t y and the s o l u t i o n and ca thode were changed a f t e r s p e c i f i c q u a n t i t i e s of charge had been p a s s e d . The i r o n w i t h the e l e c t r o l y t e a f t e r each s o l u t i o n c h a n g e , a n d c u m u l a t i v e t o t a l i r o n p r o d u c e d $ w e r e then d e t e r -m i n e d . 3 . 3 . 2 C o n t i n u o u s Flow A n o l y t e E x p e r i m e n t s A d iaphragm e l e c t r o c h e m i c a l c e l l was c o n s t r u c t e d i n 3 which the a n o l y t e chamber was v e r y s m a l l (6 cm ) . The a n o l y t e was pumped., t h r o u g h the c e l l and the f e r r a t e p r o -duced a t the anode was removed i n t h e o u t f l o w a n o l y t e s t r e a m . The a n o l y t e chamber o p e r a t e d as a s m a l l back mixed r e a c t o r ^ and thus i f the a n o l y t e f l o w r a t e was s u f f i c i e n t l y l a r g e , changes i n the r a t e o f f e r r a t e p r o d u c t i o n were d i r e c t l y m e a s u r e a b l e as changes i n the f e r r a t e c o n c e n t r a t i o n o f the o u t f l o w s t r e a m . The range o f c u r r e n t d e n s i t i e s t h i s a p p a r a t u s c o u l d o p e r a t e a t was l i m i t e d by e x c e s s i v e gas b u i l d up i n the a n o l y t e chamber and t h e a b i l i t y t o a c c u r a t e l y c o n t r o l the t e m p e r a t u r e . The a n o l y t e f l o w r a t e was l i m i t e d by the need to have m e a s u r a b l e q u a n t i t i e s o f f e r r a t e i n the o u t f l o w s t r e a m . 3 . 4 E x p e r i m e n t a l 3 . 4 . 1 E l e c t r o l y t e P r e p a r a t i o n . The e l e c t r o l y t e s o l u t i o n s were p r e p a r e d by d i s s o l v i n g 62 49 the d e s i r e d amount of a n a l y t i c a l r e a g e n t grade sodium or p o t a s s i u m h y d r o x i d e i n known volumes o f d i s t i l l e d w a t e r . 3 . 4 . 2 E l e c t r o d e M a t e r i a l s and P r e p a r a t i o n The anodes used i n the e x p e r i m e n t s were e i t h e r : 1) ' A r m c o ' I ron ( c h e m i c a l a n a l y s i s Fe 9 9 . 8 9 8 % , C - 0 . 0 1 2 % , S - 0 . 0 0 7 % , N - 0 . 0 0 4 8 % , Mn - 0 . 0 5 1 % , S i - 0 . 0 0 2 % , Cu - 0 . 0 1 5 % , Mo - 0 . 0 0 8 % , A l - 0 . 0 0 2 % ) . 2) M a s s i v e M a g n e t i t e (majo r i m p u r i t y t i t a n i u m ) . The i r o n and m a g n e t i t e spec imens were c u t and abraded to the d e s i r e d d i m e n s i o n s . S u p p o r t , as w e l l as e l e c t r i c a l c o n d u c t i o n to the sample was p r o v i d e d by P . V . C . c o a t e d 14 gauge copper w i r e . In the case of i r o n , the w i r e was s o l d e r e d d i r e c t l y to the r e a r s i d e o f the s p e c i m e n . For the m a g n e t i t e spec imens the copper w i r e was s o l d e r e d to p l a t i n u m f o i l o f s i m i l a r a r e a to the spec imens and the f o i l was g l u e d to the spec imen w i t h s i l v e r E l e c t r o d a g - 4 1 6 . The anode spec imens were then mounted i n ' Q u i c k m o u n t ' s e l f - s e t t i n g r e s i n to g i v e spec imens of s u i t -a b l e d i m e n s i o n s . The f o l l o w i n g s t a n d a r d s u r f a c e p r e p a r a t i o n was used f o r a l l i r o n s p e c i m e n s . I m m e d i a t e l y b e f o r e e l e c t r o l y s i s the 63 spec imen was a b r a d e d w i t h s u c c e s s i v e l y f i n e r a b r a s i v e -paper down to 600 grade p a p e r . The i r o n spec imens were then i m -2 mersed i n the e l e c t r o l y t e and c a t h o d i c a l l y reduced a t 10 KA/m f o r 500 s e c o n d s . The c u r r e n t was then r e v e r s e d and the e x p e r i -ment was begun . No c a t h o d i c p r e t r e a t m e n t was used w i t h magne-t i t e s p e c i m e n s . 3 . 4 . 3 Power S o u r c e s and A u x i l i a r y Equipment The power f o r c o n s t a n t c u r r e n t e x p e r i m e n t s was s u p p l i e d by e i t h e r : 1) 0 - 5 0 A , 0 - 4 0 V , H e w l e t t P a c k a r d H a r r i s o n 6269A D.C. power s u p p l y . 2) 0 - 2 0 A , 0 - 1 0 V , H e w l e t t P a c k a r d 6265B D.C. power s u p p l y . In the c o n s t a n t c u r r e n t mode the t o t a l d r i f t f o r 8 hours a t c o n s t a n t ambient i s l e s s than 0 . 0 3 % , p l u s 10 mA. The c u r r e n t was measured by r e c o r d i n g the v o l t a g e drop a c r o s s a 60 mV;@•25'amp' shunt on a Wenking PPT69 Nach P o t e n t i o -m e t e r . 3 . 4 . 4 M a t e r i a l s of C o n s t r u c t i o n The e x p e r i m e n t a l a p p a r a t u s was made e n t i r e l y o f p l a s -t i c s . P l e x i g l a s s , d e s p i t e i t s l i m i t e d s t a b i l i t y i n hot s t r o n g a l k a l i s o l u t i o n s , was found to p e r f o r m s a t i s f a c t o r i l y 64 under the e x p e r i m e n t a l c o n d i t i o n s o f the i n v e s t i g a t i o n . The s t r e n g t h and ease of b o n d i n g made i t i d e a l f o r f a b r i c a t i n g equipment of i n t r i c a t e d e s i g n . TM TM T e f l o n , Na lgene , p o l y p r o p y l e n e and p o l y v i n y l c h l o r i d e were a l s o found s u i t a b l e but t h e r e use i s l i m i t e d to p r e f o r m e d i tems such as b e a k e r s , f i t t i n g s , t u b i n g e t c . No a d h e s i v e was found s a t i s f a c t o r y f o r use i n c o n c e n t r a -TM t e d a l k a l i . S i l i c o n e r u b b e r s e a l a n t was used where a f l e x -i b l e b o n d / s e a l was r e q u i r e d , however s i l i c o n i s s l o w l y l e a c h e d out o f the s e a l a n t by s t r o n g a l k a l i n e s o l u t i o n s . 50 H y d r o p h i l l i c porous p o l y p r o p y l e n e ( C e l g a r d 2400W ) p roved s a t i s f a c t o r y as a d iaphragm m a t e r i a l . The membrane o used was 0 . 0 2 5 mm t h i c k w i t h a pore s i z e o f 2000 x 200A. T h i s m a t e r i a l i s v e r y s t r o n g and has a r e l a t i v e l y low e l e c -t r i c a l r e s i s t i v i t y . Due t o i t s t h i c k n e s s , C e l g a r d 2400W has moderate heat t r a n s f e r p r o p e r t i e s . The b u l k f l o w a c r o s s the membrane was l o w . 3 . 4 . 5 E l e c t r o c h e m i c a l C e l l s 3 . 4 . 5 . 1 B e a k e r E x p e r i m e n t s T h e , e l e c t r o c h e m i c a l c e l l c o n s i s t e d of a 400 ml Nalgene b e a k e r i n t o wh ich was p l a c e d a l a r g e r u b b e r bung. Through h o l e s i n the r u b b e r b u n g , an anode s p e c i m e n , a 3x3 cm p l a t i n u m f o i l c a t h o d e , a t e f l o n c o a t e d t h e r m i s t o r t e m p e r a -t u r e c o n t r o l l e r and a n i t r o g e n gas s p a r g e r were i n s e r t e d . A l a r g e c e n t r a l h o l e i n the bung a l l o w e d f o r gas escape and the i n s e r t i o n o f a l u g g i n c a p i l l a r y probe ( i f r e q u i r e d ) . The p l a t i n u m cathode and i r o n or m a g n e t i t e anode were immersed i n a common e l e c t r o l y t e wh ich was a g i t a t e d by n i t r o g e n gas s p a r g i n g . The e l e c t r o c h e m i c a l c e l l was p l a c e d i n a w a t e r bath and the t e m p e r a t u r e was c o n t r o l l e d : by a Thermistemp Temperature C o n t r o l l e r (Model 71 Y e l l o w s p r i n g I n s t r u m e n t C o . ) t o + 1°C, v i a the t h e r m i s t o r i n the e l e c t r o c h e m i c a l c e l l . 3 . 4 . 5 . 2 C o n t i n u o u s Flow A n o l y t e E l e c t r o c h e m i c a l C e l l The d iag rams o f the c o n t i n u o u s f l o w a n o l y t e e l e c t r o -c h e m i c a l c e l l and the c o m p l e t e a n o l y t e f l o w t r a i n a r e g i v e n i n f i g u r e s 3 . 1 and 3 . 2 . The e l e c t r o c h e m i c a l c e l l c o n s i s t s o f two d i s c r e t e p a r t s : 1) the a n o l y t e chamber and porous p o l y p r o p y l e n e d i a p h r a g m , and 2) a l a r g e r 1 l i t r e c a t h o l y t e chamber . The two s e c t i o n s are j o i n e d t o g e t h e r by a c o m p r e s s i o n c l a m p . The a n o l y t e s e c t i o n has t h r e e components : 1) The main body c o n t a i n i n g the a n o l y t e f l o w c h a n n e l , 2) an o u t e r p l a t e wh ich i s screwed 66 10cm Thermistor controller Heoter-, Cooler-, Thermometer Anolyte Chamber-, e pt Cothode Cotholyte Chomber porous polypropy-lene membrone m O (. Stirrer 3  \ ^ \ \ \ \ \ \ \ \ \ \ V V \ V . \ \ \ \ \ \ \ \ V \ \ \ \ \ ' ^ T Product out i^^ - l ron An -Luggin copillory NoOH in Not Drown to Scole 2 0 cm F i g . 3.1 Diagram of the c o n t i n u o u s f l o w a n o l y t e e l e c t r o ' c h e m i c a l c e l l . 67 electrochemical cel l peristalstic pump NaOH reservoir r—syringes—| ni (loading) |—= position of " th ree -way" cocks syringe pump (unloading) f F i g . 3 . 2 A s c h e m a t i c d i a g r a m of the a n o l y t e f l o w t r a i n to the main body t o clamp t h e porous p o l y p r o p y l e n e membrane i n p l a c e , and 3) a " Q u i c k - m o u n t 1 i r o n anode spec imen wh ich i s h e l d i n p l a c e by c o m p r e s s i o n sc rews and a l i q u i d - t i g h t s e a l i s a c h i e v e d w i t h s i l i c o n e r u b b e r s e a l a n t . The ' Q u i c k -mount ' spec imens were 7x4 cm and the anodes were t y p i c a l l y 1x3 cm. A copper s t u b , s o l d e r e d to the i r o n a n o d e , p r o t r u d e s t h r o u g h the back o f the spec imen to p r o v i d e e l e c t r i c a l c o n t a c t . The c o f f i n - s h a p e d a n o l y t e chamber has a 6 mm anode to d iaphragm d i s t a n c e . The d e s i g n i s such t h a t no s o l u t i o n m i x -i n g o c c u r s below the l e v e l o f the anode and thus the ' m i x e d 3 t a n k 1 e l e c t r o c h e m i c a l c e l l volume i s 6 cm . The a n o l y t e i s pumped t h r o u g h the a n o l y t e chamber by means of a 50 ml s y r i n g e d r i v e n by a Sage I n s t r u m e n t S y r i n g e Pump (Model 341) a t a r a t e wh ich c o u l d be s e t between 0 . 3 3 m l / h r and 13 m l / m i n . A t y p i c a l f l o w r a t e used was 3.1 ,"m.1/m'in.. The s y r i n g e s were l o a d e d by a p e r i s t a l t i c pump and the f u l l s y r i n g e s had to be mounted on the s y r i n g e pump by hand . On p a s s i n g t h r o u g h t h e a n o l y t e chamber the s o l u t i o n runs i n t o a 80 ml spec imen b o t t l e immersed i n an i c e b a t h . One 50 ml sample was p roduced per s y r i n g e l o a d and thus each sample has a volume equa l to about e i g h t t i m e s t h a t o f the mixed p o r t i o n o f the a n o l y t e chamber . Temperatu re r e g u l a t i o n o f the a n o l y t e was a c h i e v e d by m a n u a l l y s e t t i n g the t e m p e r a t u r e of the c a t h o l y t e to p r o v i d e s u f f i c i e n t t e m p e r a t u r e g r a d i e n t a c r o s s the porous p o l y p r o p y -l e n e membrane to a f f e c t heat removal o r a d d i t i o n t o the a n o -l y t e . Temperature r e g u l a t i o n t o + 1°C was p o s s i b l e f o r a c t u a l c u r r e n t s o f l e s s t h a n 5 amps. 3 . 5 A n a l y s i s T o t a l i r o n and f e r r a t e a n a l y s e s were c a r r i e d out by the methods d e t a i l e d i n Append i x B. Anode s c a l e s were examined by a number o f methods . A s c a n n i n g e l e c t r o n m i c r o s c o p y ( E . T . E . C o r p . A u t o s c a n ) was used to o b t a i n m o r p h o l o g i c a l i n f o r m a t i o n about the anode s c a l e s j s e c o n d a r y x r a y a n a l y s i s ( O r t e x Xray A n a l y s e r Model 6200) was used to q u a l i t a t i v e l y d e t e r m i n e the e l e m e n t s p r e s e n t i n the s c a l e . Of the l i g h t e l e m e n t s not d e t e c t a b l e by the p r e v i o u s e q u i p m e n t , oxygen was c o n f i r m e d to be p r e s e n t by E l e c t r o n Probe Xray M i c r o a n a l y s i s ( J E 0 L - J X A - 3 A ) . I t was not p o s s i b l e to d e t e r m i n e i f hydrogen was p r e s e n t i n t h e s c a l e . Xray d i f f r a c t i o n a n a l y s i s o f s c a l e a d h e r i n g to the anode was p e r f o r m e d u s i n g i r o n Ka r a d i a t i o n . 3 . 6 E x p e r i m e n t a l V a r i a b l e s Exami ned A l l the e x p e r i m e n t s were c o n d u c t e d a t 50°C. The e x p e r i m e n t s t o d e t e r m i n e the e f f e c t s of s u p e r f i c i a l c u r r e n t d e n s i t y on the e f f i c i e n c y o f f e r r a t e f o r m a t i o n were c a r r i e d out i n 1 4 . 3 g . m o l / 1 sodium h y d r o x i d e . 3 . 6 . 1 , ) Beaker E x p e r i m e n t s - Type 1 The e f f e c t s of the f o l l o w i n g v a r i a b l e s on the t o t a l i r o n removed f rom the a n o d e , f o r up to 3 x l 0 4 coulombs o f 2 charge passed per cm of a n o d e , were e x a m i n e d : 1) S u p e r f i c i a l c u r r e n t d e n s i t i e s o f 5 , 10 and 60 KA/m . 2 2) Anode m a t e r i a l s , i r o n and m a g n e t i t e @ 10 KA/m . 3 . 6 . 2 , Beaker E x p e r i m e n t s - Type 2 The e f f e c t s of the f o l l o w i n g v a r i a b l e s on the t o t a l 4 i r o n removed f rom the anode f o r up to 18x10 coulombs of 2 charge passed per cm o f anode were e x a m i n e d : 2 1) S u p e r f i c i a l c u r r e n t d e n s i t i e s of 5 , 1 0 , 25 and 50 KA/m . 2) Sodium h y d r o x i d e c o n c e n t r a t i o n s o f 1 , 5 , 10 and 1 4 . 3 g.moT/1• 3) 10 g . m o l / 1 p o t a s s i u m h y d r o x i d e . 4) The e f f e c t s o f the a d d i t i o n of 0 . 0 5 g . m o l / 1 N a C l . 5) The e f f e c t s o f p e r i o d i c c u r r e n t r e v e r s a l . 3 . 6 . 3 , C o n t i n u o u s F low A n o l y t e E x p e r i m e n t s The e f f e c t o f the f o l l o w i n g v a r i a b l e s on t h e c u r r e n t 4 e f f i c i e n c y o f f e r r a t e p r o d u c t i o n , f o r up to 2 . 5 x 1 0 coulombs 71 2 o f charge passed per cm o f a n o d e , were e x a m i n e d : 1) S u p e r f i c i a l c u r r e n t d e n s i t i e s o f 5 , 9 , 10 and 18 K A / m 2 . 2) Sodium h y d r o x i d e c o n c e n t r a t i o n s o f 10 and 1 4 . 3 g . m o l / I . 3 . 7 E x p e r i m e n t a l E r r o r s For a l l the e x p e r i m e n t s the e r r o r s due t o the c o n t r o l o f e x p e r i m e n t a l v a r i a b l e s (o f wh ich t e m p e r a t u r e i s p r o b a b l y the most i m p o r t a n t ) c a n n o t be q u a n t i f i e d . 3 . 7 . 1 B e a k e r E x p e r i m e n t s The measurement e r r o r s f o r the beaker e x p e r i m e n t s were m a i n l y a s s o c i a t e d w i t h the t o t a l i r o n a n a l y s i s . For i n d i v i d u a l e x p e r i m e n t s the e r r o r s were about 4 -8% and t h e s e t e n d to d i m i n i s h as the q u a n t i t y o f i r o n measured i n c r e a s e d . For the c u m u l a t i v e t o t a l i r o n , summation e r r o r s s h o u l d remain as a c o n s t a n t p e r c e n t a g e o f t o t a l c u m u l a t i v e i r o n and a r e p r o b a b l y about 10%. For the t ype 2 b e a k e r e x p e r i m e n t s t h e removal o f the anode f rom the e l e c t r o l y t e to f a c i l i t a t e c h a n g i n g the s o l u t i o n , may r e s u l t i n a l t e r a t i o n o f t h e anode s u r f a c e s t r u c t u r e a n d , c o n s e q u e n t l y , the r a t e o f i r o n removal f rom 72 the anode. The magni tude of the e r r o r s r e s u l t i n g f rom t h i s e x p e r i m e n t a l p r o c e d u r e a re unknown. 3 . 7 . 2 C o n t i n u o u s Flow A n o l y t e E x p e r i m e n t s The c o n t i n u o u s f l o w a n o l y t e e x p e r i m e n t a l r e s u l t s have e r r o r s r e s u l t i n g f rom both the a c c u r a c y of measurement and the e x p e r i m e n t a l d e s i g n . The measurement e r r o r s a r e due t o : 1) the f e r r a t e c o n c e n t r a t i o n measurements ( E r r o r = 2% > 1 x l b - 5 g . m o l / 1 ) and 2) sample volume measurements ( E r r o r = +_ 2%). The combined measurement e r r o r i s thus about + 4%. The d e s i g n of the a p p a r a t u s i s such t h a t the f e r r a t e i o n s o l u t i o n may be a t 50°C f o r s e v e r a l hundred s e c o n d s . From c h a p t e r 2 i t can be seen t h a t t h i s w i l l r e s u l t i n up to 5% f e r r a t e d e c o m p o s i t i o n f o r 1 4 . 3 g . m o l / 1 NaOH. At 10 g . m o l / 1 NaOH the d e c o m p o s i t i o n c o u l d be up to 10%. The d e c o m p o s i -t i o n o f f e r r a t e i o n s p r e v e n t e d the i n v e s t i g a t i o n , b y t h i s m e t h o d . o f the r a t e o f f e r r a t e f o r m a t i o n a t low h y d r o x i d e i o n c o n c e n t r a t i o n s . At 50°C, 1 4 . 3 g . m o l / 1 NaOH, the a c t u a l ' , amounts o f f e r r a t e d e c o m p o s i t i o n were not q u a n t i f i e d and the r e s u l t s have not been c o r r e c t e d f o r t h i s i n a c c u r a c y . Due to the e x p e r i m e n t a l d e s i g n , the i n i t i a l measurement w i l l be about 12% too l o w . D e t e c t i o n of s m a l l f l u c t u a t i o n s i n the r a t e o f f e r r a t e p r o d u c t i o n i s not p o s s i b l e . The measurement o f l a r g e changes i n the r a t e of f e r r a t e p r o d u c t i o n w i l l i n v o l v e a l a g t i m e due to m i x i n g i n the a n o l y t e chamber and d i l u t i o n i n t h e sample b u l k . 3 . 8 R e s u l t s and O b s e r v a t i o n s 3 . 8 . 1 The E f f e c t o f C u r r e n t D e n s i t y on the E f f i c i e n c y  o f F e r r a t e F o r m a t i o n 3 . 8 . 1 . 1 C o n t i n u o u s Flow A n o l y t e E x p e r i m e n t s The r e s u l t s of the c o n t i n u o u s f l o w a n o l y t e e x p e r i -ments e x a m i n i n g the e f f e c t s of s u p e r f i c i a l c u r r e n t d e n s i t y on the e f f i c i e n c y of f e r r a t e f o r m a t i o n a r e g i v e n i n t a b l e s CV13. to i 'G.17 and f i g u r e s 3 . 3 and 3 . 4 . For a l l the c u r r e n t d e n s i t i e s examined the e f f i c i e n c y of f e r r a t e f o r m a t i o n i s i n i t i a l l y l e s s than 3% and d e c l i n e s s t e a d i l y as the q u a n t i t y o f change passed i s i n c r e a s e d . The e f f i c i e n c y of f e r r a t e f o r m a t i o n f o r the i n i t i a l 6 2 Kcou lombs/cm .was e r r a t i c . 2 A f t e r the i n i t i a l 6 K coulombs/cm the e f f i c i e n c y of f e r r a t e f o r m a t i o n d e c l i n e s s t e a d i l y f o r a l l the c u r r e n t d e n s i t i e s examined to e f f e c t i v e l y ze ro a f t e r 2 0 - 2 5 2 K coulombs/cm . As can be seen f rom f i g u r e 3 . 3 the r e s u l t s a r e 74 75 F i g . 3 . 4 F e r r a t e p r o d u c t i o n r a t e v e r s u s t o t a l a n o d i c c h a r g e passed f o r s u p e r f i c i a l c u r r e n t d e n s i t i e s o f 5 , 9 and 18 K A / m 2 . r e p r o d u c i b l e . There a p p e a r s t o be a s l i g h t d e c l i n e i n the e f f i c i e n c y o f f e r r a t e f o r m a t i o n as t h e s u p e r f i c i a l c u r -r e n t d e n s i t y i s i n c r e a s e d . However the v a r i a t i o n i n the e f f i c i e n c y of f e r r a t e f o r m a t i o n . may depend to si-c e r t a i n e x t e n t on the i n d i v i d u a l n a t u r e of the spec imens and thus t h i s a p p a r e n t t r e n d may not be s i g n i f i c a n t . The measured t o t a l i r o n i n s o l u t i o n , as g i v e n i n t a b l e 3 . 1 , i s i n i t i a l l y s i m i l a r t o the measured f e r r a t e c o n -c e n t r a t i o n , but becomes p r o g r e s s i v e l y g r e a t e r than the amount of i r o n a s s o c i a t e d w i t h f e r r a t e as i thepexpenlment p r o c e e d s . T h i s c o u l d be due to s m a l l p a r t i c l e s of o x i d e b e i n g c a r r i e d away f rom the anode by the a n o l y t e s t r e a m , or due to h i g h e r a c t u a l amounts o f f e r r a t e decom-p o s i t i o n . The c u r v e s f o r the c u m u l a t i v e t o t a l f e r r a t e produced as g i v e n i n f i g u r e 3 . 5 f o r two i d e n t i c a l e x p e r i m e n t s i n d i -c a t e t h a t a l t h o u g h the r a t e s o f f e r r a t e p r o d u c t i o n a r e s i m i l a r the c u m u l a t i v e t o t a l o f s m a l l d i f f e r e n c e s i s c o n -s i d e r a b l e . The c u m u l a t i v e t o t a l f e r r a t e produced i n the - 5 2 - 2 v a r i o u s e x p e r i m e n t s range f rom 30x10 g . m o l s FeO^ /cm 2 - 5 2 - 2 anode @ 18 KA/m t o about 51x10 g .mol FeO^ /cm anode 2 @ 5 KA/m . The c u m u l a t i v e e f f e c t of e r r o r s and the i n -d i v i d u a l n a t u r e of spec imens a r e such t h a t t h e s e r e s u l t s are not a c c u r a t e i n d i c a t o r s of the r e l a t i o n s h i p between 77 Table 3.1 Comparison of the Total Iron Content of the Continuous Flow Anolyte Experimental Samples with  Those Measured as Ferrate Ions - , •- • - •(, Total Iron: Atomic Absorption Spectrophotometrically Determined. Ferrate : Spectrophotometrically Determined. i Table/ Sample Number Mean Number of Coulombs Passed coulombs cm2 v i ( x l 0 3 ) Coulombs Passed During Sample Col lect ion coulombs cm2 >: ! (x 10 3) Iron as Ferrate Total Iron Interval FeO2" g.mol cm2 (x 10" 5 ) Rate of Ferrate Production g.mol - 1 _ Q coulomb (x 10 ) Interval Total Iron g.mol cm2 (x 10" 5 ) Rate of Total Iron Leaving'thenAnode g.mol Q coulomb (x 10 ) C.14-1 0.55 1.1 4.38 39.78 4.517 41.07 C.14-4 C.14-15 3.68 16.28 1.15 1.15 4.65 1.18 38.85 10.27 4.577 1.684 39.80 14.64 C.14-20 21.94 1.125 0.15 1.30 0.555 4.93 C.16-2 1.26 0.825 3.71 45.00 3.996 48.44 C.16-12 9.49 0.825 2.36 28.62 3.033 36.76 C.16-20 16.11 0.825 0.80 9.76 1.312 15.90 -C. 1.8-3 : 2.88 1.1 5:i 2.40 20.44 2.969 25.82 C.18-10 11.17 1.15 1.79 15.58 2.408 20.94 C.18-22 : 25.03 1.15 0.09 0.76 ::. 3.737 3.25 78 C h a r g e P a s s e d . c o u l o m b s ( x l O ) c m 2 o f A n o d e F i g . 3 . 5 C u m u l a t i v e t o t a l f e r r a t e p r o d u c e d ? 10 KA/m v e r s u s t o t a l a n o d i c charge p a s s e d . 79 s u p e r f i c i a l c u r r e n t d e n s i t y and the amount of f e r r a t e p r o -d u c e d . 3 . 8 . 1 . 2 Beaker E x p e r i m e n t s - Type 1 The e x p e r i m e n t a l r e s u l t s o f the b e a k e r e x p e r i m e n t s to d e t e r m i n e the t o t a l i r o n removed f rom the anode w i t h q u a n t i t y o f charge passed a r e g i v e n i n t a b l e s G.19 to G,2<r and f i g u r e s 3 . 6 to 3 . 8 . For Armco i r o n spec imens t h e t o t a l i r o n removed f rom the anode i s i n i t i a l l y s i m i l a r f o r a l l the c u r r e n t d e n s i t i e s e x a m i n e d . Most of the r e s u l t s a r e c o n s i s t e n t w i t h a t r e n d i n wh ich t h e b u l k o f the i r o n i s removed f rom the anode 2 d u r i n g t h e f i r s t 1 0 - 2 0 K c o u l o m b s / c m a f t e r wh ich the r a t e of i r o n removal d e c l i n e s r a p i d l y . The q u a n t i t i e s o f i r o n removed f rom the anodes f o r t h i s t r e n d , a r e s i m i l a r to the q u a n t i t i e s o f f e r r a t e i o n s measured i n the p r e v i o u s e x p e r i -ments . In a few c a s e s the q u a n t i t y o f i r o n removed f rom the anode was l a r g e r than t h a t s u g g e s t e d by the g e n e r a l t r e n d . A l t h o u g h a c e r t a i l n amount o f s c a t t e r i s e x p e c t e d , due to. ..the i n d i v i d u a l n a t u r e o f t h e s p e c i m e n s , the s p r e a d o b s e r v e d i s too l a r g e to be d i s m i s s e d as not s i g n i f i c a n t . 3 . 8 . 1 . 3 Beaker E x p e r i m e n t s - Type 2 The c u m u l a t i v e t o t a l i r o n removed f rom the i r o n 80 8 0 0 Charge Passed. Coulombs ( x l O 3 ) cm of Anode 3 . 6 I ron removed f rom the anode @ 5 KA/m v e r s u s t o t a l a n o d i c charge p a s s e d . 81 8 0 0 7 0 0 -6 0 0 -I o * 5 0 0 X) o c < 4 0 0 -300 -c o - 200 o I— 100 o O A r r n c o Iron • M a g n e t i t e O O V 1 _ & to 15 2 0 2 5 30 Charge P a s s e d . C o u l o m b s  . c m 2 of A n o d e ( x l O 3 ) F i g . 3 . 7 I ron removed f rom the i r o n and m a g n e t i t e anodes © 10 K A / m 2 . 82 8 0 0 Charge Passed . Coulombs cm 2 of Anode F i g . 3 . 8 I ron removed f rom the anode @ 60 KA/m v e r s u s t o t a l a n o d i c charge p a s s e d . anode as a f u n c t i o n of change p a s s e d , as d e t e r m i n e d by t ype 2 b e a k e r e x p e r i m e n t s , a re g i v e n i n t a b l e s C . 2 3 to f C . 2 6 and f i g u r e 3 . 9 . The r e s u l t s s u g g e s t t h a t the q u a n t i t y o f i r o n removed f rom the anode i n c r e a s e s s t e a d i l y as the q u a n t i t y of charge passed i s i n c r e a s e d . T h e ' q u a n t i t y o f i r o n removed f rom the anode a p p e a r s to be i n d e p e n d e n t o f the s u p e r f i c i a l c u r -r e n t d e n s i t y u s e d . The i n i t i a l r a t e o f f e r r a t e f o r m a t i o n a p p e a r s to d e c l i n e to a s t e a d y - s t a t e v a l u e a f t e r 2 0 - 3 0 2 K coulombs/cm . The c u m u l a t i v e t o t a l i r o n removed f rom anode a f t e r 2 30 K c o u l o m b s / c m i s o f a s i m i l a r magn i tude to the r e s u l t s of s e c t i o n s 3 . 8 . 1 . 1 and 3 . 8 . 1 . 2 . However the i n i t i a l r a t e o f i r o n removal d e t e r m i n e d by t h i s method i s l e s s than t h a t d e t e r m i n e d i n the p r e v i o u s m e t h o d s . The s u b s t a n t i a l s t e a d y s t a t e r a t e o f t o t a l i r o n removal f rom the anode f o r l a r g e r q u a n t i t i e s o f charge passed i s not s u g g e s t e d by the s t e a d y -s t a t e ex.p-erimeints d e s c r i bed i n the e a r l i e r s e c t i o n s . 3 . 8 . 1 . 4 Summary of the R e s u l t s o f t h e E x p e r i m e n t s I n v e s t i -g a t i n g the E f f e c t s o f S u p e r f i c i a l C u r r e n t D e n s i t y on the E f f i c i e n c y of F e r r a t e F o r m a t i o n The r e s u l t s f rom the t h r e e d i f f e r e n t e x p e r i m e n t a l t e c h n i q u e s s u g g e s t s t h a t the e f f i c i e n c y o f f e r r a t e f o r m a t i o n 84 0 5 0 Cumulotive Charge Passed. 100 coulombs J50 c m 2 of Anode (x I0 3 ) F ig . 3.9 Total cumulative iron removed from the anode versus tota l anodic charge passed for s u p e r f i c i a l current densi t ies of 5 , 10, 25 and 50 KA/m2. 85 and i t s d e c l i n e w i t h i n c r e a s i n g q u a n t i t i e s o f charge passed has v e r y l i t t l e dependence on the s u p e r f i c i a l c u r r e n t d e n s i -2 t i e s f o r s u p e r f i c i a l c u r r e n t d e n s i t i e s between 5 and 60 KA/m . The c o n t i n u o u s f l o w a n o l y t e e x p e r i m e n t s s u g g e s t s the - 9 i n i t i a l r a t e o f f e r r a t e f o r m a t i o n i s about 4 0 - 5 0 (x lO ) g . m o l -/cpul'omb. and f a l l s s t e a d i l y t o v e r y low l e v e l s a f t e r 2 0 - 2 5 2 K c o u l o m b s / c m . In c o n j u n c t i o n w i t h the beaker e x p e r i m e n t s the r e s u l t s s u g g e s t most o f t h e i r o n removed f rom the anode e n t e r s s o l u t i o n as f e r r a t e i o n s . The t ype 1 beaker e x p e r i -ments s u g g e s t t h a t i n c r e a s e d i r o n removal f rom the anode o c c u r s i n i s o l a t e d c a s e s . D e s p i t e the r e s e r v a t i o n s as to the v a l i d i t y o f e x p e r i -menta l t e c h n i q u e s of the t ype 2 b e a k e r e x p e r i m e n t s , the r e s u l t s were i n p a r t i a l agreement w i t h t h e r e s u l t s of the o t h e r e x p e r i m e n t s . I t i s not known i f the measured s t e a d y s t a t e i r o n removal a t q u a n t i t i e s o f charge g r e a t e r than 2 30 K c o u l o m b s / c m i s a c c u r a t e o r i f the form the i r o n e n t e r -i n g s o l u t i o n r e m a i n s u n i f o r m . 3 . 8 . 2 The E f f e c t o f A l k a l i Type and C o n c e r i t r a t i o n The e f f e c t s of a l k a l i c o n c e n t r a t i o n on the c u m u l a t i v e t o t a l i r o n removed f rom the a n o d e , as d e t e r m i n e d by t y p e 2 b e a k e r e x p e r i m e n t s a r e g i v e n i n t a b l e s f"G.25 and CC•.27 to C: ;30 and f i g u r e 3 . 1 0 . 86 Cumulative Charge Passed, coulombs (x lO 3 ) cm2 of Anode .3 .10 The e f fec t of a l k a l i type and concentration on the cumula-t i ve to ta l iron removed from the anode with quantity of anodic charge passed. The c u m u l a t i v e t o t a l i r o n removed f rom the anode w i t h q u a n t i t y o f charge passed i s s t r o n g l y dependent on the sodium h y d r o x i d e c o n c e n t r a t i o n . The t o t a l i r o n removed f rom the anode a f t e r 30 Kcoulombs/cm (see t a b l e C 31 and f i g u r e 3 . 1 1 ) d e c r e a s e s a p p r o x i m a t e l y l i n e a r l y as the sodium h y d r o x i d e c o n c e n t r a t i o n i s d e c r e a s e d f o r c o n c e n t r a t i o n s between 1 4 . 3 and 1 g . m o l / 1 . A l t h o u g h the a c c u r a c y o f t h e s e e x p e r i m e n t s i s l i m i t e d , the magni tude o f the e f f e c t o f h y d r o x i d e c o n c e n t r a t i o n on the amount of i r o n removed f rom the anode i s such - t h a t j t h e r e s u l t s a re p r o b a b l y s i g n i f i c a n t . From t h i s t r e n d i t can be deduced, ; t h a t the r a t e o f f e r r a t e f o r m a t i o n must a l s o d e c r e a s e as the sodium h y d r o x i d e c o n -c e n t r a t i o n i s d e c r e a s e d . The s i m i l a r i t y of the r e s u l t s f o r 10 g . m o l / 1 NaOH and 10 g . m o l / 1 KOH s u g g e s t t h a t removal o f i r o n f rom the anode i s not a f f e c t e d by the t ype of a l k a l i u s e d . C o n t i n u o u s f l o w a n o l y t e e x p e r i m e n t a l r e s u l t s f o r the r a t e o f f e r r a t e f o r m a t i o n a t 1 4 . 3 g . m o l / 1 NaOH and 10 g . m o l / 1 NaOH a t 10 KA/m 2 a re g i v e n i n t a b l e 1 C..1 3 and f i g u r e : 3 . 1 , and t a b ! e GC.,18 and f i g u r e 3 . 1 2 , r e s p e c t i v e l y . D e s p i t e the d i f f e r e n t f e r r a t e d e c o m p o s i t i o n r a t e s a t the d i f f e r e n t sodium h y d r o x i d e c o n c e n t r a t i o n s (see c h a p t e r 2) 88 F ig . 3.11 Cumulative to ta l iron removed from the anode a f te r 30 K s 2 @ 10 KA/m versus the sodium hydroxide concentration. 89 cm^ of Anode F ig . 3.12 Ferrate production rate versus to ta l anodic charge passed. 2 Super f i c ia l current density = 10 KA/m [NaOH] = 10 g.mol/1. the d e c l i n e i n measured r a t e of f e r r a t e p r o d u c t i o n a t the l o w e r sodium h y d r o x i d e c o n c e n t r a t i o n i s s i g n i f i c a n t . The r e s u l t i s c o n s i s t e n t w i t h the r a t e of f e r r a t e f o r m a t i o n b e i n g dependent on the a l k a l i c o n c e n t r a t i o n . 3 . 8 . 3 The E f f e c t s of Sodium C h l o r i d e A d d i t i o n s The c u m u l a t i v e t o t a l i r o n removed f rom an i r o n anode f o r a 1 4 . 3 g . m o l / 1 NaOH w i t h 0 . 0 5 g . m o l / 1 NaCl e l e c t r o l y t e , as d e t e r m i n e d by Type 2 b e a k e r e x p e r i m e n t s i s g i v e n i n T a b l e C .32 and f i g u r e 3 . 1 3 . The r e s u l t s s u g g e s t the p r e s e n c e of s m a l l amounts o f c h l o r i n e i n s o l u t i o n does not enhance the r a t e of i r o n removal f rom t h e anode . The e f f e c t s o f c h l o r i n e , i f they had been p r e s e n t , would most p r o b a b l y be due t o : l ) enhanced s c a l e f a i l u r e and/or 2) c h e m i c a l r e a c t i o n s r e s u l t i n g f rom h y p o c h l o r i t e g e n e r a t e d a t the anode . 3 . 8 . 4 The E f f e c t s of C u r r e n t R e v e r s a l The r e s u l t s o f a c u r r e n t r e v e r s a l e x p e r i m e n t i n wh ich an ' Armc'o' i r o n spec imen was r e p e a t e d l y e l e c t r o l y z e d a n o d i c a l l y @ 10 KA/m f o r 5000 seconds f o l l o w e d by c a t h o d i c 2 r e d u c t i o n a t 10 KA/m f o r 500 seconds a r e g i v e n i n t a b l e C .33 and f i g u r e 3 . 1 4 . The r a t e o f i r o n removal f rom the anode was found to remain c o n s t a n t and be o f s i m i l a r magni tude to t h a t i n i t i a l l y F ig . 3.13 The e f fec ts of chlor ide addit ions on the cumulative to ta l i ron removed from the anode @ 10 KA/m2 with quantity of anodic charge passed. 92 8 0 0 t O x X3 O c < o C\l E o 6 0 0 4 0 0 -c o a> > o E o 2 0 0 5 6 —I 7 8 Cur rent C y c l e s . Total meosured iron — (including that resulting from cothodisotion) Iron due to onodic dissolution (calculated ) A 10 KA/m 2 -no current reversal O 10 KA/ m 2 - 5 0 0 s current reversol every 5 0 0 0 s . 0 10 2 0 3 0 4 0 A n o d i c C h a r g e P o s s e d . c o u l o m b c m 2 of Anode 5 0 6 0 ( x l O 3 ) F ig . 3.14 The ef fec ts of current reversal on the cumulative to ta l 2 iron removed, from the anode @ 10 KA/m with quantity of anodic charge passed. 93 2 e x p e r i e n c e d i n spec imens a n o d i c a l l y e l e c t r o l y z e d a t 10 KA/m . F e r r a t e f o r m a t i o n was o b s e r v e d i n a l l the a n o d i c c u r r e n t c y c l e s . P e r i o d i c c u r r e n t r e v e r s a l had the e f f e c t of p r e -v e n t i n g the d e c r e a s e i n i r o n removal r a t e (and f e r r a t e f o r m a t i o n r a t e ) n o r m a l l y e x p e r i e n c e d . 3 . 8 . 4 M a g n e t i t e Anodes The t o t a l i r o n removed f rom m a g n e t i t e anodes e l e c t r o -2 l y z e d a t 10 KA/m , as d e t e r m i n e d by t ype 1 b e a k e r e x p e r i m e n t s a r e g i v e n i n T a b l e C .22 and f i g u r e 3 . 7 (shown i n s e c t i o n 3 . 8 . 1 . 2 ) . The t o t a l i r o n removed f rom the m a g n e t i t e anodes was a p p r o x i m a t e l y one t e n t h t h a t removed f rom i r o n anodes under s i m i l a r e x p e r i m e n t a l c o n d i t i o n s . No f e r r a t e f o r m a t i o n was v i s i b l e f rom m a g n e t i t e a n o d e s . 3 . 9 S c a l e M o r p h o l o g i e s 3 . 9 . 1 I ron Anodes The t y p e o f s c a l e f o r m e d , and the change i n morpho logy w i t h the q u a n t i t y of charge passed i s s i m i l a r f o r a l l the s u p e r f i c i a l c u r r e n t d e n s i t i e s e x a m i n e d . I n i t i a l l y a c o h e r e n t b l a c k o x i d e (or; h y d r o x i d e ) l a y e r formed on the i r o n anode . Secondary x.-.nay a n a l y s i s i n d i c a t e d i r o n (see f i g u r e 3 . 1 5) and oxygen were the major components o f the anode l a y e r ; iog.Courrts Energy ( keV) Anode Treatment 20ksec at I O k A / m 2 /. s x.. - b . K Ca K " F e K ^ Cu Fig . 3.15 S.E .M. X-ray Analyzer spectra for the duplex anode scale a) the upper layer b) > the lower layer 95 i t i s p r o b a b l e hydrogen i s a l s o p r e s e n t but t h i s c o u l d not be c o n f i r m e d e x p e r i m e n t a l l y . The anode l a y e r was xtray amorphous. 2 A f t e r about 5 K coulombs/cm p i t t i n g and a p p a r e n t d i s -s o l u t i o n of the i n i t i a l s c a l e i s v i s i b l e (see f i g u r e s 3 . 1 6 to 3 . 1 8 ) . As the q u a n t i t y o f charge passed i s i n c r e a s e d the amount of o r i g i n a l s c a l e ( d i s c e r n a b l e by the p o l i s h marks) d e c r e a s e s . The a r e a s where the o r i g i n a l o x i d e l a y e r i s a b s e n t have a rough b roken a p p e a r a n c e . A f t e r 2 0 - 3 0 Kcoulombs/cm' the amount o f o r i g i n a l o x i d e s t i l l p r e s e n t on the spec imen i s s m a l l . F i g u r e s 3 . 1 7 c and d i n d i c a t e detachment o f the r e -m a i n i n g i s l a n d s o f s c a l e may r e s u l t f rom d i s s o l u t i o n beneath 2 t h e s e a r e a s . The anode s u r f a c e p r e s e n t a f t e r 30 Kcou lombs/cm i s p r e d o m i n a n t l y a rough g r a n u l a r l a y e r . The l a y e r does not a l t e r i t s g e n e r a l l y morpho logy w i t h f u r t h e r e l e c t r o l y s i s . The change i n the s c a l e t h i c k n e s s and s t r u c t u r e w i t h i n c r e a s i n g q u a n t i t i e s o f charge passed was s i m i l a r f o r a l l the s u p e r f i c i a l c u r r e n t d e n s i t i e s e x a m i n e d . The c o h e r e n t i n i t i a l l y formed anode l a y e r (see f i g u r e s 3 . 1 9 a and b) has a f i n e , n o r j u l a r , h i g h s u r f a c e a r e a m i c r o s t r u c t u r e . P i t t i n g i s e v i d e n t i n the upper few m i c r o n s o f the anode l a y e r . Wi th i n c r e a s e d a n o d i z a t i o n new p i t s f o r m i n the i n i t i a l l a y e r and the e x i s t i n g p i t s appear to i n c r e a s e i n s i z e . The p i t s c o a l e s c e as the i n t e r m e d i a t e m a t e r i a l i s d i s s o l v e d or s p a l l e d . Figure 3.16 Electron micrographs of the anode scales formed with increasing quantities of charge passed § 5 kA/te2 I xlOO magnification). c) 20 kcoulanbs/cm d) 30 kcoularnbs/crn Figure 3.17 Electron micrographs of the anode scales formed with increasing 2 quantities of charge passed @ 10 kA/m C-xlQ.0 magnification). c) 20 kcoulcmbs/cm d) 30 kco\ilaiibs/ciri Figure 3.18 Electron micrographs of the anode scales formed with increasing 2 quantities of charge passed I 60 kA/m ( xlOO magnification) . Figure 3.19 High magnification electron micrographs of different oxide Cor hydroxide) structures formed on the anode. a) anode layer present after 2 ksec § 10 kA/m2 (x4200 magnification) b) same as above (x21 000 magnifica-tion) c) exposed lower anode layer after 2 10 ksec §10 kA/m (x4000 magnificat-ion) 100 The p i t s do not appear t o i n c r e a s e s i g n i f i c a n t l y i n d e p t h . The i n i t i a l s c a l e wh ich forms appears d u p l e x i n n a t u r e , w i t h the o u t e r and i n n e r l a y e r s o f s i m i l a r t h i c k n e s s . No major c o m p o s i t i o n a l d i f f e r e n c e s between the two l a y e r s c o u l d be d e t e c t e d by s e c o n d a r y x - r a y a n a l y s i s (see f i g u r e 3 . 1 5 ) . A f t e r 5 K c o u l o m b s / ( c m of anode) the anode l a y e r i s about 15 yin t h i c k . The c r a c k i n g and b u l k anode l a y e r d i s s o l u t i o n (o r s p a l l i n g ) appears to o c c u r p r i m a r i l y i n the o u t e r l a y e r . The t h i c k n e s s o f anode o x i d e o r h y d r o x i d e o b s e r v e d to be removed f r o m the anode i s much g r e a t e r than the depth of the p i t t i n g o b s e r v e d on the i n i t i a l anode l a y e r . The growth o f the i n n e r o x i d e l a y e r a p p e a r s a c c e l e r a t e d where t h e o u t e r l a y e r i s f r a c t u r e d or m i s s i n g ( see f i g u r e 3 . 2 0 a and b ) . D u r i n g the p e r i o d o f d i s s o l u t i o n (o r s p a l l i n g ) the u n a f f e c t e d a r e a s of d u p l e x s c a l e r e a c h t h i c k n e s s e s of up to 50. pm. The exposed i n n e r l a y e r a p p e a r s rugged and broken o r g r a n u l a r (see f i g u r e 3 . 1 9 c ) , and i s r e d i n c o l o u r . T h i s l a y e r e x h i b i t s some s u r f a c e p i t t i n g and c r a c k i n g but does not have the n o d u l a r , h i g h s u r f a c e a r e a m i c r o s t r u c t u r e o f the i n i t i a l s u r f a c e . I t i s p o s s i b l e t h i s i n n e r o x i d e l a y e r c o r r e s p o n d s the u l t i m a t e anode l a y e r wh ich i s p r e s e n t a f t e r e x t e n d e d p e r i o d s o f e l e c t r o l y s i s . The anode c r o s s s e c t i o n i n f i g u r e 3 . 2 0 c c o r r e s p o n d s t o t h e spec imen i n f i g u r e 3 . 1 7 d and s u g -g e s t s the f i n a l anode l a y e r , p r e s e n t a f t e r e x t e n d e d p e r i o d s 101 mounting anode Fe plastic layer of e l e c t r o l y s i s , i s g r a n u l a r and r e l a t i v e l y t h i n ( 5 - 1 0 ym). It p r o b a b l e , . t h a t the amount of i r o n a s s o c i a t e d w i t h the o x i d e , o r h y d r o x i d e l a y e r i s o f a s i m i l a r magn i tude to the t o t a l i r o n removed f rom the anode . I f the anode l a y e r i s m a g n e t i t e 3 and r e l a t i v e l y compact w i t h a d e n s i t y of 5 g/cm , a 50 ym t h i c k l a y e r wou ld c o n t a i n about 320 x 1 0 " g . m o l s o f i r o n per cm o f anode l a y e r ( c . f . the r e s u l t s of s e c t i o n 3 . 8 . 1 ) . I t i s p o s s i b l e the anode l a y e r i s an i r o n o x i d e or h y d r o -x i d e of d i f f e r e n t c o m p o s i t i o n , and i t c o u l d be l e s s d e n s e ; thus the q u a n t i t y . ;of\ i r o n a s s o c i a t e d w i t h the anode l a y e r c o u l d be l e s s . The e f f e c t s o f the sodium h y d r o x i d e c o n c e n t r a t i o n on the s c a l e morpho logy can be seen i n f i g u r e 3 . 2 1 . The i n i t i a l o x i d e l a y e r appears to c r a c k and d i s s o l v e , o r s p a l l , more r e a d i l y as the sodium h y d r o x i d e c o n c e n t r a t i o n i s i n c r e a s e d . F i g u r e 3 . 2 1 a f o r 1 g . m o l / 1 sodium h y d r o x i d e i n d i c a t e s t h a t even a f t e r v e r y l a r g e q u a n t i t i e s o f charge have been passed the anode i s c o v e r e d w i t h a t e n a c i o u s s c a l e . The s c a l e i s t y p i c a l l y 5 - 1 0 ym t h i c k and r e f o r m s as a compact l a y e r even when s p a l l i n g has o c c u r r e d . The p h o t o g r a p h s 3 . 2 1 b to d i n d i c a t e t h a t , f o r sodium c) 10 g.mol/1. 26 kcoulcmbs/cm d) 14.3 g.mol/1. 25 kcoulcrribs/cm Figure 3.21 Electron micrographs of the anode scales formed at various sodium hydroxide concentrations between 1 and 14.3 g,mol/l. (xlOO magnification). 104 h y d r o x i d e c o n c e n t r a t i o n s between 5 and 1 4 . 3 g . m o l s / 1 the s c a l e s formed have s i m i l a r m o r p h o l o g i e s . The e x t e n t o f c r a c k i n g , d i s s o l u t i o n and s p a l l i n g , f o r s i m i l a r q u a n t i -t i e s o f c h a r g e p a s s e d , became p r o g r e s s i v e l y g r e a t e r as the sodium h y d r o x i d e c o n c e n t r a t i o n was i n c r e a s e d . In a l l c a s e s the s c a l e s a re d u p l e x i n n a t u r e . The growth of the anode l a y e r and i t s s t r u c t u r e appear s i m i l a r to t h o s e examined i n the c u r r e n t d e n s i t y s t u d i e s . The d u p l e x l a y e r , where i t i s not a f f e c t e d by d i s s o l u t i o n or s p a l l i n g grows to t h i c k -n e s s e s o f 2 0 - 5 0 ym. The f i n a l anode l a y e r p r e s e n t a f t e r f a i l u r e o f the i n i t i a l s c a l e was g r a n u l a r but i t s t h i c k n e s s was not d e t e r m i n e d . There i s some i n d i c a t i o n t h a t f o r sodium h y d r o x i d e c o n c e n t r a t i o n s between 5 and 1 4 . 3 g . m o l / 1 the main e f f e c t o f d e c r e a s i n g the h y d r o x i d e c o n c e n t r a t i o n i s to d e c r e a s e the r a t e o f the f i l m growth, and f i l m b r e a k -down p r o c e s s e s . 3 . 9 . 2 Magnet i t e Anodes The o x i d e (o r h y d r o x i d e ) 1 a y e r s formed d u r i n g the e l e c t r o l y s i s o f m a g n e t i t e anodes appears to d e p e n d , t o a l a r g e e x t e n t , on the c r y s t a l o r i e n t a t i o n of the s u r f a c e . For the spec imen i n f i g u r e 3 . 2 2 t h e r e a re two t y p e s of g r a i n s wh ich a r e v i s i b l e , t h o s e which a r e rough and b r o w n i s h i n c o l o u r and t h o s e which a re smooth and grey i n c o l o u r . 105 h i Figure 3.22 Electron micrographs of a magnetite anode surface after 30 ksec § 10 kA/m2. a) Anode surface (xll magnification) b) Area b (x440 magnification) c) Area c (x440 magnification) .—1 - -mm m H 5 106 F i g u r e 3 . 2 2 b c o r r e s p o n d s to the brown r e g i o n wh ich can be seen to c o n s i s t o f a mesh o f c r y s t a l s between which i s the b r o w n i s h o x i d e . Some removal o f m a t e r i a l f rom t h e s e r e g i o n s i s e v i d e n t . F i g u r e 3 . 2 2 c c o r r e s p o n d s t o the g rey a r e a s which have a compact t e n a c i o u s s c a l e on wh ich the o r i g i n a l p o l i s h marks a re v i s i b l e . In g e n e r a l , the anode l a y e r s wh ich forms on the m a g n e t i t e a re l e s s than 5 pm t h i c k , how-e v e r i n the brown r e g i o n up to 10 ym t h i c k n e s s o f the o r i g i n a l m a g n e t i t e i s m i s s i n g . The r e s u l t s s u g g e s t most of the i r o n removal f rom the anode comes f rom t h e s e brown r e g i o n s . 3 . 1 0 Summary of the E x p e r i m e n t a l R e s u l t s o f C h a p t e r 3 In g e n e r a l , t h e s e s t u d i e s i n d i c a t e t h a t : 1) f e r r a t e i o n s a r e p roduced a t low c u r r e n t e f f i c i e n c i e s (43%) on i r o n a n o d e s , and a r e not p roduced a t a l l on m a g n e t i t e a n o d e s ; 2) the amount of f e r r a t e t h a t can be p roduced f rom a g i v e n anode s u r f a c e i s l i m i t e d , because the r a t e o f f e r r a t e p r o d u c t i o n d e c l i n e s to z e r o a f t e r about 2 25 K c o u l o m b s / c m of c u r r e n t has p a s s e d ; 3) t h e r e i s an i n d i c a t i o n t h a t p e r i o d i c c u r r e n t r e v e r s a l can p r e v e n t the s t e a d y d e c l i n e i n f e r r a t e p r o d u c t i o n r a t e ; 4) the c u r r e n t e f f i c i e n c y of - f e r r a t e p r o d u c t i o n <•>.':• i s i n d e p e n d e n t o f c u r r e n t d e n s i t y ; 5) the t o t a l amount o f f e r r a t e t h a t can be produced b e f o r e t h e r a t e f a l l s to z e r o i s i n d e p e n d e n t o f c u r r e n t d e n s i t y ; the s c a l e of i r o n o x i d e o r h y d r o x i d e formed on an i r o n anode i s a l s o p roduced a t a v e r y low c u r r e n t e f f i c i e n c y ( 1 - 3 % ) ; t h i s a n o d i c s c a l e i s amorphous , t h i c k e n s s l o w l y , and an o u t e r l a y e r i s d i s s o l v e d o r s p a l l s o f f as the s c a l e g r o w s ; s c a l e t h i c k n e s s can v a r y o v e r the anode s u r f a c e by a s u b s t a n t i a l f a c t o r ; the dominant a n o d i c p r o c e s s i s oxygen e v o l u t i o n , a c c o u n t i n g f o r 95% or more o f the a n o d i c c u r r e n t . 108 C h a p t e r 4 THE ELECTROCHEMICAL BEHAVIOUR OF IRON IN CONCENTRATED SODIUM HYDROXIDE SOLUTIONS 4 . 1 T e c h n i q u e s Used Three e x p e r i m e n t a l t e c h n i q u e s were used to examine the e l e c t r o c h e m i c a l p o t e n t i a l o f the anode and i t s b e h a v i o u r w i t h v a r i a t i o n s i n the s u p e r f i c i a l c u r r e n t d e n s i t y and the q u a n -t i t y of charge p a s s e d . The methods used w e r e : 1) P o t e n t i o d y n a m i c t e c h n i q u e s 2) - P o t e n t i o s t a t i c t e c h n i q u e s 3) C o n s t a n t c u r r e n t t e c h n i q u e s . E x p l a n a t i o n o f the d i f f e r e n t t e c h n i q u e s are g i v e n i n s t a n d a r d e l e c t r o c h e m i c a l t e x t b o o k s (see r e f e r e n c e s 36 and 5 1 ) . 4 . 2 The Measurement o f the E l e c t r o c h e m i c a l P o t e n t i a l . For a l l the e x p e r i m e n t a l t e c h n i q u e s u s e d , the m e a s u r e -ment o f the anode ( w o r k i n g ) e l e c t r o d e p o t e n t i a l w i t h r e s p e c t to a r e f e r e n c e e l e c t r o d e was a c h i e v e d u s i n g a L u g g i n c a p i l -l a r y tube p r o b e . A s c h e m a t i c d i a g r a m of the e x p e r i m e n t a l s e t -u p , i n t h i s case f o r the c o n s t a n t c u r r e n t t e c h n i q u e , a r e g i v e n i n f i g u r e 4 . 1 and the components o f t h e measured p o t e n t i a l as g i v e n i n f i g u r e 4 . 2 . A -DC POWER SUPPLY 109 Working Electrode Anodejgj Counter Electrode Pt POTENTIO-METER p: CHART RECORDER "REFERENCE ELECTRODE , -LUGGIN CAPILLARY TUBE Reference Electrode Circuit Counter Electrode Circuit F ig . 4.1 Schematic diagram of anode potent ial measurement for the constant current method. +ve E(V) 1) 2) 3) 4) 5) 6) Distonce Not to Scole *Totol (meosured) j)oJum- Refjrence_Electrpd_e _ ,6 Potentiol Potent ial drop in the conductors. Potent ial drop in the specimen. Potent ial drop across the 'oxide' layer . Potent ia l drop due to the electrochemical reactions at the anode (includes anodic equi l ibr ium potent ial w . r . t . the reference e lectrode) . Potent ia l drop in the e lec t ro l y te between the anode and luggin c a p i l l a r y t i p . L iquid junct ion potential F ig . 4.2 A schematic diagram of the components of potential measured in the reference electrode c i r c u i t , for a working anode. A l l the v o l t a g e components o f the measured a n o d i c p o t e n t i a l , e x c e p t f o r the e q u i l i b r i u m anode p o t e n t i a l and the l i q u i d j u n c t i o n p o t e n t i a l , a r e due to the b u l k f l o w o f c u r r e n t between the c a t h o d e ( c o u n t e r e l e c t r o d e ) and the anode ( w o r k i n g e l e c t r o d e ) . The e q u i l i b r i u m anode p o t e n t i a l w i l l depend on the anode m a t e r i a l ( o r o x i d e c o a t i n g ) and the e l e c t r o l y t e c o m p o s i t i o n and t e m p e r a t u r e . The l i q u i d j u n c t i o n p o t e n t i a l s o c c u r because of the r e q u i r e m e n t of charge n e u t r a l i t y a c r o s s the c o n c e n t r a t i o n g r a d i e n t s e t -up a t the i n t e r f a c e between d i f f e r e n t e l e c t r o l y t e s c o n -t a i n i n g i o n i c s p e c i e s o f d i f f e r e n t m o b i l i t y . At low c u r -r e n t d e n s i t i e s the l i q u i d j u n c t i o n p o t e n t i a l i s the major e r r o r i n v o l v e d i n the measurement o f the anode e l e c t r o -c h e m i c a l r e a c t i o n k i n e t i c s p o t e n t i a l . At h i g h c u r r e n t d e n s i t i e s the ohmic and n o n - o h m i c r e s i s t a n c e s o f t h e f o l l o w -i n g can be s i g n i f i c a n t : 1) the c o n d u c t o r s , 2) the s p e c i -men, 3) o x i d e l a y e r s on the ftnoxke s u r f a c e , and 4) the e l e c t r o -l y t e . The r e s u l t a n t v o l t a g e s a r e t h u s components o f the measured p o t e n t i a l . The i n d i v i d u a l dependence o f t h e s e v a r i o u s components on c u r r e n t d e n s i t y , and the p o s s i b i l i t y of v a r i a t i o n d u r i n g the e x p e r i m e n t means the p o t e n t i a l component due t o the anode e l e c t r o c h e m i c a l r e a c t i o n k i n e t i c s i s not r e a d i l y d e t e r m i n a b l e . 4 . 3 The M a g n i t u d e of the Measured Components o f A n o d i c P o t e n t i a l A b s o l u t e d e t e r m i n a t i o n of the i n d i v i d u a l v o l t a g e components o f the measured anode p o t e n t i a l i s not n e c e s -s a r y to deduce q u a l i t a t i v e l y what components of t h e v o l t a g e are v a r y i n g d u r i n g the e x p e r i m e n t a l i n v e s t i g a t i o n . I t i s n e c e s s a r y t o d e t e r m i n e wh ich components a r e i m p o r t a n t and the a p p r o x i m a t e magn i tude o f the v a r i o u s v o l t a g e component . For the e x p e r i m e n t a l c o n d i t i o n s c o n s i d e r e d the p o t e n t i a l drop due to ohmic r e s i s t a n c e s i n the c o n d u c t o r s i s n e g l i g i b l e (<5mV). The l i q u i d j u n c t i o n p o t e n t i a l w i l l be c o n s t a n t , i f not n e g l i g i b l e . Three v o l t a g e components can thus v a r y d u r i n g the c o u r s e o f an e x p e r i m e n t : 1) E l e c t r o l y t e r e s i s t a n c e . 2) R e s i s t a n c e s due to s u r f a c e l a y e r f o r m a t i o n on the anode . 3) The o v e r p o t e n t i a l o f the combined e l e c t r o c h e m i c a l r e a c t i o n s on the anode . These v a r i o u s components must be c o n s i d e r e d i n d i v i d u a l l y . 4 . 3 . 1 The P o t e n t i a l Drop Due t o E l e c t r o l y t e R e s i s t a n c e The e l e c t r i c a l c o n d u c t i v i t y of the b u l k e l e c t r o l y t e 52 can r e a d i l y be m e a s u r e d . The p rob lem however , w i t h gas e v o l v i n g e l e c t r o d e s , i s t h a t the e l e c t r o l y t e p r o p e r t i e s a t the e l e c t r o d e can be c o n s i d e r a b l y d i f f e r e n t f rom t h o s e of the b u l k s o l u t i o n . The e l e c t r o l y t e c o m p o s i t i o n , t e m p e r a -t u r e and gas f r a c t i o n a t the i n t e r f a c e may a l l be r a d i c a l l y 47 53 d i f f e r e n t f rom the b u l k s o l u t i o n . ' 112 53 Kuhn and S t e v e n s o n have r e p o r t e d t h a t the p o t e n t i a l measured a t a g a s s i n g e l e c t r o d e by s t a n d a r d l u g g i n c a p i l -l a r y t e c h n i q u e s (see f i g u r e 4 . 3 ) has two components : 1) t h a t due to b u l k e l e c t r o l y t e r e s i s t i v i t y wh ich i s u n i f o r m to w i t h i n 0 . 5 mm of the e l e c t r o d e , and 2) an i n n e r r e s i s t a n c e a s s o c i a t e d i n some way w i t h the e l e c t r o d e . Kuhn and 53 S t e v e n s o n have s u g g e s t e d the i n n e r r e s i s t a n c e i s due to e l e c t r i c f i e l d c o n s t r u c t i o n caused by a f i n e l a y e r o f bub -b l e s on the anode . The i n n e r r e s i s t a n c e term was found t o i n c r e a s e f rom 50 to 200 mV as the c u r r e n t d e n s i t y was i n -2 c r e a s e d f rom 300 t o 2500 A/m f o r oxygen e v o l u t i o n f rom a p l a t i n u m anode i n 0 . 1 M H^SO^. An e x p e r i m e n t a l i n v e s t i g a t i o n would be r e q u i r e d t o a c c u r a t e l y d e t e r m i n e the magn i tude of the p o t e n t i a l drop due t o e l e c t r o l y t e r e s i s t a n c e s . However f o r c o n s t a n t c u r r e n t e x p e r i m e n t s , the p o t e n t i a l drop due to e l e c t r o l y t e r e s i s t a n c e s w i l l be e f f e c t i v e l y c o n s t a n t once an e q u i l i b r i u m gas c o n c e n t r a -t i o n has b u i l t up i n any sys tem c o n s i d e r e d . To Reference Electrode T B A) S t a n d a r d ( f r o n t s i d e ) l u g g i n c a p i l l a r y tube p o s i t i o n . B) ' B a c k s i d e ' l u g g i n c a p i l l a r y tube p o s i t i o n . F i g u r e 4 . 3 Anode Electrolyte A l t e r n a t i v e l u g g i n c a p i l l a r y tube p o s i t i o n s . 113 The use of a b a c k s i d e l u g g i n c a p i l l a r y tube (see 54 f i g u r e 4 . 3 ) h a s ) been p roposed to e l i m i n a t e e l e c t r o l y t e r e s i s t a n c e s f rom the measured anode p o t e n t i a l . The t i p o f the l u g g i n c a p i l l a r y tube i s f l u s h w i t h the anode s u r f a c e ; 54 I t has been r e p o r t e d t h a t t h i s s e t u p g i v e s c o n s i s t e n t p o t e n t i a l measurements on n o n - g a s s i n g e l e c t r o d e s . O ther 55 w o r k e r s have r e p o r t e d the p o t e n t i a l s measured by t h i s t e c h n i q u e a r e c o n s i s t e n t l y h i g h . The s u i t a b i l i t y o f t h e b a c k s i d e l u g g i n c a p i l l a r y tube f o r the measurement o f the e l e c t r o d e p o t e n t i a l o f h i g h c u r r e n t d e n s i t y ' g a s s i n g ' e l e c t r o d e s i s not known. 4 . 3 . 2 P o t e n t i a l Drop A c r o s s an Anode S u r f a c e L a y e r The growth o f an o x i d e (o r h y d r o x i d e ) l a y e r on the anode s u r f a c e w i l l r e s u l t i n a r e s i s t a n c e p o t e n t i a l drop a c r o s s t h i s 1 a y e r and t h u s the measured p o t e n t i a l w i l l i n -c r e a s e . The e x a c t c o m p o s i t i o n and r e s i s t i v i . t y c p / f the l a y e r which forms i s not known. The p o t e n t i a l drop a c r o s s the f i l m w i l l i n c r e a s e as the f i l m t h i c k e n s ; i n t h e o r y a 60 ym 2 t h i c k f i l m f o r a c u r r e n t d e n s i t y o f 10 KA/m c o u l d have a p o t e n t i a l drop r a n g i n g f rom 0 . 0 6 mV ( m a g n e t i t e p = 1 0 ~ 4 56 56 ohm.tn) to 160 V ( H a e m e t i t e p = 270 ohm.m). L a r g e p o t e n t i a l d rops a c r o s s t h e l a y e r may r e s u l t i n the f o r m a -t i o n o f m e t a s t a b l e f i l m s o f unknown e l e c t r i c a l p r o p e r t i e s and the anode l a y e r r e s i s t a n c e may be n o n - o h m i c i f i o n i c c o n d u c t i o n c u r r e n t s a r e s i g n i f i c a n t . 114 Changes i n the measured anode p o t e n t i a l due to anode s u r f a c e l a y e r f o r m a t i o n and growth may or may not be easy to d e t e c t i n h i g h c u r r e n t d e n s i t y c o n s t a n t c u r r e n t e x p e r i -ments . W i t h o u t o t h e r e x p e r i m e n t a l v e r i f i c a t i o n c a r e must be t a k e n i n a s s i g n i n g p o t e n t i o s t a t i c b e h a v i o u r i , - t o s c a l e t h i c k e n i n g . 4 . 3 . 3 Anode P o t e n t i a l Component Due t o the E l e c t r o c h e m i c a l  R e a c t i ons 44 The r e p o r t e d T a f e l e q u a t i o n f o r oxygen e v o l u t i o n f rom i r o n i n a l k a l i s o l u t i o n i s : n (V) = 0 . 3 5 + 0 . 0 7 l o g 1 Q i i = A/m 2 - 4 . 1 2 Thus f o r an anode a t a c u r r e n t d e n s i t y o f 10 KA/m the r e -s u l t a n t anode o v e r p o t e n t i a l due t o the k i n e t i c s o f the oxygen e v o l u t i o n r e a c t i o n would be 630 mV. T y p i c a l l y l e s s than 5% o f the c u r r e n t r e s u l t s i n f e r r a t e or o x i d e f o r m a t i o n . I f t h e s e r e a c t i o n s c e a s e d the r i s e i n the a n o d i c p o t e n t i a l due to i n c r e a s e d oxygen e v o l u t i o n to compensate wou ld be i n the r e g i o n of 2 mV. The o n l y o b s e r v a b l e changes i n the a n o d i c p o t e n t i a l component r e s u l t i n g f rom the e l e c t r o c h e m i c a l r e a c t i o n s w i l l be due t o : 1) changes i n the mechanism or k i n e t i c s of 115 oxygen e v o l u t i o n or 2) l a r g e r e d u c t i o n s i n a c t u a l c u r r e n t d e n s i t i e s due to i n c r e a s e s i n s u r f a c e a r e a . Anode p o t e n t i a l measurements f o r h i g h c u r r e n t d e n s i t y s t u d i e s y i e l d no d i r e c t i n f o r m a t i o n about the e l e c t r o c h e m i c a l f o r m a t i o n of f e r r a t e i o n s . 4 . 4 E x p e r i m e n t a l 4 . 4 . 1 E x p e r i m e n t a l A p p a r a t u s The e x p e r i m e n t a l a p p a r a t u s was the same as t h a t used i n the beaker e x p e r i m e n t s (see page(6.4.) w i t h two e x c e p t i o n s : 1) a l u g g i n c a p i l l a r y - r e f e r e n c e e l e c t r o d e c i r c u i t was i n c o r p o r a t e d i n the a p p a r a t u s to measure the p o t e n -t i a l o f the anode d u r i n g the e x p e r i m e n t , 2) f o r p o t e n t i o d y n a m i c and p o t e n t i o s t a t i c e x p e r i m e n t s a P o t e n t i o s t a t (EG & G PARC Model 3504 c o r r o s i o n m e a s u r e -ment c o n s o l e ) was used to s u p p l y power of up to 1 A . The p o t e n t i a l measurement a c c u r a c y was + 2 mV and r e g u l a t i o n of the p o t e n t i a l was v i a t h e r e f e r e n c e e l e c t r o d e - 1 u g g i n c a p i l l a r y tube c i r c u i t . 4 . 4 . 2 L u g g i n C a p i l l a r y Tube Probe Narrow bare p o l y e t h y l e n e t u b i n g ( I n t r o m e d i c Co . ) was found to work s a t i s f a c t o r i l y as a l u g g i n C a p i l l a r y t u b e . Two s i z e s o f t u b i n g were u s e d : 1) 0 . 9 mm I .D . - 1 . 3 mm O.D. and 2) 0 . 6 mm I . D . - 1 mm O.D. The c a p i l l a r y tube was c o n n e c t e d to a 50 ml s y r i n g e wh ich was f i l l e d w i t h sodium h y d r o x i d e o f the same c o n c e n t r a t i o n as the b u l k e l e c t r o l y t e ( 1 4 . 3 g . m o l / 1 NaOH). The s y r i n g e was both a r e s e r v o i r fear the e l e c t r o l y t e p a s s i n g t h r o u g h the s m a l l bore t u b i n g and a tank t o c o n t a i n the r e f e r e n c e e l e c t r o d e . ' ; By i n c r e a s i n g the h y d r o s t a t i c head o f t h e s y r i n g e s u f f i c i e n t l y to e n s u r e a f l o w r a t e t h r o u g h the t u b i n g o f 1 0 - 5 0 m l / h r , e l e c t r i c a l c o n d u c t i v i t y t h r o u g h the l u g g i n c a p i l l a r y was m a i n t a i n e d even when the c a p i l l a r y t i p was c l o s e to the gas e v o l v i n g anode . Two p o s i t i o n s f o r the p l a c e m e n t o f the t i p o f the l u g g i n c a p i l l a r y tube were examined (see f i g u r e 4 . 3 ) : 1) the l u g g i n c a p i l l a r y tube t i p was p l a c e d about 5.:imm i n f r o n t o f the a n o d e , and 2) the back s i d e l u g g i n c a p i l -l a r y tube p o s i t i o n . The f o r m e r l u g g i n c a p i l l a r y tube p o s i -t i o n was used i n the e x p e r i m e n t s u n l e s s o t h e r w i s e s t a t e d . 4 . 4 . 3 R e f e r e n c e E l e c t r o d e The r e f e r e n c e e l e c t r o d e used f o r a l l e x p e r i m e n t s was a m e r c u r y - m e r c u r y o x i d e i n 1 4 . 3 g . m o l / 1 NaOH e l e c t r o l y t e . 57 The method o f p r e p a r a t i o n i s o u t l i n e d by Ives and Janz . These e l e c t r o d e s were found to be h i g h l y r e p r o d u c i b l e i n s t r o n g a l k a l i s o l u t i o n s . The e l e c t r o c h e m i c a l p o t e n t i a l o f a l l the e l e c t r o d e s p r e p a r e d were the same w i t h i n + 1 mV. The e l e c t r o c h e m i c a l p o t e n t i a l a t 23°C f o r the Hg/HgO | 1 4 . 3 g . m o l / 1 NaOH w i t h r e s p e c t to a s a t u r a t e d c a l o m e l e l e c t r o d e w w i t h 1 4 . 3 g . m o l / 1 NaOH as the j o i n i n g e l e c t r o l y t e was - 2 0 2 +_ 1 mV. Less than 1 mV d r i f t was o b s e r v e d i n 5 h o u r s . For t h e e x p e r i m e n t s , the b u l k e l e c t r o l y t e and the r e f e r e n c e e l e c t r o d e e l e c t r o l y t e a r e t h e same and thus l i q u i d j - ' unc t ion p o t e n t i a l s a r e v i r t u a l l y e l i m i n a t e d . Any a t t e m p t to measure the s t a n d a r d e l e c t r o c h e m i c a l p o t e n t i a l of t h e e l e c t r o d e H g / H g O j l A . 3 g . m o l / 1 NaOH w i t h r e s p e c t to a known r e f e r e n c e e l e c t r o d e , such as a c a l o m e l e l e c t r o d e , w i l l i n v o l v e l i q u i d j u n c t i o n p o t e n t i a l s . The l i q u i d j u n c t i o n p o t e n t i a l between 1 4 . 3 g . m o l / 1 NaOH and a c a l o m e l e l e c t r o d e ( s a t u r a t e d KC1) as c a l c u l a t e d by the P l a n k - H e n d e r s o n Equa -3 6 t i o n i s about 15 mV. The l i q u i d j u n c t i o n p o t e n t i a l can be c o n s i d e r e d to remain c o n s t a n t . Thus the a p p r o x i m a t e r e v e r s i b l e e l e c t r o c h e m i c a l p o t e n t i a l f o r the m e r c u r y -mercury o x i d e ( 1 4 . 3 g . m o l / 1 NaOH) e l e c t r o d e : HgO + H 2 0 + 2 e~ = Hg + 2 OH" [NaOH] = 1 4 . 3 g . m o l / 1 - 4 . 2 as measured a g a i n s t the s a t u r a t e d c a l o m e l e l e c t r o d e i s : Hg/HgClJKCl (Saturated); NaOH (.14.3 g.mol/1) | HgO/Hg EHg/HgO}l4.'3 g.mol/1 .NaOH * Measured + ECalomel + E LJP ~ 4 " 3 * - 202 + 2 4 2 + 1 5 - + 55 mV T h i s can be compared w i t h the s t a n d a r d r e v e r s i b l e e l e c t r o -49 c h e m i c a l p o t e n t i a l f o r m e r c u r y - m e r c u r y o x i d e a Q H ~ = 1 of + 98 mV. E x t r a p o l a t i n g the thermodynamic c a l c u l a t i o n s of Macdona ld and M c K u b r e 2 7 f rom 10 g . m o l / k g NaOH t o 1 6 . 7 g . m o l / k g ( 1 4 . 3 g.moT/1) NaOH g i v e s E Hg/HgO [ 1 4 . 3 g . m o l / 1 NaOH * + 150 mV. 4 . 5 E x p e r i m e n t s Conducted A l l the e x p e r i m e n t s were c o n d u c t e d at 50°C u s i n g 1 4 . 3 g . m o l / 1 sodium h y d r o x i d e e l e c t r o l y t e . U n l e s s o t h e r w i s e s t a t e d , a l l p o t e n t i a l s a re w i t h r e s p e c t t o a m e r c u r y - m e r c u r y o x i d e i n 1 4 . 3 g . m o l / 1 NaOH r e f e r e n c e e l e c t r o d e a t 23°C. 4 . 5 . 1 I ron E l e c t r o d e s The f o l l o w i n g e x p e r i m e n t s a r e r e p o r t e d i n s e c t i o n 4 . 6 . 1) P o t e n t i o d y n a m i c E x p e r i m e n t s - t h e v a r i a t i o n s i n s u p e r -f i c i a l c u r r e n t d e n s i t i e s , d u r i n g the f o l l o w i n g anode p o t e n t i a l c y c l e s , were m e a s u r e d . a) The a n o d i c p o t e n t i a l o f a c l e a n i r o n spec imen was c y c l e d f rom - 1 . 2 5 V to - + 1 V and back to - 1 . 2 5 V @ 1 mV/s. b) The p o t e n t i a l o f an i r o n anode w h i c h had p r e -2 v i o u s l y been a n o d i z e d f o r 2 7 . 5 K sec @ 10 KA/m was c y c l e d f rom t h e c o r r o s i o n p o t e n t i a l o f the spec imen ( - - 0 . 5 V) to + 1 V and back t o - 1 . 2 5 V @ 1 mV/s. 119 2) C o n s t a n t C u r r e n t E x p e r i m e n t s - the v a r i a t i o n s i n the anode p o t e n t i a l as i n c r e a s i n g q u a n t i t y of charge was p a s s e d , were measured f o r a s u p e r f i c i a l c u r r e n t d e n s i t y 2 of 10 KA/m . U s i n g b a c k s i d e l u g g i n t e c h n i q u e s s i m i l a r measurements were made f o r s u p e r f i c i a l c u r r e n t d e n -s i t i e s o f 5 , 10 and 18 K A / m 2 . 3) P o t e n t i o s t a t i c E x p e r i m e n t s - a c l e a n i r o n spec imen was e l e c t r o l y z e d a t + 1 . 6 V f o r 30 K s e c . The v a r i a t i o n s i n the s u p e r f i c i a l c u r r e n t d e n s i t y d u r i n g the e x p e r i -ment were m e a s u r e d . 4 . 5 . 2 M a g n e t i t e E l e c t r o d e s The f o l l o w i n g e x p e r i m e n t s u s i n g m a g n e t i t e anodes a r e r e p o r t e d i n s e c t i o n 4 . 6 . 4 . 1) P o t e n t i o d y n a m i c E x p e r i m e n t - the v a r i a t i o n s ;i,n the s u p e r f i c i a l c u r r e n t ' d e n s i t y were measured d u r i n g the c y c l i n g o f the anode p o t e n t i a l f rom - 1 . 6 V to + 2 V and back to - 1 . 6 V @ 1 mV/s. 2) P o t e n t i o s t a t i c E x p e r i m e n t s - the s u p e r f i c i a l c u r r e n t d e n s i t y , and i t s v a r i a t i o n w i t h t i m e , a t an a n o d i c p o t e n t i a l o f +1.2 V was m e a s u r e d . The e f f e c t s o f c a t h o d i z a t i o n p r i o r t o a n o d i c e l e c t r o l y s i s a t +,!!•. 2 V were a l s o e x a m i n e d . C a t h o d i z a t i o n s o f 5 and 10 Kssec @ - 1 . 6 V were u s e d . 120 4 . 6 R e s u l t s and A n a l y s i s 4 . 6 . 1 P o t e n t i o d y n a m i c S t u d i e s o f I ron The c u r r e n t d e n s i t y v e r s u s p o t e n t i a l p l o t f o r an i n i t i a l l y c l e a n i r o n spec imen i s g i v e n i n f i g u r e 4 . 4 . For the i n c r e a s i n g p o t e n t i a l sweep ( l i n e a), t h e r e a r e f o u r d i s t i n c t r e g i o n s . Below - 1 V the c a t h o d i c c u r r e n t i s p r i m a r i l y due t o hydrogen e v o l u t i o n . At p o t e n t i a l s g r e a t e r than - I V a n o d i c c u r r e n t s p r e d o m i n a t e . Between - 1 and - 0 . 7 5 V the i r o n was a c t i v e ( d i s s o l u t i o n ) and p a s s i v a t i o n o c c u r r e d above - 0 . 7 5 V . From - 0 . 5 to + 0 . 5 V a p a s s i v e l a y e r i s p r e s e n t on the anode and the s t e a d y c u r r e n t i s due to e i t h e r , o r b o t h , e l e c t r o n i c and i o n i c c o n d u c t i o n t h r o u g h t h e l a y e r . At p o t e n -t i a l s o f g r e a t e r than 0 . 6 V oxygen e v o l u t i o n o c c u r s on the anode . The p a s s i v e f i 1 m i s s t i l l p r e s e n t i n t h i s r e g i o n and thus the c o n d u c t i o n of charge t h r o u g h t h i s l a y e r must p r i m a r i l y 46 be e l e c t r o n i c . The p r o c e s s e s which a re i n v o l v e d i n the p a s s i v a t i o n o f i r o n a r e i n d i s p u t e (see s e c t i o n 1 . 4 ) . The hump ( I ) and p l a t e a u x ( I I ) o b s e r v e d a t p o t e n t i a l s between - 1 and 0 . 5 V a r e not i n d i s a g r e e m e n t w i t h t h o s e r e p o r t e d by Macdona ld and 41 42 McKubre . ' I t i s p r o b a b l e the f i r s t peak i s a s s o c i a t e d w i t h the f o r m a t i o n o f an i r o n (.II) o x i d e o r h y d r o x i d e . The second peak may be a s s o c i a t e d ' w-i th [the f o r m a t i o n o f h i g h e r o x i d e or h y d r o x i d e s p e c i e s F e ~ Q 4 , Fe(.QH)q, FeQ'Q-H. e t c . I t 121 1-0 0-5 0-0 -0-5 -1-0 Electrode Potential.Volts.vs.Hg/HgO. F ig . 4.4 Potentiodynamic plot for an i n i t i a l l y clean iron specimen. i s n o t known i f d i s s o l v e d i n t e r m e d i a t e s p e c i e s were p r e s e n t . At h i g h anode p o t e n t i a l s the mechanism o f oxygen e v o l u t i o n and the r o l e o f the p a s s i v e l a y e r i s not w e l l u n d e r s t o o d . The e l e c t r o n i c and i o n i c c u r r e n t s due to s c a l e growth or d i s s o l u t i o n i n t h i s r e g i o n a r e o b s c u r e d by the l a r g e oxygen e v o l u t i o n c u r r e n t s . The T a f e l s l o p e f o r oxygen 2 e v o l u t i o n i s about 0 . 0 7 5 l o g ^ l (A/m ) . At v e r y h i g h c u r -r e n t d e n s i t i e s r e s i s t a n c e s i n the e l e c t r o l y t e becomes s i g n i -f i c a n t and the s l o p e o f the E v e r s u s l o g ^ l p l o t d e v i a t e s f rom l i n e a r i t y . The r e v e r s e sweep of p o t e n t i a l ( l i n e b, f i g u r e 4 . 4 ) g i v e s some i n d i c a t i o n as t o what anode o x i d a t i o n p r o c e s s e s have been o c c u r r i n g on the f o r w a r d p o t e n t i a l sweep. The decay o f the r a t e of oxygen e v o l u t i o n as the p o t e n t i a l i s reduced i s o f s i m i l a r magn i tude to the i n c r e a s e s o b t a i n e d on the f o r w a r d sweep. The p a s s i v e l a y e r c u r r e n t a t p o t e n -t i a l s below t h a t r e q u i r e d f o r oxygen e v o l u t i o n i s about an o r d e r of magni tude l o w e r than i n the f o r w a r d sweep. T h i s c o u l d be due to the i n c r e a s e d e l e c t r i c a l o r i o n i c r e s i s t i v i t y o f the anode p a s s i v e l a y e r , p r o b a b l y by t h i c k e n i n g d u r i n g the p e r i o d o f oxygen e v o l u t i o n . The c a t h o d i c p r o c e s s e s o f the r e v e r s e sweep p r e d o m i n a t e 123 a t p o t e n t i a l s more n e g a t i v e than - 0 . 2 5 V. The p o s i t i v e s h i f t o f 0 . 7 5 V f rom the f o r w a r d , sweep i s due to the r e -d u c t i o n o f h i g h e r v a l e n c y i r o n o x i d e s (o r h y d r o x i d e s ) wh ich have been formed on the s u r f a c e . The i n i t i a l s h o u l d e r i s p o s s i b l y due t o d e f e c t movements i n the anode l a y e r . The second s h o u l d e r ( I I I ) a t about - 0 . 7 5 V c o r r e s p o n d s t o t h a t r e -42 p o r t e d by McKubre t o be the r e d u c t i o n of F e , , 0 3 to Fe^O^. McKubre a l s o s u g g e s t e d t h a t the r e d u c t i o n of Fe^O^ to FetOH)^ o c c u r s a t a l l p o t e n t i a l s l e s s than - -TV and i s not v i s i b l e as a peak . The v e r y r e v e r s i b l e peak ( IV ) a t - - I V i s 42 r e p o r t e d to be due to the r e d u c t i o n of FeOOH to F e ( 0 H ) 2 -The c u r r e n t s due to f u r t h e r r e d u c t i o n of the o x i d e (o r h y d r o -x i d e ) l a y e r a re o b s c u r e d by the hydrogen e v o l t u i o n c a t h o d i c c u r r e n t . The p o t e n t i o d y n a m i c p l o t o b t a i n e d u s i n g an anode spec imen wh ich had been a n o d i c a l l y e l e c t r o l y z e d f o r 2 7 . 5 2 K sec @ 10 KA/m i s g i v e n i n f i g u r e 4 . 5 . S e v e r a l d i f f e r e n c e s a re e v i d e n t between the o x i d i z e d and the i n i t i a l l y c l e a n i r o n s p e c i m e n s . For the o x i d i z e d s p e c i m e n , the c u r r e n t i n the p a s s i v e r e g i o n i s i n i t i a l l y h i g h e r than f o r the f r e s h , i r o n s p e c i m e n , but i t d e c l i n e s to a p p r o x i m a t e l y an o r d e r o f magn i tude l e s s than the f r e s h i r o n spec imens p a s s i v e l a y e r c u r r e n t when the oxygen e v o l u t i o n 124 J I I L 10 0 5 0 - 0 5 -10 Electrode Potential. VoltsvsHg/HgO F ig . 4.5 Potentiodynamic plot for an iron anode previously anodized for 27.5 K sec @ 10 KA/m2. 125 commences. I t i s s u g g e s t e d t h a t the e f f e c t s o f b u i l d i n g up a p a s s i v e o x i d e l a y e r l o w e r s the charge c o n d u c t i o n a c r o s s the l a y e r . The i n i t i a l l y h i g h e r c u r r e n t s c o u l d be due t o the r e o x i d a t i o n o f the s u r f a c e and r e e s t a b l i s h m e n t o f i n -t e r n a l d e f e c t s t r u c t u r e s wh ich d u r i n g the p e r i o d between the i n i t i a l o x i d a t i o n and the p o t e n t i o d y n a m i c t e s t had e q u i l i -b r i a t e d to some e x t e n t . The oxygen e v o l u t i o n r e a c t i o n on the a n o d i z e d spec imen i s e f f e c t i v e l y t h e same; as on the ; i n i t i a l l y c l ean i r o n s p e c i m e n . S l i g h t d i f f e r e n c e s may r e s u l t f rom changes i n s u r f a c e a r e a d u r i n g o x i d i z a t i o n . The anode l a y e r on t h e o x i d i z e d spec imen i s r e l a t i v e l y c o n d u c t i v e . On the r e v e r s e p o t e n t i a l sweep c a t h o d i c p r o c e s s e s due t o t h e r e d u c t i o n of o x i d e s (o r h y d r o x i d e s ) p r e d o m i n a t e a t p o t e n t i a l s more n e g a t i v e than + 0 . 2 V and the magn i tude o f the r e d u c t i o n c u r r e n t s a r e much l a r g e r than t h o s e o b t a i n e d f o r the p o t e n t i o d y n a m i c a l l y t e s t e d c l e a n i r o n s p e c i m e n . L a r g e r r e d u c t i o n c u r r e n t s a r e e x p e c t e d i f more h i g h e r v a l e n c y i r o n o x i d e i s p r e s e n t on the o x i d i z e d s p e c i m e n . H i g h e r r e d u c t i o n c u r r e n t s would a l s o be e x p e c t e d i f the p a s s i v e f i l m has l e s s s t r u c t u r e and i s more e a s i l y p e n e t r a t e d by the e l e c t r o l y t e as i t r e d u c e s . 125 4 . 6 . 1 . 1 Summary of the P o t e n t i o d y n a m i c S t u d i e s o f I ron The p o t e n t i o d y n a m i c s t u d i e s of i r o n i n d i c a t e t h a t no majo r e l e c t r o c h e m i c a l d i f f e r e n c e s e x i s t f o r oxygen e v o -l u t i o n f r o m : 1) a t h i c k o x i d e (o r h y d r o x i d e ) l a y e r formed a f t e r e x t e n d e d p e r i o d s o f a n o d i c e l e c t r o l y s i s , and 2) the t h i n i n i t i a l l y formed anode l a y e r . The anode l a y e r has c o n s i d e r a b l e c o n d u c t i v i t y even a f t e r e x t e n d e d p e r i o d s o f a n o d i c e l e c t r o l y s i s . The t h i c k l a y e r s formed by e x t e n d e d a n o d i z a t i o n a t h i g h c u r r e n t d e n s i t y s u p p o r t c o n s i d e r a b l y h i g h e r c a t h o d i c c u r r e n t s than do t h e t h i n q u i c k l y formed 1 a y e r s . 4 . 6 . 2 C o n s t a n t C u r r e n t S t u d i e s o f I ron The r e s u l t s o f c o n s t a n t c u r r e n t e x p e r i m e n t s c o n d u c t e d 2 2 a t 10 KA/m f o r 30 and 45 K c o u l o m b s / c m a r e g i v e n i n f i g u r e 4 . 6 . P o t e n t i a l n o i s e ( n o t shown i n f i g u r e 4 . 6 ) i s p r o b a b l y due to bubb le e f f e c t s a t the l u g g i n c a p i l l a r y t i p . The r e -s u l t s s u g g e s t the p o t e n t i a l b e h a v i o r can be d i v i d e d i n t o t h r e e r e g i o n s . Reg ion I, wh ich e x t e n d s f o r the f i r s t 4 2 K c o u l o m b s / c m , d u r i n g wh ich t h e r e i s a g e n e r a l d e c r e a s e i n measured p o t e n t i a l . The i n i t i a l anode p o t e n t i a l v a r i e d but i n both c a s e s the p o t e n t i a l d e c l i n e d to about + 1 .25 V. Reg ion II i s e v i d e n t as a g e n e r a l i n c r e a s e i n p o t e n t i a l . The r e s u l t s f o r both runs a re q u i t e s i m i l a r w i t h the anode p o t e n t i a l i n c r e a s i n g to about + 1 .46 V a f t e r a t o t a l o f 15 to 20 K c o u l o m b s / c m . Reg ion I I I a p p e a r s to be the e s t a -b l i s h m e n t o f a s t e a d y s t a t e . A s l i g h t d e c l i n e i n p o t e n t i a l was o b s e r v e d a f t e r which the p o t e n t i a l f l u c t u a t e s around a mean v a l u e . 4 . 6 . 2 . 1 B a c k s i d e L u g g i n C a p i l l a r y Tube R e s u l t s The anode p o t e n t i a l measurements o b t a i n e d u s i n g b a c k s i d e l u g g i n c a p i l l a r y t u b e s a r e c o n s i d e r a b l y d i f f e r e n t f rom t h o s e o b t a i n e d u s i n g the l u g g i n c a p i l l a r y tube i n the s t a n d a r d p o s i t i o n . F i g u r e 4 . 7 c o n t a i n s s e c t i o n s o f the ••-2 p o t e n t i a l p l o t o b t a i n e d d u r i n g a 5 KA/m c o n s t a n t c u r r e n t 2 e x p e r i m e n t o f 1 2 . 5 Kcou lombs/cm d u r a t i o n . The measured p o t e n t i a l undergoes a s e r i e s o f r e g u l a r f l u c t u a t i o n s . I n i t i a l l y the f l u c t u a t i o n s r e p e a t t h e m s e l v e s e v e r y 30 s e c o n d s . As the q u a n t i t y o f c u r r e n t passed i s i n c r e a s e d the r e g u l a r i t y and f r e q u e n c y of the f l u c t u a t i o n s f a l l . The 2 i n i t i a l anode p o t e n t i a l measured a t 875 coulomb/cm was about 0 . 8 3 V and t h i s r o s e t o about 0 . 8 7 V a f t e r 1 2 . 5 2 K coulomb/cm . S i m i l a r t r e n d s were o b s e r v e d i n the r e s u l t s o f e x p e r i -2 m e n t s c o n d u c t e d a t 10 and 18 KA/m . In a l l c a s e s r e g u l a r f l u c t u a t i o n s i n the measured p o t e n t i a l were o b s e r v e d but the f r e q u e n c y i n c r e a s e d as the c u r r e n t d e n s i t y was i n c r e a s e d . At 10 KA/m 2 the i n i t i a l measured p o t e n t i a l was 0 . 9 5 V. At 130 ? 18 KA/m the measured p o t e n t i a l was- about 1 . 0 5 V f o r the 2 f i r s t 7 K coulombs/cm and then r o s e g r a d u a l l y to 1 . 1 5 V 2 a f t e r 15 K c o u l o m b s / c m . I t i s p o s s i b l e the l a r g e f l u c t u a t i o n s i n measured p o t e n t i a l a r e due to the f o r m a t i o n , growth and detachment of gas b u b b l e s a t the h y d r o p h o b i c l u g g i n c a p i l l a r y t i p , how-e v e r i t i s not o b v i o u s how t h i s c o u l d r e s u l t i n the e x a c t shape o f the f l u c t u a t i o n s o b t a i n e d . O ther p o s s i b l e c a u s e s , such as s p a l l i n g , n u c l e a t i o n o f new phases o r l o c a l i z e d changes i n e l e c t r o l y t e c o n d u c t i v i t y a r e l e s s l i k e l y . 4 . 6 . 2 . 2 Compar ison of t h e ' B a c k s i d e ' and ' F r o n t s i d e ' L u g g i n Capi11 a r y Probe R e s u l t s 2 At 10 KA/m the anode p o t e n t i a l s measured u s i n g the b a c k s i d e l u g g i n c a p i l l a r y tube probe were 0 . 4 - 0 . 5 V l e s s than t h a t measured by t h e c o n v e n t i o n a l l u g g i n c a p i l -l a r y tube p r o b e . The h i g h e r v a l u e f o r the f r o n t s i d e l u g -g i n c a p i l l a r y tube w i l l be due to p o t e n t i a l drops i n the e l e c t r o l y t e between the anode and t h e l u g g i n t i p . I t i s p o s s i b l e t h a t the b a c k s i d e l u g g i n probe does not measure the t r u e anode p o t e n t i a l ; the i n i t i a l anode p o t e n t i a l measured i n c r e a s e s by about 0 . 2 V as the c u r r e n t d e n s i t y i n c r e a s e s 2 f rom 5 t o 18 KA/m however i f the p r e v i o u s l y d e t e r m i n e d T a f e l s l o p e of 0 . 0 7 5 (V.) i s v a l i d an anode p o t e n t i a l i n c r e a s e of - 0 . 0 4 V would be e x p e c t e d . 131 A d e c l i n e i n anode p o t e n t i a l c o r r e s p o n d i n g to Reg ion I i n the f r o n t s i d e l u g g i n c a p i l l a r y e x p e r i m e n t s was not o b s e r v e d i n the b a c k s i d e l u g g i n c a p i l l a r y e x p e r i m e n t s . The r i s e i n anode p o t e n t i a l c o r r e s p o n d i n g to Reg ion II was found t o be l a r g e r i n f r o n t s i d e l u g g i n c a p i l l a r y e x p e r i m e n t s than b a c k s i d e l u g g i n c a p i l l a r y e x p e r i m e n t s . 4 . 6 . 3 P o t e n t i o s t a t i c S t u d i e s o f I ron The v a r i a t i o n i n c u r r e n t d e n s i t y w i t h t ime f o r an i r o n spec imen h e l d a t an a n o d i c p o t e n t i a l o f + 1 .6 V i s g i v e n i n f i g u r e 4 . 8 . A f t e r 500 seconds c a t h o d i z a t i o n a t - 1 .6 V the spec imen i s a n o d i c a l l y e l ectro.1 y z e d a t 1 .6 V. The p o t e n t i o -s t a t i c b e h a v i o r can be d i v i d e d i n t o t h r e e r e g i o n s . In the f i r s t r e g i o n wh ich l a s t s f o r about 2 . 5 K sec the s u p e r f i c i a l 2 c u r r e n t d e n s i t y r i s e s f rom 1 1 . 5 t o 1 3 . 6 KA/m . The second r e g i o n , wh ich e x t e n d s up t o about 20 K s e c , i s e v i d e n t as a s low d e c l i n e i n s u p e r f i c i a l c u r r e n t d e n s i t y to about 9 . 5 2 KA/m . Reigaojiiothr'ee i s r e p r e s e n t e d by the e f f e c t i v e l y c o n -s t a n t s u p e r f i c i a l c u r r e n t d e n s i t y which i s p r e s e n t f o r the r e m a i n d e r o f the e x p e r i m e n t . 4 . 6 . 4 M a g n e t i t e Anodes 4 . 6 . 4 . 1 P o t e n t i o d y n a m i c S t u d i e s of M a g n e t i t e The p o t e n t i o d y n a m i c p l o t f o r m a g n e t i t e (see f i g u r e 4 . 9 ) i s o f a s i m i l a r shape to t h a t o b t a i n e d f o r i r o n . The F ig . 4.8 Potent iostat ic p lot for an iron anode @ + 1.6 V. 133 F ig . 4.9 Potentiodynamic p lot for a magnetite specimen. 134 i n i t i a l c o r r o s i o n p o t e n t i a l o f the immersed spec imen was - - 0 . 5 V. The e x p e r i m e n t was begun a t - 1 .6 V and thus the p a s s i v a t i o n p e a k s may r e s u l t f rom the i n i t i a l c a t h o d i z a -t i o n f o r m i n g a reduced l a y e r on the spec imen s u r f a c e . The r e g i o n o f p a s s i v a t i o n and the p o t e n t i a l o f oxygen e v o l u t i o n a r e s i m i l a r to t h o s e of the i r o n s p e c i m e n . The r a p i d d e v i a -t i o n f rom l i n e a r i t y of the l o g ^ l v e r s u s E p l o t i s d u e , most l i k e l y , to the lower c o n d u c t i v i t y o f the m a g n e t i t e . For the r e v e r s e p o t e n t i a l sweep, no s t r u c t u r e i s e v i d e n t i n the i n -c r e a s i n g c a t h o d i c c u r r e n t c u r v e p r i o r to the e v o l u t i o n of h y d r o g e n . 4 . 6 . 4 . 2 P o t e n t i o s t a t i c a n d C a t h o d i z a t i o n S t u d i e s of  M a g n e t i t e The e f f e c t s o f v a r i o u s p e r i o d s o f p o t e n t i o s t a t i c c a t h o d i z a t i o n @~1.6 V on the p o t e n t i o s t a t i c a n o d i c c u r r e n t d e n s i t i e s a t + 1 .2 V a re shown i n f i g u r e 4 . 1 0 . The c u r v e f o r z e r o c a t h o d i z a t i o n i s e f f e c t i v e l y c o n -s t a n t a t an a n o d i c c u r r e n t d e n s i t y of 9Q0 A/m . The e f f e c t of i n c r e a s i n g the p e r i o d o f c a t h o d i ' z a t i o n i s to i n c r e a s e the i n i t i a l a n o d i c c u r r e n t d e n s i t y . The a n o d i c c u r r e n t d e n s i t y decays to l e v e l s s i m i l a r to t h o s e found i n u n -c a t h o d i : z e d spec imens but the decay t ime and c o n s e q u e n t l y the q u a n t i t y of charge under the peak i n c r e a s e s w i t h i n c r e a s i n g p e r i o d s o f c a t h o d i z a t i o n . No f e r r a t e f o r m a t i o n was e v i d e n t 135 CM i < JSC u >^ -o .tz o c < CD Q 0 o c Q> o i_ 3 o o o -2 0 10 Cothodisat ion T i m e . H a 0 6V) b c 5 ksec 10 ksec 2 0 3 0 4 0 Time, seconds. ( x l O 3 ) Fig. 4.10 Magnetite anodes - the ef fects of periods of cathodization on the potent iostat ic anodic current dens i t ies . 136 f rom the u n c a t h o d i z e d m a g n e t i t e s p e c i m e n . A f t e r c a t h o d i z a -t i o n f e r r a t e i o n f o r m a t i o n was o b s e r v e d d u r i n g the a n o d i c e l e c t r o l y s i s o f the m a g n e t i t e s p e c i m e n . C a t h o d i z a t i o n r e s u l t s i n the r e d u c t i o n of the s u r f a c e of the m a g n e t i t e to l o w e r v a l e n c y i r o n o x i d e s o r m e t a l l i c i r o n . The amount of i r o n formed w i l l i n c r e a s e w i t h t i m e o f c a t h o d i z a t i o n but the e x a c t n a t u r e of the r e l a t i o n s h i p i s not known. There a r e a p p r o x i m a t e l y 122 and 208 coulombs o f charge per cm of anode under the a n o d i c peaks f o r 5 and 10 K seconds o f c a t h o d i z a t i o n r e s p e c t i v e l y . These q u a n t i t i e s of charge would be a s s o c i a t e d w i t h t h e o x i d a t i o n , t o m a g n e t i t e , o f m e t a l l i c i r o n l a y e r s 38 and 57 ym t h i c k r e s p e c t i v e l y . I t was not v e r i f i e d i f t h e s e l a y e r s were i n f a c t p r e s e n t , o r i f p a r t o f the a n o d i c c u r r e n t peaks were due to i n c r e a s e d oxygen e v o l u t i o n a t the reduced s u r f a c e . 137 C h a p t e r 5 DISCUSSION 5 .1 Review o f R e s u l t s The f o l l o w i n g r e s u l t s a re of s i g n i f i c a n c e i n u n d e r -s t a n d i n g the p r o c e s s e s wh ich a re o c c u r r i n g d u r i n g the e l e c t r o c h e m i c a l f o r m a t i o n o f f e r r a t e i o n s : 1) The d e c o m p o s i t i o n o f f e r r a t e s o l u t i o n s ii:s s t r o n g l y dependent on the t e m p e r a t u r e and h y d r o x y l i o n c o n -c e n t r a t i o n o f the e l e c t r o l y t e s o l u t i o n . Decom-p o s i t i o n i s r a p i d a t t e m p e r a t u r e s g r e a t e r than 50°C and a t h y d r o x y l i o n c o n c e n t r a t i o n s o f 5 g . m o l / 1 and l e s s . S o l i d p a r t i c l e s o f l o w e r o x i d a t i o n s t a t e i r o n o x i d e or h y d r o x i d e are c a t a l y t i c to the d e c o m p o s i -t i o n r e a c t i o n ( c h a p t e r 2 ) . 2) I n i t i a l l y l e s s than 3% o f the a n o d i c c u r r e n t i s a s s o c i a t e d w i t h the f o r m a t i o n o f f e r r a t e i o n s . The amount o f f e r r a t e formed was e r r a t i c d u r i n g the 2 f i r s t 5 K c o u l o m b s of charge passed per cm o f anode and then d e c l i n e d to e f f e c t i v e l y z e r o f e r r a t e f o r m a -2 t i o n a t about 2 0 - 2 5 Kcou lombs/cm anode ( s e c t i o n 3 . 8 . 1 . 1 ) . A few p e r c e n t of the c u r r e n t passed i s a s s o c i a t e d w i t h o x i d e (or h y d r o x i d e ) f o r m a t i o n on the anode . Most of the c u r r e n t (> 95%) i s a s s o c i a t e d w i t h oxygen e v o l u t i o n . The c u r r e n t e f f i c i e n c y o f f e r r a t e f o r m a t i o n has v e r y l i t t l e dependence ( i f a t a l l ) , on the s u p e r f i c i a l c u r r e n t d e n s i t y f o r c u r r e n t d e n s i t i e s between 5 and 60 KA/m ( s e c t i o n 3 . 8 . 1 . 4 ) . The t o t a l q u a n t i t y o f f e r r a t e produced appears to depend o n l y on the t o t a l charge passed up to f e r r a t e f o r m a t i o n c e s s a t i o n a t 2 0 - 2 5 K cou l ombs / cm 2 anode*, 4 0 0 - 5 0 0 x 1 0 " 6 g . m o l / c m 2 anode was t y p i c a l l y p roduced ( s e c t i o n 3 . 8 . 1 . 1 ) . The q u a n t i t i e s of i r o n removed f rom the anodes w i t h i n c r e a s i n g q u a n t i t i e s o f c h a r g e passed Were not w e l l d e f i n e d . In most cases the amount o f i r o n w i t h the s o l u t i o n hha'd . 1e£ csimi:llain•,• magni tude sto ,uari.'d' the r a t e of removal f o l l o w e d a s i m i l a r d e c l i n e , t o , t h a t o f f e r r a t e f o r m a t i o n . In the o t h e r c a s e s the t o t a l i r o n removed f rom the anode was l a r g e r than s u g g e s t e d by the above t r e n d ( s e c t i o n 3 . 8 . 1 . 2 ) . The amount o f i r o n removed f rom the anode appears to be d i r e c t l y p r o p o r t i o n a l to the h y d r o x y l i o n c o n -c e n t r a t i o n f o r c o n c e n t r a t i o n s between 1 and 1 4 . 3 g . m o l / 1 NaOH. The amount of f e r r a t e produced appears to show a s i m i l a r dependence ( s e c t i o n 3 . 8 . 2 ) . The v i s u a l o b s e r v a t i o n s s u g g e s t t h a t d e c r e a s i n g the h y d r o x y l i o n c o n c e n t r a t i o n decreases , the r a t e o f anode d i s s o l u t i o n and/or s p a l l i n g and p o s s i b l y a l s o the r a t e o f s c a l e growth [ s e c t i o n 3 . 9 . 1 ) . P e r i o d i c c u r r e n t r e v e r s a l r e s t o r e s the r a t e o f f e r -r a t e p r o d u c t i o n to the i n i t i a l l e v e l s (.1-3%) and a l l o w s c o n t i n u o u s f e r r a t e p r o d u c t i o n ( s e c t i o n 3 . 8 . 4 ) . The o x i d e o r h y d r o x i d e l a y e r s wh ich formed on the anode c o u l d not be i d e n t i f i e d . The i n i t i a l l a y e r i s r e l a t i v e l y smooth, and c o h e r e n t but has a f i n e , h i g h s u r f a c e a r e a , n o d u l a r m i c r o s t r u c t u r e . Uneven d i s -s o l u t i o n and p o s s i b l y s p a l l i n g , o f the i n i t i a l l a y e r 2 was e v i d e n t a f t e r about 5 Kcou lombs/cm a n o d e ; t h i s l a y e r was g r a d u a l l y removed f rom the anode d u r i n g the nex t 1 5 - 2 5 K coulombs/cnf anode . D u r i n g t h i s p e r i o d the o r i g i n a l l a y e r i s d u p l e x i n n a t u r e and t h i c k e n s t o 3 0 - 5 0 ym. A f t e r t h i s p e r i o d the anode i s c o v e r e d w i t h a rugged g r a n u l a r o x i d e (o r h y d r o x i d e ) l a y e r about 5 - 1 0 ym t h i c k , wh ich may have been n u c l e a t e d below the o r i g i n a l anode l a y e r , p o s s i b l y o r i g i n a t i n g f rom the l o w e r d u p l e x l a y e r ( s e c t i o n 3 . 9 ) . From p o t e n t i o d y n a m i c s t u d i e s o f i r o n no major d i f -f e r e n c e s between oxygen e v o l u t i o n f rom newly p a s -s i v a t e d and e x t e n s i v e l y a n o d i z e d i r o n spec imens were d e t e c t e d ( s e c t i o n 4 . 6 . 1 ) . The c o n s t a n t c u r r e n t and p o t e n t i o s t a t i c s t u d i e s of i r o n i n d i c a t e d t h a t the anode b e h a v i o u r c o u l d be 140 d i v i d e d i n t o t h r e e r e g i o n s . The i n i t i a l r e g i o n p (Reg ion I) e x t e n d s f o r about the f i r s t 5 Kcou lombs/cm anode and i s c h a r a c t e r i z e d by the i n c r e a s i n g ease w i t h wh ich some o r a l l the e l e c t r o c h e m i c a l p r o c e s s e s a r e o c c u r r i n g . T h i s was e v i d e n t as a d e c l i n e i n anode p o t e n t i a l f o r c o n s t a n t c u r r e n t e x p e r i m e n t s and an i n c r e a s e i n s u p e r f i c i a l c u r r e n t d e n s i t y f o r p o t e n t i o s t a t i c e x p e r i m e n t s . Reg ion I I , wh ich e x t e n d s 2 f rom the end o f Reg ion I to 15 to 20 Kcou lombs/cm a n o d e , was e v i d e n t as a g r a d u a l i n c r e a s e i n anode p o t e n t i a l i n c o n s t a n t c u r r e n t e x p e r i m e n t s and a g r a d u a l d e c l i n e i n the s u p e r f i c i a l c u r r e n t d e n s i t y i n p o t e n t i o s t a t i c e x p e r i m e n t s . Reg ion I I I i s o b s e r v e d as a p p r o x i m a t e l y s t a b l e anode b e h a v i o u r . For c o n s t a n t c u r r e n t e x p e r i m e n t s a s l i g h t d e c l i n e i n anode p o t e n t i a l 2 o v e r a p e r i o d o f a few K' coulombs/cm anode was sometimes o b s e r v e d p r i o r to s t a b i l i z a t i o n ( s e c t i o n s 4 . 6 . 2 and 4 . 6 . 3 ) . 10) The r e s u l t s o f the e x p e r i m e n t s u s i n g m a g n e t i t e anodes ( s e c t i o n s 3 . 8 . 5 , 3 . 9 . 2 and 4 . 6 . 4 ) i n d i c a t e t h a t , f o r u n t r e a t e d a n o d e s , i r o n d i s s o l u t i o n i s low and f e r r a t e f o r m a t i o n was not o b s e r v e d . The amount o f d i s s o l u t i o n a p p e a r s to depend on the t ype o f m a g n e t i t e g r a i n i n v o l v e d , p o s s i b l y due to c r y s t a l 1 o g r a p h i c o r i e n t a -t i o n or c h e m i c a l c o m p o s i t i o n . R a d i c a l e l e c t r o -c h e m i c a l b e h a v i o u r d i f f e r e n c e s between i r o n and / 141 m a g n e t i t e anodes were not o b s e r v e d . C a t h o d i c r e d u c -t i o n o f t h e m a g n e t i t e s u r f a c e r e s u l t e d i n f e r r a t e f o r m a t i o n and i n c r e a s e d a n o d i c d i s s o l u t i o n . 5 . 2 The Mechanism of F e r r a t e F o r m a t i o n The p r e s e n c e o f a s u b s t a n t i a l o x i d e (o r h y d r a t e d o x i d e ) l a y e r on the anode d u r i n g the p e r i o d o f f e r r a t e f o r m a t i o n s u g g e s t s t h a t t h i s l a y e r i s i n some way r e l a t e d to the f e r -r a t e f o r m a t i o n mechanism. The e x p e r i m e n t a l anode p o t e n t i a l s a r e t h e r m o d y n a m i c a l l y s u f f i c i e n t (> 0 . 6 V . S H E . ) to o x i d i z e the anode l a y e r ( i r o n o x i d e s or h y d r a t e o x i d e s ) to f e r r a t e i o n s . 1 0 S e v e r a l a u t h o r s ^ ' ^ have s u g g e s t e d t h a t m e t a l s such as ch romium, vanadium and manganese, whose h i g h e s t o x i d a t i o n s t a t e s i n a l k a l i n e s o l u t i o n s a r e s o l u b l e , a r e e a s i l y p a s s i v a t e d a t medium a n o d i c p o t e n t i a l s but a t more p o s i t i v e ( o x i d i z i n g ) p o t e n t i a l s the l o w e r o x i d e s d i s s o l v e as s o l u b l e o x y a n i o m s . A l t h o u g h t h i s may be t r u e the p r e s e n c e o f an anode l a y e r o f i n t e r m e d i a t e o x i d a t i o n s t a t e o x i d e s i n the i r o n - s o d i u m h y d r o x i d e sys tem i s not i m p o s s i b l e . I f the f o r m a t i o n o f f e r r a t e i o n s f rom m e t a l l i c i r o n i s k i n e t i c a l l y s l o w e r than the f o r m a t i o n r a t e o f the t h e r m o d y n a m i c a l l y l e s s s t a b l e l o w e r o x i d e s o f i r o n , s u r f a c e o x i d e growth w i l l o c c u r . Once formed on the a n o d e , the o x i d a t i o n o f t h e s e l o w e r o x i d a t i o n s t a t e i r o n s p e c i e s w i l l be i m p o r t a n t . The b u i l d up o f an 142 o x i d e (o r h y d r o x i d e ) f i l m w i l l o c c u r i f the r a t e o f o x i d e (o r h y d r o x i d e ) f o r m a t i o n exceeds t h a t o f o x i d e (o r h y d r o -x ide ) d i s s o l u t i o n . I t i s p o s s i b l e t h a t t h e anode l a y e r formed has a h i g h degree of m i c r o p o r o s i t y . The n o d u l a r h i g h s u r f a c e a r e a m i c r o s t r u c t u r e o f the anode s u r f a c e (see f i g u r e 3 . 1 9 a and o b) c o u l d o b s c u r e m i c r o p o r o s i t y ( < 100 A) i n t h e upper few m i c r o n s o f the amorphous anode l a y e r . The h i g h c o n d u c t i v i t y and the r e l a t i v e l y f a s t growth o f the anode l a y e r a r e c o n -s i s t e n t w i t h a porous s t r u c t u r e . Porous anode f i l m s have been o b s e r v e d , p r i m a r i l y i n 5 8 sys tems where p a r t i a l d i s s o l u t i o n by the e l e c t r o l y t e o c c u r s . The pores i n amorphous anode f i l m s formed on a l u m i n i u m a n o d i z e d i n weak a c i d s a r e g e n e r a l l y v e r y r e g u l a r , w i t h ° 9 11 d i a m e t e r s o f l e s s than 350 A and d e n s i t i e s o f 10 to 10 2 pores/cm . The pore d i a m e t e r and d e n s i t i e s were dependent on both the s o l u t i o n c o m p o s i t i o n and the f o r m i n g v o l t a g e ( 2 0 - 1 2 0 V ) . The porous f i l m s were o b s e r v e d t o c o n s i s t o f a porous o u t e r l a y e r up to s e v e r a l m i c r o n s t h i c k , and an impermeable base l a y e r up to 0 . 2 ym t h i c k . I t i s b e l i e v e d p o r o s i t y o c c u r s by uneven d i s s o l u t i o n o f the i n i t i a l b a r -r i e r l a y e r accompanied by i n c r e a s e d c u r r e n t d e n s i t i e s a t the t h i n n e d r e g i o n s . I t has a l s o been s u g g e s t e d t h a t h e a t i n g 143 i n the pores f u r t h e r a i d s the p r o c e s s . P o r o s i t y has a l s o been o b s e r v e d i n t i n , z i n c , cadmium and magnes ium.anode f i l m s . The r a t e o f f e r r a t e f o r m a t i o n w i l l depend on both the k i n e t i c s o f the f e r r a t e f o r m a t i o n r e a c t i o n and t h e a r e a o r number o f s i t e s a t wh ich f e r r a t e f o r m a t i o n i s o c c u r r i n g . The a b i l i t y to t r a n s f e r an i r o n s p e c i e s i n the anode l a y e r to the s o l u t i o n as a s t a b l e f e r r a t e i o n w i l l depend on the r e l a t i v e s t a b i l i t y o f the i r o n s p e c i e s a t the two s i t e s and the energy b a r r i e r t o t h e t r a n s f e r e n c e . The i r o n i o n s a t r e l a t i v e l y h i g h energy s i t e s such as a t o m i c s t e p s , g r a i n b o u n d a r i e s , p o i n t d e f e c t s , i n t e r -s t i t i a l s e t c . s h o u l d be o x i d i z e d p r e f e r e n t i a l l y o v e r i r o n 59 atoms i n the s t a b l e o x i d e l a t t i c e . F e r r a t e f o r m a t i o n f rom a m e t a s t a b l e o x i d e l a t t i c e s h o u l d be e n e r g e t i c a l l y e a s i e r than f rom a s t a b l e o x i d e l a t t i c e . P o r e s , i f they e x i s t e d , wou ld a l s o be s i t e s o f h i g h e r energy i r o n atoms and thus c o u l d be i m p o r t a n t to f e r r a t e f o r m a t i o n . Changes i n the c o n d i t i o n s (such as c u r r e n t d e n s i t y , t e m p e r a t u r e and [ O H - ] ) i n the p o r e s may a l s o promote f e r r a t e f o r m a t i o n . A s u f f i c i e n t number o f the r e q u i r e d oxygen s p e c i e s must be a v a i l a b l e a t a s u i t a b l e s i t e , f o r f e r r a t e f o r m a t i o n to o c c u r . I t i s p r o b a b l e t h a t the a c t i v e oxygen s p e c i e s i s an i n t e r m e d i a t e i n the oxygen e v o l u t i o n r e a c t i o n . A s c h e m a t i c d i a g r a m o f the p o s s i b l e r e l a t i o n s h i p between t h e oxygen and f e r r a t e f o r m a t i o n r e a c t i o n s i s g i v e n i n f i g u r e 5 . 1 . A d i r e c t dependence of the f e r r a t e f o r m a t i o n r e a c t i o n on the oxygen e v o l u t i o n r e a c t i o n i s c o n s i s t e n t w i t h t h e o b s e r v e d dependence o f f e r r a t e f o r m a t i o n on the s u p e r f i c i a l c u r r e n t d e n s i t y . I t i s p o s s i b l e t h a t monoatomic oxygen i s the i n t e r m e d i a t e species in the oxygen e v o l u t i o n r e a c t i o n a n d , c o n -s e q u e n t l y , the a c t i v e oxygen s p e c i e s i n the f e r r a t e f o r m a -t i o n r e a c t i o n . Thus f o r the f e r r a t e f o r m a t i o n r e a c t i o n , i t c o u l d be t h a t on a s t a t i s t i c a l l y d e t e r m i n e d number o f i r o n s i t e s s u f f i c i e n t oxygen c o n c e n t r a t i o n e x i s t s f o r f e r r a t e f o r m a t i o n , i n s t e a d o f oxygen e v o l u t i o n , to o c c u r . A l t e r n a t i v e l y a t t r a c t i v e f o r c e s between a h i g h e r energy i r o n s i t e and an i n t e r m e d i a t e oxygen s p e c i e s may s t a b i l i z e i t l o n g enough f o r a d d i t i o n a l oxygen c a p t u r e and f e r r a t e f o r m a t i o n t o o c c u r . I t i s f e a s i b l e t h a t the a c t i v e oxygen s p e c i e s c o u l d fi n be some o t h e r s p e c i e s ( such as the p e r o x y l i o n ) w h i c h i s g e n e r a t e d as an. i n t e r m e d i a t e i n , o r c o n c u r r e n t l y w i t h , the oxygen e v o l u t i o n r e a c t i o n . W h i c h e v e r oxygen s p e c i e s i s i n v o l v e d i n the f e r r a t e f o r m a t i o n r e a c t i o n , the uneven a t t a c k of the anode l a y e r may be due to h i g h e r c u r r e n t d e n s i t i e s , o c c u r r i n g at^spec/i f i c Fe FeO, Electrolyte 'OH"' ( f re) —^^OH"( i f involved) Fe^M>FeOi" K FeO, H O FeO, Overall Reactions 4 OH"- 0 2 + 2H20 + 4 e" Fe + 8 0 H ~ - FeO^" + 4 H 2 0 + 6 e* Fe + 2xOH~ - FeO x + xH 20 + 2x e" '02 (Fe) K F e 0 2 " KFeO x P e02 *e~Fe = Rate of oxygen formation = Rate of oxygen supply to the iron react ions. = Rate of fer rate formation. = Rate of oxide (or hydroxide) formation. = Flux of electrons from oxygen ox idat ion . = Flux of electrons from i ron ox idat ion . = Act ive oxygen species on anode surface. Fe* = Act ive i ron species on the anode surface. F i g . 5.1 A schematic diagram of the possible re la t ionsh ip between oxygen evolut ion and fe r ra te formation. 146 s i t e s . The s t r o n g dependence of the f e r r a t e f o r m a t i o n and d e c o m p o s i t i o n r e a c t i o n s on the h y d r o x y l i o n c o n c e n t r a t i o n s u g g e s t s t h a t the s t a b i l i t y o f the f e r r a t e i o n i n s o l u -t i o n i s o f g r e a t i m p o r t a n c e to the k i n e t i c s of the f e r -r a t e f o r m a t i o n r e a c t i o n . U n l e s s the f e r r a t e i o n i s s t a b i l i z e d i n the s o l u t i o n , s u f f i c i e n t l y to escape f rom the a n o d e , f e r r a t e f o r m a t i o n w i l l not be o b s e r v e d to o c c u r . As the h y d r o x y l i o n c o n c e n t r a t i o n i s i n c r e a s e d , the number of o c c u r r e n c e s where a f o r m i n g f e r r a t e i o n i s s t a b i l i z e d w i l l be i n c r e a s e d . The r a t e d e t e r m i n i n g s t e p f o r the f e r r a t e f o r m a t i o n r e a c t i o n c o u l d be the a v a i l a b i l i t y o f the i r o n and/or oxygen s p e c i e s . The q u a n t i t y of f e r r a t e p roduced may depend on the number of o c c u r r e n c e s where the f e r r a t e i o n formed i s s t a b i l i z e d i n s o l u t i o n , and/or the number o f s i t e s a t wh ich i t i s b e i n g p r o d u c e d . 5 . 2 . 1 The Ro le of the Anode L a y e r i n the F e r r a t e F o r m a t i o n  Mechanism The anode l a y e r c o u l d i n t e r a c t i n the f e r r a t e f o r m a -t i o n mechanism i n a number o f w a y s . : 1) the i r o n o x i d e (o r h y d r o x i d e ) c o u l d be d i r e c t l y o x i d i z e d to f e r r a t e i o n s (see e q u a t i o n 5 .1 and f i g u r e 5 . 2 a ) . FeOv + (8-2x) OH" = FeO 2 : M + (4-x) H ?0 + (6-2x) e" - 5 . 1 x 0xide H a q l L 2) the anode s c a l e c o u l d be a l a y e r t h r o u g h which i r o n i o n s d i f f u s e and m i g r a t e to be o x i d i z e d to f e r r a t e a t the o x i d e (o r h y d r o x i d e ) s u r f a c e (see e q u a t i o n 5 . 2 and f i g u r e 5 . 2 b ) F e 0 x i d e + 8 0 H ~ * F e 0 4 ( a q ) + 4 ¥ + ^ e " ~ 5 ' 2 3) i f m i c r o p o r o s i t y e x i s t s , the f e r r a t e f o r m a t i o n c o u l d o c c u r w i t h i n the pores and the b u l k oxygen f o r m a t i o n c o u l d o c c u r on the s u r f a c e o f t h e anode l a y e r (see f i g u r e 5 . 2 c ) . P r e f e r e n t i a l f e r r a t e f o r m a t i o n w i t h i n the pores c o u l d be due to the c h e m i c a l and p h y s i c a l e n v i r o n m e n t , w i t h i n the p o r e s . i n a n anode l a y e r ( o p e r a t i n g a t h i g h c u r r e n t d e n s i t i e s ) b e i n g r a d i c a l l y d i f f e r e n t f rom the b u l k s o l u t i o n . F e r r a t e f o r m a t i o n w i t h i n the pores c o u l d o c c u r by e i t h e r o f the two p r e v i o u s l y m e n t i o n e d anode l a y e r i n t e r a c t i o n s i n the f e r -r a t e f o r m a t i o n mechanisms o r , more i m p r o b a b l y , by a mechanism 12 i n v o l v i n g d i s s o l v e d i r o n i n t e r m e d i a t e s p e c i e s . Tousek has s u g g e s t e d f e r r a t e f o r m a t i o n o c c u r s by the o x i d a t i o n of 2 - 42 F e 9 0 . i o n s p roduced i n the p o r e s . McKubre d e t e c t e d Fe a) E l e c t r o l y t e /OH" (if involved ) ^FeO| -0vera11 fer rate formation react ion : FeOx + (8-2X)0H" = FeO^" (4-x)H 2 0 +(6-2x) e" Fe b) F e O y pFe y ne* Electrolyte CT .OH" (if involved) > F e O £ Fe _ J X FeO„ =*H20 Overall fer rate formation react ion : F e y + + 80H" = FeO2" + 4H20 + (6-y) e" 148 Fe E lectro lyte O H -0 species possibly involved in ferrote formotion Fe* = Iron atom taking part in the react ion . 0* = Oxygen species taking part in the react ion . F e y + = A migrating iron ion within the anode layer . m, n, p, q , r. s . •= fluxes of electrons and iron species across the anode layer . F ig .5 .2 A schematic diagram of the possible interact ions between the anode layer and the fer rate formation mechanism. a) Direct anode layer ox idat ion . b) The oxidation of iron species d i f fus ing and migrating through the anode layer c) Ferrate formation in pores. 149 d i s s o l v e d i r o n i n t e r m e d i a t e s ( s u g g e s t e d s p e c i e s Fe(OH)^) d u r i n g t h e f o r m a t i o n o f p a s s i v e f i l m s . A g e n e r a l i z e d r e a c t i o n sequence f o r f e r r a t e f o r m a t i o n by such a mechanism i s g i v e n i n e q u a t i o n s 5 . 3 a and b. F e o x i d e + 2 x 0 H " = F e 0 x " + x H 2 ° + m e " " 5 ' 3 a pore x •-•« 4 m = 0 ( c h e m i c a l r e a c t i o n ) m > 1 ( e l e c t r o c h e m i c a l r e a c t i o n ) FeOj." + (8-2x)0H~ = FeO?/ • x + (4-x)H 9 0 + ne" -5 .3b xp o r e ->vaq; £ I ron s p e c i e s c o u l d be s o l u b l i z e d as t r i v a l e n t i r o n s p e c i e s w i t h i n t h e p o r e , wh ich c o u l d then be f u r t h e r o x i d i z e d to f e r r a t e a t the pore mouth . I f f e r r a t e f o r m a t i o n o c c u r s by d i r e c t o x i d a t i o n o f the anode s u r f a c e the i n v o l v e m e n t o f i n t e r m e d i a t e v a l e n c y s o l u b l e i r o n s p e c i e s i s i m p r o b a b l e . The e l e c t r o c h e m i c a l o x i d a t i o n o f such an i n t e r m e d i a t e , i f i t e x i s t e d , would not fi l be o b s e r v a b l e due to the F r a n k - C o n d o n P r i n c i p l e which s t a t e s t h a t e l e c t r o n movements a r e s u f f i c i e n t l y more r a p i d than i o n i c movements such t h a t on the t ime s c a l e o f an e l e c t r o n i c o c c u r r e n c e the i o n s can be c o n s i d e r e d s t a t i o n a r y . Thus f o r the o x i d a t i o n of a l o w e r o x i d a t i o n s t a t e i o n i r o n c h e m i c a l l y bonded to the a n o d e , to a f e r r a t e i o n , the 150 i o n , : w i l l be o b s e r v e d to e n t e r s o l u t i o n as an i r o n (V I ) ; s p e c i e s , as the e l e c t r o c h e m i c a l o x i d a t i o n o f any i n t e r m e d i a t e v a l e n c y i r o n s p e c i e s wou ld be too r a p i d t o be o b s e r v e d . Chemica l o x i d a t i o n ^of., s o l u b l e l o w e r o x i d a t i o n s t a t e i r o n s p e c i e s to f e r r a t e i o n s c o u l d o n l y o c c u r at the anode s u r f a c e (and thus be u n o b s e r v a b l e ) because the c h e m i c a l c o n d i t i o n s o u t s i d e the r e g i o n of i n f l u e n c e o f the anode a r e not s u f f i c i e n t l y o x i d i z i n g . The t o t a l f e r r a t e p roduced c o u l d be due to one o r more o f t h e t h r e e s u g g e s t e d anode l a y e r i n t e r a c t i o n s i n the f e r r a t e f o r m a t i o n mechanism. The q u a n t i t i e s o f i r o n a s s o c i -a t e d w i t h the i n i t i a l anode l a y e r (see page 102)which i s o b s e r v e d -.oto . d e t a c h : u f r o m : ; t'hei anode l a y e r i s a s i g n i f i -c a n t p r o p o r t i o n o f the t o t a l f e r r a t e and t o t a l i r o n d e t e c t e d i n the e x p e r i m e n t s . T h i s s u g g e s t s a l a r g e p r o p o r t i o n of the d e t a c h e d anode l a y e r i s o x i d i z e d to f e r r a t e as opposed to s p a l l e d o f f . The e l e c t r o n m i c r o g r a p h s s u g g e s t s t h i s may o c c u r by a m a c r o s c o p i c p i t t i n g mechanism a t s e l e c t i v e s i t e s . U n i f o r m d i s s o l u t i o n by a s i m i l a r mechanism as the anode l a y e r t h i c k e n s c o u l d acc:o.un.t f o r the r e m a i n d e r o f the f e r r a t e f o r m e d . Thus the r e s u l t s s u g g e s t d i r e c t d i s s o l u -t i o n o f the anode l a y e r p roduces a s u b s t a n t i a l p r o p o r t i o n , i f no t a l l , o f the f e r r a t e o b s e r v e d . A p r o p o r t i o n of the f e r r a t e i o n s c o u l d form by the o x i d a t i o n o f i r o n s p e c i e s wh ich have m i g r a t e d t h r o u g h the anode l a y e r . I t i s d i f f i c u l t to see how a s i g n i f i c a n t - 9 - 7 2 f l u x (10 - 10 g .mols/cm . s e c ) o f i o n s c o u l d o c c u r a c r o s s a r e l a t i v e l y t h i c k (> 10 ym) anode l a y e r . A s i m p l i s t i c e q u a t i o n f o r the f l u x of i r o n s p e c i e s a t any p o i n t a c r o s s an i n e r t l a y e r by c h e m i c a l d i f f u s i o n and e l e c t r o n i c m i g r a -t i o n i s g i v e n by e q u a t i o n 5 . 4 : D x + c Z F ^ - . D dc F e x + . Fe x Fe x Fe x d* _ 5 : 4 Fe x Fe x dx RT dx J x + F l u x of i r o n i o n s f rom the m e t a l l i c i r o n to Fe' - 2 - 1 the s o l u t i o n i n t e r f a c e g . m o l . m s x+ 2 - 1 D . - D i f f u s i v i t y o f the s p e c i e s Fe m s Fe x X+ - "3 c , C o n c e n t r a t i o n o f s p e c i e s Fe g . m o l . m Fe x x = P o s i t i o n a c r o s s an anode l a y e r t h i c k n e s s X.m P o t e n t i a l g r a d i e n t a c r o s s t h e anode l a y e r 1 d§ dx .V m The anode l a y e r i s o b s e r v e d to be r e l a t i v e l y c o n d u c t i v e ( t h e p o t e n t i a l g r a d i e n t a c r o s s the anode l a y e r i s p o s s i b l y i n t h e o r d e r o f 1 0 4 V/m (see page ;157)) and thus the d r i v i n g f o r c e f o r i o n i c m i g r a t i o n s h o u l d be s m a l l . The d i f f u s i v i t y of p o i n t d e f e c t oxygen t h r o u g h a p a s s i v e l a y e r o f ^ e2®3 152 (or Fe^O^) a t room t e m p e r a t u r e was c a l c u l a t e d by Chao e t a l . - 3 0 2 to be 10 m / s . The d i f f u s i v i t y o f the i r o n s p e c i e s would have t o be 1 0 - 2 0 o r d e r s o f magni tude g r e a t e r t o p roduce a s i g n i f i c a n t f l u x o f i r o n i o n s . I t i s p o s s i b l e t h e d i f -f u s i n g i r o n s p e c i e s c o u l d be i n t e r s t i t i a l s formed a t the m e t a l - o x i d e i n t e r f a c e but i t s t i l l seems i m p r o b a b l e the f l u x of i o n s wou ld be s i g n i f i c a n t . However , c o n s i d e r a b l e q u a n t i -t i e s o f o x i d e o r h y d r o x i d e are a s s o c i a t e d w i t h the anode l a y e r and t h i s must f o r m , t o some e x t e n t , by s o l i d - s t a t e d i f f u s i o n . A mechanism i n v o l v i n g i o n i c d i f f u s i o n / m i g r a t i o n c o u l d o c c u r i n c o n j u n c t i o n w i t h m i c r o p o r o s i t y . Very t h i n o x i d e o r h y d r o x i d e f i l m s c o u l d be p r e s e n t a t the base o f the pores t h u s a l l o w i n g a s i g n i f i c a n t i o n i c f l u x f o r f e r r a t e f o r m a t i o n . The d i f f u s i o n o f f e r r a t e i o n s out o f the pore c o u l d be i m p o r t a n t to the r a t e o f f e r r a t e f o r m a t i o n by t h i s mechan ism. I f pores e x i s t t h i s mechanism c o u l d however , be r e s p o n s i b l e f o r a s i g n i f i c a n t p r o p o r t i o n of the f e r r a t e p r o d u c e d . A l t e r n a t i v e l y m i c r o p o r o s i t y c o u l d enhance the d i r e c t o x i d a t i v e d i s s o l u t i o n o f t h e anode l a y e r p o s s i b l y by v a r i a t i o n s i n the c h e m i c a l or p h y s i c a l p r o p e r t i e s of the pore e l e c t r o l y t e . However , as the anode i s u n e v e n l y a t t a c k e d and the m i c r o s t r u e t u r e o f the i n i t i a l l a y e r a p p e a r s s i m i l a r a t both the s i t e s o f r a p i d a t t a c k and the r e l a t i v e l y 153 u n a t t a c k e d a r e a s , such a mechanism appears l e s s i m p o r t a n t than d i r e c t o x i d a t i o n o f the anode l a y e r a t p r e f e r r e d s i t e s . 5 . 3 The D e c l i n e o f the E f f i c i e n c y o f F e r r a t e F o r m a t i o n w i t h I n c r e a s i n g Q u a n t i t i e s o f Charge Passed From the e x p e r i m e n t a l r e s u l t s i t i s a p p a r e n t t h a t the f o r m a t i o n o f f e r r a t e o c c u r s d u r i n g the p e r i o d where the measured anode p a r a m e t e r s a r e t r a n s i e n t . Tii'Thr'ee . ; r e g i o n s of anode b e h a v i o u r a re e v i d e n t i n the f e r r a t e f o r m a t i o n e x p e r i m e n t a l r e s u l t s . Reg ion I wh ich e x t e n d s f o r about 2 5 Kcoulorr ibs/cm and i s c h a r a c t e r i z e d by the i n c r e a s e d ease w i t h which some, or a l l , the e l e c t r o c h e m i c a l p r o c e s s e s a r e o c c u r r i n g i s e v i d e n t as a p e r i o d i n wh ich e r r a t i c q u a n t i t i e s of f e r r a t e a r e p r o d u c e d . Reg ion I I wh ich e x t e n d s 2 f rom R e g i o n I to about 20 K c o u l ombs/cm arid,, i s e v i d e n t as a g r a d u a l i n c r e a s e i n t h e measured anode p o t e n t i a l f o r the c o n s t a n t c u r r e n t e x p e r i m e n t , a p p r o x i m a t e l y c o r r e s p o n d s to the r e g i o n where t h e r a t e of f e r r a t e f o r m a t i o n d e c l i n e s s t e a d i l y to e f f e c t i v e l y z e r o . R e g i o n I I I wh ich c o r r e s p o n d s to g r e a t e r q u a n t i t i e s o f charge passed and i s e v i d e n t as a r e g i o n of a p p r o x i m a t e l y c o n s t a n t anode p o t e n t i a l ( a l t h o u g h a s l i g h t d e c l i n e i n anode p o t e n t i a l may i n i t i a l l y be o b s e r v e d ) c o r -r e s p o n d s to the p e r i o d where the r a t e o f f e r r a t e f o r m a t i o n and anode d i s s o l u t i o n i s l o w . The v i s u a l o b s e r v a t i o n s o f the s c a l e morpho logy c o r r e l a t e w i t h the t h r e e d i s t i n c t p e r i o d s o f anode b e h a v i o u r . Reg ion I c o r r e s p o n d s to the p e r i o d where the i n i t i a l l y formed anode l a y e r i s c o h e r e n t and r e l a t i v e l y u n a t t a c k e d . Reg ion II c o r r e s p o n d s t o the p e r i o d where g e n e r a l anode l a y e r d i s s o l u t i o n and p o s s i b l y s p a l l i n g i s o c c u r r i n g . Reg ion I I I c o r r e s p o n d s to the p e r i o d where the anode i s m o s t l y c o v e r e d w i t h an u l t i m a t e , r e l a t i v e l y u n r e a c t i v e anode l a y e r . I t i s p o s s i b l e the s u r f a c e a r e a i n c r e a s e s o b s e r v e d to o c c u r w i t h the growth of the i n i t i a l anode l a y e r c o n -t r i b u t e t o the d e c l i n e i n anode p o t e n t i a l e v i d e n t as Reg ion I. The d e c l i n e i n p o t e n t i a l may a l s o be the r e s u l t o f the i n i t i -a l l y formed anode l a y e r t r a n s f o r m i n g , d u r i n g t h i s p e r i o d , i n t o a more s t a b l e o x i d e (o r h y d r o x i d e ) l a y e r which i s G a t a l y t i c a l l y a b e t t e r s u r f a c e f o r oxygen e v o l u t i o n . At h i g h anode p o t e n t i a l s the i n i t i a l anode l a y e r w i l l be formed v e r y r a p i d l y on a l l a v a i l a b l e s i t e s and thus i s p r o b a b l y not the most s t a b l e f o r m . The d r i v i n g f o r c e f o r the r e -s t r u c t u r i n g o f the anode l a y e r , p o s s i b l y by n u c l e a t i o n and g r o w t h , wou ld e x i s t and the r e d u c t i o n i n oxygen o v e r -p o t e n t i a l c o u l d o c c u r . The e r r a t i c f e r r a t e p r o d u c t i o n measurements o b s e r v e d d u r i n g t h i s p e r i o d a re not i n c o n -s i s t e n t w i t h random anode l a y e r r e s t r u c t u r i n g , however g r e a t e r q u a n t i t i e s of f e r r a t e ;raay be e x p e c t e d f rom the i n i t i a l m e t a s t a b l e anode l a y e r . 155 The f o r m a t i o n of m i c r o p o r o s i t y , i f i t e x i s t s , would a l s o o c c u r i n Reg ion I, p o s s i b l y c o n c u r r e n t l y w i t h a r e s t r u c t u r i n g of the anode l a y e r . I f m i c r o p o r o s i t y i s b e n e f i c i a l t o f e r r a t e f o r m a t i o n e r r a t i c f e r r a t e f o r m a t i o n r e s u l t s c o u l d o c c u r d u r i n g the e s t a b l i s h m e n t of a porous s t r u c t u r e . M i c r o p o r o s i t y would a l s o be c o n s i s t e n t w i t h the growth o f a r e l a t i v e l y t h i c k (up to 15 ym) anode l a y e r d u r i n g Reg ion I, w i t h o u t a l a r g e a s s o c i a t e d anode l a y e r r e s i s t a n c e component . A l t e r n a t i v e l y the d e c l i n e i n anode p o t e n t i a l o b s e r v e d d u r i n g Reg ion I may be o b s c u r i n g an i n c r e a s i n g p o t e n t i a l component a s s o c i a t e d w i t h the o x i d e (o r h y d r o x i d e ) l a y e r g r o w t h . The g r a d u a l i n c r e a s e i n anode p o t e n t i a l c o r r e s p o n d i n g to Reg ion II i n the c o n s t a n t c u r r e n t e x p e r i m e n t s i s c o n -s i s t e n t w i t h an i n c r e a s e d r e s i s t i v i t y , and c o n s e q u e n t l y v o l t a g e component* o f the anode l a y e r . The o b s e r v e d b r e a k d o w n , and p o s s i b l y h i g h e r c o n d u c t i v i t y , o f the i n i t i a l anode l a y e r s u g g e s t s t h a t i t i s the growth of an i n n e r l a y e r wh ich r e s u l t s i n the i n c r e a s e d a n o d i c p o t e n t i a l . I t i s p o s s i b l e t h i s i n n e r l a y e r c o r r e s p o n d s to the f i n a l anode l a y e r w h i c h may. d e v e l o p f r o m , o r b e l o w , the l o w e r l a y e r o f the i n i t i a l l y o b s e r v e d d u p l e x s c a l e . I n c r e a s e s i n the anode p o t e n t i a l due t o the i n c r e a s e d oxygen c u r r e n t d e n s i t y r e s u l t i n g f rom the r e d u c t i o n i n the f e r r a t e f o r m a t i o n r e a c t o n w i l l be i n s i g n i f i c a n t . I t seems i m p r o b a b l e . . t h a t the i n c r e a s e i n the measured p o t e n t i a l i s due t o i n c r e a s e d oxygen e v o l u t i o n o v e r p o t e n t i a l s because the p o t e n t i o d y n a m i c r e s u l t s s u g g e s t the p r e s e n c e o f the anode l a y e r does not s i g n i f i c a n t l y a l t e r the k i n e t i c s o f the oxygen e v o l u t i o n r e a c t i o n . N e g l e c t i n g the o b s e r v e d uneven d i s s o l u t i o n and p o s s i b l y s p a l l i n g of the i n i t i a l anode l a y e r an a p p r o x i m a t e c a l c u l a t i o n of the p o t e n t i a l g r a d i e n t a c r o s s the s c a l e f rom the p o t e n t i a l measurements o f Reg ion I I f o r the c o n s t a n t p c u r r e n t e x p e r i m e n t s a t 10 KA/m (see f i g u r e 4 . 6 ) can be done i f one assumes t y p i c a l s c a l e t h i c k n e s s e s (see s e c t i o n 3 . 9 ) . At 5 K cou lomb/cm 2 V •- 1 . 2 4 V o l t s T h i c k n e s s = x = 1 5 x 1 0 " m At 2 0 K c o u l o m b / c m 2 V = 1 . 4 6 V o l t s T h i c k n e s s = x = 40 x 1 0 " m The r a t e o f s c a l e growth w i t h r e s p e c t to charge (q) i s : ±* = 1 . 7 x 1 0 " 9 m , 2 - 5 . 6 a q cou lomb/cm" The r a t e of , anode p o t e n t i a l i n c r e a s e w i t h r e s p e c t to c h a r g e The p o t e n t i a l g r a d i e n t a c r o s s the anode l a y e r i s t h u s : dV = dV d a = 1 .5 x T0~ 5 V . R _ Q - D . T I dx dq dx 1 . 7 x 1 0 m - 1 0 4 V m" 1 - 5 . 9 2 Thus a t 10 KA/m the r e s i s t i v i t y o f the anode l a y e r would b e : 2 p = 1 0 4 - • - ^ v — = 1 n m - 5 . 1 0 m 10 A 56 T h i s can be compared w i t h t h a t r e p o r t e d f o r m a g n e t i t e ^FegO^ = 1 0 ~ 4 ohm.m. The anode l a y e r has c o n s i d e r a b l e c o n d u c t i v i t y and the r e s i s t a n c e c o u l d be due to a much t h i n n e r anode l a y e r o f h i g h e r r e s i s t i v i t y . The r e l a t i v e l y u n i f o r m anode p o t e n t i a l c o r r e s p o n d i n g to Reg ion I I I i n the c o n s t a n t c u r r e n t e x p e r i m e n t s i s c o n -s i s t e n t w i t h the o b s e r v e d s t a b i l i z a t i o n o f the u l t i m a t e anode l a y e r . S l i g h t d e c l i n e s i n anode p o t e n t i a l s o b s e r v e d at the b e g i n n i n g of Reg ion I I I c o u l d be due to s l i g h t r e d u c -t i o n s i n the anode l a y e r t h i c k n e s s o r s l i g h t s u r f a c e m o d i f i -c a t i o n s . D i f f e r e n t c h e m i c a l and/or p h y s i c a l p r o p e r t i e s of an anode l a y e r wh ich forms d u r i n g the p e r i o d o f t r a n s i e n t a n o d i c measurements appears t o be the most p r o b a b l e r e a s o n 158 f o r the d e c l i n e i n the e f f i c i e n c y o f f e r r a t e f o r m a t i o n . The e x p e r i m e n t a l r e s u l t s s u g g e s t t h e u l t i m a t e anode l a y e r forms beneath the i n i t i a l l a y e r and thus p r o b a b l y o c c u r s by s o l i d s t a t e d i f f u s i o n . I t i s p o s s i b l e t h i s l a y e r i s a com-p a c t u n h y d r a t e d i r o n o x i d e . The rugged s t r u c t u r e o f the exposed i n n e r l a y e r (see f i g u r e 3.19c:) appears d i f f e r e n t f r o m the i n i t i a l l y formed anode l a y e r . The s u r f a c e o f the exposed anode l a y e r e x h i b i t s a c e r t a i n amount o f s u r f a c e p i t t i n g but i n g e n e r a l a p p e a r s e v e n l y a t t a c k e d . The r e s u l t s f rom m a g n e t i t e anodes i n d i c a t e t h a t the r a t e o f f e r r a t e f o r m a -t i o n f rom c e r t a i n i r o n o x i d e s can be l o w . The absence o f m i c r o p o r o s i t y c o u l d i n i t s e l f g r e a t l y reduce the r e a c t i v i t y of an anode l a y e r . How e x a c t l y the f o r m a t i o n o f a c h e m i c a l l y o r s t r u c t u r -a l l y d i f f e r e n t anode l a y e r c o u l d r e s u l t i n a d e c l i n e i n the e f f i c i e n c y of f e r r a t e f o r m a t i o n would depend on the mechanism of f e r r a t e f o r m a t i o n . I f as the e x p e r i m e n t a l r e s u l t s s u g g e s t * , a l a r g e p r o p o r t i o n of the f e r r a t e formed i s by d i r e c t o x i d a t i o n of the i n i t i a l anode l a y e r , the g r a d u a l e x p o s u r e o f a r e l a -t i v e l y u n r e a c t i v e anode l a y e r would r e s u l t i n t h e d e c l i n e i n the e f f i c i e n c y of f e r r a t e f o r m a t i o n . The d i s s o l u t i o n of the i n i t i a l l a y e r i s uneven and thus the a r e a or number o f s i t e s on the anode l a y e r were r e l a t i v e e n e r g e t i c i r o n atoms are a v a i l a b l e c o u l d d e c l i n e as the q u a n t i t y o f charge passed i s i n c r e a s e d . I f a s i m i l a r q u a n t i t y of i n i t i a l r e l a t i v e l y r e a c t i v e o x i d e (o r h y d r o x i d e ) was formed f o r a l l the s u p e r -f i c i a l c u r r e n t d e n s i t i e s i n v e s t i g a t e d , and most o f t h i s i n i t i a l l a y e r was o x i d i z e d to f e r r a t e s i m i l a r q u a n t i t i e s of f e r r a t e would be e x p e c t e d i n a l l c a s e s . I t i s not o b v i o u s why the d e c l i n e i/ri the e f f i c i e n c y of f e r r a t e f o r m a t i o n w i t h i n c r e a s i n g q u a n t i t i e s o f charge passed s h o u l d be i n d e p e n d e n t of s u p e r f i c i a l c u r r e n t d e n s i t y . I f a p r o p o r t i o n o f the f e r r a t e i o n s a re formed by the o x i d a t i o n of i r o n s p e c i e s wh ich have m i g r a t e d t h r o u g h the anode l a y e r , the growth o f an anode l a y e r t h r o u g h wh ich i o n i c d i f f u s i o n and m i g r a t i o n i s low would r e d u c e the r a t e o f f e r r a t e f o r m a t i o n . I f t h i s mechanism o p e r a t e s t h r o u g h m i c r o p o r o s i t y the g r o w t h of such a l a y e r below the pores would s i m i l a r l y reduce the r a t e o f f e r r a t e f o r m a t i o n . A c c o r d i n g to V e t t e r the p o t e n t i a l g r a d i e n t s and c o n s e -quently fhe i o n i c m i g r a t i o n a c r o s s an o x i d e l a y e r o f c o n s t a n t c o m p o s i t i o n i s i n d e p e n d e n t o f t h i c k n e s s . Thus the n u c l e a -t i o n of a l a y e r o f r e d u c e d i o n i c c o n d u c t i v i t y s h o u l d cause immediate r e d u c t i o n i n the m i g r a t i o n f l u x . T h i c k e n i n g of t h i s l a y e r wou ld o n l y d i m i n i s h the c h e m i c a l d i f f u s i o n a c r o s s t h i s l a y e r . I f f e r r a t e f o r m a t i o n o c c u r s e x c l u s i v e l y w i t h i n the pores i n the anode l a y e r the g r a d u a l removal o f t h i s l a y e r and r e p l a c e m e n t by an n o n - p o r o u s l a y e r would r e s u l t i n the g r a d u a l d e c l i n e i;h the e f f i c i e n c y o f f e r r a t e f o r m a t i o n . T h i s mechanism r e l i e s e x c l u s i v e l y on s t r u c t u r a l as opposed to c h e m i c a l d i f f e r e n c e s i n the anode l a y e r s and t h u s would a l s o be v a l i d i f the detachment of the upper l a y e r o c c u r r e d by s p a l l i n g as opposed to d i s s o l u t i o n . 5 . 4 G e n e r a l C o n c l u s i o n s on t h e F e r r a t e F o r m a t i o n R e a c t i o n In g e n e r a l the r a t e o f f e r r a t e f o r m a t i o n w i l l depend on the a v a i l a b i l i t y o f e i t h e r : 1) s u f f i c i e n t i r o n and oxygen s p e c i e s to p roduce the f e r r a t e i o n s , or 2) the s i t e s a t wh ich the r e a c t i o n can o c c u r . The s t r o n g dependence of the r e a c t i o n on the h y d r o x y l i o n c o n c e n t r a t i o n s u g g e s t s t h a t t h e s t a b l i z a t i o n o f the f e r r a t e i o n i n s o l u t i o n , a t the s i t e o f i t s p r o d u c t i o n , g r e a t l y a f f e c t s the i q u a n t i t y o f f e r r a t e p r o d u c e d . The e f f i c i e n c y of f e r r a t e p r o d u c t i o n depends on the r e l a t i v e k i n e t i c s of the f e r r a t e and oxygen e v o l u t i o n r e a c t i o n s and t h e i r v a r i a t i o n w i t h i n c r e a s e d q u a n t i t i e s of charge p a s s e d . I t does not appear p o s s i b l e t o e x p l a i n the b e h a v i o u r and k i n e t i c s o f the f e r r a t e f o r m a t i o n r e a c t i o n s o l e l y i n terms o f the a v a i l a b i l i t y o f the oxygen s p e c i e s ; the a v a i l a b i l i t y o f s u i t a b l e i r o n s p e c i e s , o r s i t e s f o r the r e a c t i o n t o o c c u r , a l s o a p p e a r s c r i t i c a l . The d e c l i n e i n the c u r r e n t e f f i c i e n c y of f e r r a t e f o r m a t i o n w i t h i n c r e a s e d q u a n t i t i e s of charge passed i s p r o b a b l y due t o the r e d u c t i o n i n the number o f o c c u r r e n c e s or s i t e s where c o n d i t i o n s a r e s u i t a b l e t o p roduce f e r r a t e i o n s . I t i s p r o b a b l e t h a t the f o r m a t i o n and e x p o s u r e of an anode l a y e r o f d i f f e r e n t p h y s i c a l s t r u c t u r e and/or c h e m i c a l c o m p o s i t i o n to the i n i t i a l l y formed anode l a y e r , r e s u l t s i n t h e d e c l i n e i n the number o f o c c u r r e n c e s or s i t e s f rom wh ich f e r r a t e f o r m a t i o n can o c c u r . 162 C h a p t e r 6 CONCLUSIONS The e f f e c t s o f the e x p e r i m e n t a l v a r i a b l e s on the s t a b i l " i t y o f f e r r a t e i o n s i n a l k a l i n e s o l u t i o n s were d e t e r m i n e d . The k i n e t i c s o f the f e r r a t e d e c o m p o s i t i o n r e a c t i o n a r e d e p e n -dent on the f e r r a t e i o n c o n c e n t r a t i o n , the r e s u l t s were a p p r o x i m a t e d to f i r s t or 3/2 o r d e r r e a c t i o n k i n e t i c s . The a c t i v a t i o n energy o f the f e r r a t e d e c o m p o s i t i o n r e a c t i o n was d e t e r m i n e d to be 66 K j / g . m o l . The d e c o m p o s i t i o n r e a c t i o n has an a p p r o x i m a t e l y l i n e a r dependence on the h y d r o x y l i o n c o n c e n t r a t i o n between 5 and 1 4 . 3 g . m o l / 1 NaOH. F e r r i c h y d r o -x i d e s u s p e n s i o n s a re c a t a l y t i c to the f e r r a t e d e c o m p o s i t i o n r e a c t i o n . The s t a b i l i t y o f f e r r a t e i o n s i s f a v o u r e d by low t e m p e r a t u r e s and f e r r a t e i o n c o n c e n t r a t i o n s , and h i g h h y d r o x y l i o n c o n c e n t r a t i o n s . An e l e c t r o c h e m i c a l c e l l was d e s i g n e d to measure the r a t e o f f e r r a t e f o r m a t i o n w i t h q u a n t i t y o f charge p a s s e d . U s i n g t h i s c e l l and o t h e r s t a n d a r d t e c h n i q u e s the e f f e c t s o f c u r -r e n t d e n s i t y and a l k a l i c o n c e n t r a t i o n on the e f f i c i e n c y o f f e r r a t e f o r m a t i o n from i r o n anodes, a t 50°C-were d e t e r m i n e d . The r e s u l t s i n d i c a t e t h a t t h e e f f i c i e n c y o f f e r r a t e f o r m a t i o n , 163 2 a t c u r r e n t d e n s i t i e s between 5 and 60 KA/m i s low (< 3%). The dominant a n o d i c p r o c e s s i s oxygen e v o l u t i o n , a c c o u n t i n g f o r 95% o r more o f the a n o d i c c u r r e n t . The c u r r e n t e f f i c i -ency o f f e r r a t e f o r m a t i o n g r a d u a l l y d e c l i n e s to z e r o a f t e r 2 0 - 3 0 Kcou lombs/em of c h a r g e has been p a s s e d . The e f f i -c i e n c y o f f e r r a t e f o r m a t i o n and the d e c l i n e i n the e f f i c i -ency o f f e r r a t e f o r m a t i o n w i t h i n c r e a s i n g q u a n t i t i e s of charge passed i s i n d e p e n d e n t o f the s u p e r f i c i a l c u r r e n t d e n s i t y . S i m i l a r q u a n t i t i e s o f f e r r a t e were p roduced a t a l l the c u r r e n t d e n s i t i e s i n v e s t i g a t e d . There a r e i n d i c a -t i o n s t h a t p e r i o d i c c u r r e n t r e v e r s a l can p r e v e n t the s t e a d y d e c l i n e i n t h e f e r r a t e p r o d u c t i o n r a t e . The e f f i c i e n c y o f f e r r a t e p r o d u c t i o n i n c r e a s e s w i t h i n c r e a s i n g h y d r o x y l i o n c o n c e n t r a t i o n . A s m a l l p r o p o r t i o n o f the t o t a l c u r r e n t r e s u l t s i n i r o n o x i d e o r h y d r o x i d e f o r m a t i o n on the anode . X - r a y d i f f r a c t i o n and e l e c t r o n m i c r o s c o p y s t u d i e s s u g g e s t t h i s l a y e r i s amor -phous and i n i t i a l l y has a h i g h s u r f a c e a r e a n o d u l a r m i c r o -s t r u c t u r e . The l a y e r t h i c k e n s s l o w l y and an o u t e r l a y e r i s d i s s o l v e d , o r s p a l l s o f f , as the anode l a y e r grows ( r e a c h -i n g t h i c k n e s s e s o f 50 ym i n p l a c e s ) . The exposed i n n e r o x i d e (o r h y d r o x i d e ) has a rugged g r a n u l a r m i c r o s t r u c t u r e . A f t e r l o n g p e r i o d s o f e l e c t r o l y s i s the anode l a y e r s tend to be r e l a t i v e l y t h i n ( < 10 ym). 164 Three d i s t i n c t t r e n d s i n the e l e c t r o c h e m i c a l b e h a v i o u r o f the anode d u r i n g e l e c t r o l y s i s were o b s e r v e d . F o r the 2 i n i t i a l few K coulombs/cm of charge passed the e l e c t r o -c h e m i c a l p r o c e s s e s o c c u r w i t h i n c r e a s i n g e a s e . The anode 2 b e h a v i o u r s o b s e r v e d f o r the f o l l o w i n g 1 0 - 1 5 Kcou lombs/cm o f charge p a s s e d are c o n s i s t e n t w i t h an i n c r e a s i n g r e s i s -t a n c e component a s s o c i a t e d w i t h the o x i d e (o r h y d r o x i d e ) l a y e r on the anode . For g r e a t e r q u a n t i t i e s o f c h a r g e passed a s t e a d y s t a t e anode b e h a v i o u r i s o b s e r v e d . I ron d i s s o l u t i o n f rom u n t r e a t e d m a g n e t i t e anodes i s low and f e r r a t e i o n s were not d e t e c t e d . C a t h o d i c r e d u c t i o n of the m a g n e t i t e anode s u r f a c e r e s u l t s i n f e r r a t e f o r m a t i o n and i n c r e a s e d anode d i s s o l u t i o n . The e x p e r i m e n t a l r e s u l t s do not i d e n t i f y the p r e c i s e mechanism of f e r r a t e f o r m a t i o n . The r e s u l t s s u g g e s t t h a t the mechanism of f e r r a t e f o r m a t i o n i s such t h a t the k i n e t i c s cannot be e x p l a i n e d s o l e l y i n terms o f the oxygen e v o l u t i o n k i n e t i c ^ the a v a i l a b i l i t y o f : 1) an i r o n s p e c i e s , o r 2) s i t e s a t wh ich f e r r a t e f o r m a t i o n can o c c u r , i s a l s o i m p o r t a n t . I t i s t e n t a t i v e l y p roposed t h a t f e r r a t e i o n s form f r o m , or w i t h i n the i n i t i a l l a y e r w h i c h forms on the a n o d e ; the g r a d u a l removal o f t h i s anode l a y e r w i t h the a s s o c i a t e d e x p o s u r e of a s t r u c t u r a l l y o r c h e m i c a l l y d i f f e r e n t o x i d e o r h y d r o x i d e b e -low r e s u l t s i n t h e g r a d u a l d e c l i n e o f the e f f i c i e n c y o f f e r r a t e f o r m a t i o n . 165 C h a p t e r 7 RECOMMENDATIONS FOR FUTURE STUDY F u r t h e r work i s r e q u i r e d to d e t e r m i n e the mechanism of f e r r a t e f o r m a t i o n f rom i r o n anodes i n s t r o n g a l k a l i n e s o l u -t i o n s . The f u r t h e r s t u d i e s recommended a r e : 1) an i n v e s t i g a t i o n i n t o the c h e m i c a l and s t r u c t u r a l p r o p e r t i e s o f the anode l a y e r s which fo rm on the e l e c t r o d e , 2) a d i r e c t i n v e s t i g a t i o n i n t o the c h e m i c a l and e l e c t r o -c h e m i c a l p r o c e s s e s o c c u r r i n g a t the a n o d e - s o l u t i o n i n t e r f a c e ; i t may be p o s s i b l e to use low c u r r e n t d e n s i t y t e c h n i q u e s f o r t h i s i n v e s t i g a t i o n , a l t e r n a t i v e h i g h c u r r e n t d e n s i t y t e c h n i q u e s may have to be d e v e l o p e d . The e f f e c t s o f t e m p e r a t u r e and h i g h f r e q u e n c y c u r r e n t r e v e r s a l on the e f f i c i e n c y o f f e r r a t e f o r m a t i o n s h o u l d a l s o be i n v e s t i g a t e d . 166 REFERENCES 1 . N .E . T u f f r e y . 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Tex tbook o f P h y s i c a l C h e m i s t r y . 2nd E d i t i o n . D. Van N o s t r a n d Co. I n c . New York ( 1 9 4 6 ) . 6 2 . C. Y. C h a o , L. F. L i n and D..D. M a c d o n a l d . J . E l e c t r o -chem. S o c . 1 29 pp . 1 8 7 4 - 1 879 ( 1 9 8 2 ) . 6 3 . J . M. S c h r e y e r , G.W. Thompson and L . T . Ockerman. A n a l . Chem. _22 pp. 1 4 2 6 - 1 427 (1 9 5 0 ) . 6 4 . J . M . S c h r e y e r , G.W. Thompson and L . T . Ockerman. A n a l . Chem. 22 pp .691 - 6 9 2 ( .1950). 6 5 . A . S . V e n k a t a d r i , W.F . Wagner and H.H. B a u e r . A n a l . Chem. 43 p p . 1 1 1 5 - 1 1 1 9 (1971 ) A p p e n d i x A REACTIONS AND EQUILIBRIA PERTAINING TO THE POTENTIAL - pH DIAGRAM FOR THE IRON - WATER SYSTEM AT 25°C Append ix A REACTIONS AND EQUILIBRIA PERTAINING TO THE POTENTIAL - pH DIAGRAM FOR THE IRON - WATER SYSTEM AT 2 5 Q C A. - l S u b s t a n c e s C o n s i d e r e d O x i d a t i o n S p e c i e s AG°* Number , K c a l l s  g .mol 0 Fe 0 +2 F e ( O H ) 2 - 5 5 . 8 8 +2 F e 2 + - 2 0 . 3 0 +2 H FeO^ - 4 0 . 6 3 +3 F e ( 0 H ) 3 - 1 6 1 . 9 3 +3 F e 3 + - 2 . 5 3 +3 F e ( O H ) 2 - 1 0 6 . 2 +3 F e 0 H 2 + - 5 5 . 9 1 + 6 Fe O4" - 7 7 . 0 *A11 d a t a i s f rom P o u r b a i x e t a l . e x c e p t AG° FeO^~ 9 which i s f rom Wood. The a c t i v i t y of i r o n c o n t a i n i n g s p e c i e s i n s o l u t i o n _ 3 t a k e n as 10 M. F e r r o u s and f e r r i c h y d r o x i d e s , Fe(OH) and F e ( 0 H)o have been c o n s i d e r e d the s t a b l e o x i d e s . F e 2 + + 2H20 HFeO~ + 3H + [HFeOZ] log — — 7 ~ =• ->31.58 + 3 pH; pH [Fe ] s 10.53 F e 3 + + H20 FeOH 2 + + H + 2+ log ^¥e0f. J = - 2 . 4 3 + pH; PH = 2.43 [ F e 3 + ] FeOH 2 + + H20 -> 2 + H + log [Fe(0H) 2] [Fe(OH) 2 +] •4.69 + pH; pH = 4.69 Fe 2+ r- 3+ Fe + e E = 0.771 + 0.0591 log 1 0J [ F e 2 + ] E = 0.771 F e 2 + + H90 Fe(0H) 2 + + H + + e" 2+ E = 0.914 - 0.0591 pH + 0.0591 log EFeCQH)- ^ [ F e Z + ] E = 0.914 - 0.059 pH 173 2 c v v ' " 2 6) F e2 + + 2H„0 -> Fe(QH)t + 2H + + e [Fe(OH)t] E = 1.191 - 0.1182 pH + Q.Q591 log [ F e 2 + ] E = 1 . 1 9 1 - 0.1182 pH 7) HFeO" + H + Fe(0H) 2 + e" [Fe(OH)t] E = -0 .675 + 0.0591 pH + 0.0591 log d [Fe02H~] E = -0 .675 + 0.0591 pH 8) HFeO" + 2H20 -> FeO 2 - + 5H + e" [FeO 2 - ] E = 1.376 - 0.0738 pH + 0.0148 log — [HFe02] E = 1.376 - 0.0738 pH 9) . F e 3 + + 4H90 -»• FeO 2 - + 8H + + 3 e" [FeO 2 - ] E = 2.201 - 0.1580 pH + 0.0197 log ^ r -[ F e 3 + ] E = 2.201 - 0.1580 pH 174 10) Fe(0H) 2 + + 3H20 FeO2" + 7H + + 3 e" [FeO 2"] . E = 2.153 - 0.1379 pH + 0.0197 log H 0 , " [Fe(0H) 2 + ] E = 2.153 - 0.1379 pH 11) Fe(0H) 2 + 2H20 - FeO2" + 6H + + 3 e" [FeO 2"] E = 2.060 - 0.1182 pH + 0.0197 log 4 [Fe(OH)J] E = 2.060 - 0.1182 pH A.2.2 Two So l id Substances 12) Fe + 2H20 Fe(0H) 2 + 2H + + 2 e" E = -0 .047 - 0.0591 pH 13) Fe + 3H20 Fe(OH)3 + 2H + + 2 e" E = 0.059 - 0.0591 pH 14) Fe(0H) 2 + H20 -> Fe(0H) 3 + 2H + + 2 e" E = 0.271 - 0.0591 pH 175 A .2 .3 One So l id Substance arid One Pissolved Substance 15) Fe 2 * + 2H20 Fe(0H) 2 + 2H + log [ F e 2 + ] = 13.29 - 2 pH pH = 8.145 16) Fe(0H) 2 -> HFeO~ + H + log [HFeO~] = -18.30 + pH pH = 15.3 17) F e 3 + + 3H^0 -* Fe(0H) 3 + 3 H + log [ F e 3 + ] = 4.84 - 3 pH pH = 2.613 18) Fe(0H) 2 + + 2H20 Fe(0H) 3 + 2 H + log [Fe(0H) 2 + ] = 2.41 - 2 pH pH = 2.705 Fe(0H) 2 + H20 -> Fe(.0H)3 + H + log [Fe(0H) 2] = . - 2 . 2 8 - pH pH = 0.72 2+ Fe Fe + 2 e E = -0.440 + 0.0295 log [ F e 2 + ] E = -0.529 Fe + 2H20 -»• HFeO~ + 3H + + 2 e" E = 0.493 - 0.0886 pH + 0.0295 log [HFe02] E = 0.405 - 0.0886 pH Fe F e 3 + + 3 e" E = -0 .037 + 0.0197 log [ F e 3 + ] E = -0.096 F e 2 + + 3H20 Fe(_0H)3 + 3H + + e" E = .1.057 - 0.1773 pH - 0.0591 log [ F e 2 + ] E = 1.234 - 0.1773 pH HFeO" + H20 -»• Fe(0H) 3 + e~ E = -0.810 - 0.0591 log [HFeOgJ E = -0 .633 Fe + 4H20 = FeO2" + 8H + + 6 e" E = 1.0823 - 0.0788 pH + 0.0099 log [FeO 2 - ] E = 1.0526 - 0.0788 pH Fe(OH)2 + 2H20 = FeO2" + 6H + + 4 e" E .= 1.647 - 0.1182 pH + 0.01485 log [FeO 2"] E = 1.602 - 0.1182 pH Fe(0H) 3 + H20 = FeO2" + 5H + + 3 e" E = 2.106 - 0.0984 pH + 0.0197 log [FeO^"] E = 2.047 - 0.0984 pH 178 Append ix B THE ANALYSIS OF FERRATE CONTAINING SOLUTIONS 179 Append i x B THE ANALYSIS OF FERRATE CONTAINING. SOLUTIONS . B.1 Methods A v a i l a b l e An o v e r a l l v iew o f the methods o f f e r r a t e a n a l y s i s i s Q g i v e n by M i l l e r . The methods a v a i l a b l e can be d i v i d e d i n t o t h r e e c a t e g o r i e s 1) c h e m i c a l , 2) s p e c t r o p h o t o m e t r i c and 3) e l e c t r o c h e m i c a l . ..;.B:. 2 C h e m i c a l A n a l y s i s The chromium ( I I I ) - F e r r o u s , A r s e n i t e ( I I I ) -B r o m a . t e 6 4 and A r s e n i t e ( I I I ) - C e r a t e 6 4 methods were e x a m i n e d . In a l l t h r e e methods the f e r r a t e i o n i s used to o x i d i z e the r e d u c e d s p e c i e s (chromium ( I I I ) o r A r s e n i t e ( I I I ) ) under h i g h l y a l k a l i n e c o n d i t i o n s . For the chromium ( I I I ) -f e r r o u s method the s o l u t i o n i s then a c i d i f i e d a n d . t h e o x i d i z e d chromium (V I ) s p e c i e s i s back t i t r a t e d w i t h a f e r r o u s s o l u t i o n . For the a r s e n i t e (111 )}„ methods the u n o x i d i z e d e x c e s s , a r s e n i t e ( I I I ) i s back t i t r a t e d w i t h e i t h e r bromate or c e r a t e s t a n d a r d s o l u t i o n s . None of t h e above methods p roved s a t i s f a c t o r y f o r the a n a l y s i s o f d i l u t e f e r r a t e s o l u t i o n s . In a l l eases the f e r -r a t e - r e d u c e d s p e c i e s r e a c t i o n was i n c o m p l e t e and thus the c a l c u l a t e d f e r r a t e c o n c e n t r a t i o n s were too l o w . 180 I f s o l i d f e r r a t e samples were used the chromium ( I I I ) - f e r r o u s method was found to g i v e s a t i s f a c t o r y a n a l y t i c a l r e s u l t s . The s o l i d p o t a s s i u m f e r r a t e was p r e p a r e d u s i n g the 8 h y p o c h l o r i t e method of M i l l e r , (see page'1'4). The r e s u l t s of f o u r chromium ( I I I ) - f e r r o u s a n a l y s e s o f s o l i d p o t a s s i u m f e r r a t e samples were c o n s i s t e n t w i t h i n + 2%. The r e s u l t s o f chromium ( I I I ) - f e r r o u s a n a l y s e s o f s o l u t i o n s p r e p a r e d f rom measured q u a n t i t i e s of s o l i d p o t a s s i u m f e r r a t e were t y p i c a l l y 20% l o w e r than t h o s e o f s o l i d p o t a s s i u m f e r r a t e a n a l y s e s . ; . B . 2 . 1 The Chromium ( I I I ) - F e r r o u s Method f o r t h e A n a l y s i s o f S o l i d F e r r a t e The chromium ( I I I ) - f e r r o u s method as p r o p o s e d 6 3 by S c h r e y e r e t a l . r e l i e s on the c h r o m i t e i o n b e i n g q u a n t i -t a t i v e l y o x i d i z e d by f e r r a t e i o n s . C r (0H)~ + F e O 2 - = Cro|~ + F e ( 0 H ) 3 + OH" On a c i d i f i c a t i o n the chromate i s c o n v e r t e d to d i c h r o m a t e . The d i c h r o m a t e i s then reduced i n i a. back t i t r a t i o n w i t h a s t a n d a r d i z e d f e r r o u s s o l u t i o n . 181 B . 2 . 1 . 1 A n a l y t i c a l P r o c e d u r e 3+ 1) Mix 5 ml o f 0 . 1 . M Cr ( c h r o m i c c h l o r i d e h e x a h y d r a t e ) w i t h 20 ml o f s a t u r a t e d sodium h y d r o x i d e s o l u t i o n and 5 ml o f d i s t i l l e d w a t e r . 2) Add 0 . 1 5 t o 0 . 2 g o f s o l i d p o t a s s i u m f e r r a t e and s w i r l u n t i l d i s s o l v e d . 3) Add 150 ml o f d i s t i l l e d wate r and a c i d i f y w i t h 60 ml o f 1 - 5 s u l p h u r i c a c i d and 15 ml o f s u l p h u r i c - p h o s p h o r i c a c i d m i x t u r e (240 H'20 : 60 H 2 $0 4 ; 150 H 3 P0 4 ) . 4) Add 5 t o 6 drops o f sodium d i p h e n y l amine s u l p h o n a t e i n d i c a t o r and t i t r a t e i m m e d i a t e l y w i t h 0 . 0 8 5 N f e r r o u s ammonium s u l p h a t e s o l u t i o n . (The f e r r o u s s o l u t i o n must be s t a n d a r d i z e d , i m m e d i a t e l y b e f o r e u s e , w i t h a s t a n d a r d d i c h r o m a t e s o l u t i o n . ) The p e r c e n t a g e f e r r a t e i n t h e sample i s then c a l c u l a t e d as f o l l o w s : [ml of F e 2 + x NFe 2 + ] x RMM K ? Fe0. Percent K 2FeO. = — x 100 3000 x wt. of sample (grams) B.3 Spectrophometric Analysis F e r r a t e i o n s o l u t i o n s a r e i n t e n s e l y c o l o u r e d and obey the B e e r - L a m b e r t Law: A = l o g 1 Q - j | = ee l 182 A = A b s o r b a n c e 1^ = I n i t i a l l i g h t i n t e n s i t y I = T r a n s m i t t e d l i g h t i n t e n s i t y C = S p e c i e s c o n c e n t r a t i o n ( g . m o l / 1 ) 1 = Length (cm) g =• M o l a r a b s o r b a n c e c o e f f i c i e n t — ... B. 3 . 1 Cal i b r a t i o n B e f o r e s p e c t r o p h o t o m e t r y a n a l y s i s of f e r r a t e i o n s o l u t i o n s the s p e c t r o p h o t o m e t e r must be c a l i b r a t e d . In t h i s case a Bausch and Lomb S p e c t r o p h o t o m e t e r (Model - S p e c t r o n i c 88) and 1 cm path l e n g t h q u a r t z c e l l was us-edr.: ^:\~J. For f e r r a t e s o l u t i o n s i n the range o f sodium h y d r o x i d e c o n c e n t r a t i o n s used e x p e r i m e n t a l l y the maximum a b s o r b a n c e was found to o c c u r a t 515 nm. The s t a n d a r d f e r r a t e , s o l u t i o n s were p r e p a r e d f rom chromium ( I I I ) - f e r r o u s method a n a l y z e d s o l i d p o t a s s i u m f e r r a t e s a m p l e s . The a b s o r b a n c e was d i r e c t l y p r o p o r t i o n a l to f e r r a t e c o n c e n t r a t i o n ; up to an a b s o r b a n c e of 0 . 9 . The m o l a r a b s o r b a n c e c o e f f i c i e n t was d e t e r m i n e d , f rom the ave rage of 5 r e s u l t s , a s : g . m o l . c m A 935 + 2% 1 i t r e s e Cl g . m o l . c m 183 An e a s i e r way o f e x p r e s s i n g t h i s f o r a 1 cm path l e n g t h i s [ F e O 2 - ] 2 — = 1 .07 x 10 J g . m o l / 1 A b s o r b a n c e U n i t For maximum a c c u r a c y measurements were made, i f p o s -s i b l e , w i t h a b s o r b a n c e s a t between 0 . 4 and 0 . 7 , however the i n s t r u m e n t c h a r a c t e r i s t i c s were such t h a t the l o w e r l i m i t o f - 4 f e r r a t e c o n c e n t r a t i o n a c c u r a t e l y d e t e c t a b l e was 10 g . m o l / 1 - 5 w i t h a machine s e n s i t i v i t y o f + 10 g . m o l / 1 . . i . B . 4 The E l e c t r o c h e m i c a l A n a l y s i s o f F e r r a t e Ion S o l u t i o n s 65 V e n k a t a d r i e t a l . found t h a t by u s i n g c y c l i c v o l t a -metry t e c h n i q u e s f e r r a t e c o n c e n t r a t i o n s down to between - 5 - 6 2 . 5 x 10 and 2 . 5 x 10 g . m o l s / 1 c o u l d be d e t e r m i n e d . A r o t a t i n g i r o n w o r k i n g e l e c t r o d e was used w i t h a p o t e n t i a l sweep r a t e of 5 mV/s. For, weak f e r r a t e s o l u t i o n s i n 1 0 . 5 g . m o l / 1 K0H the h e i g h t of the c a t h o d i c c u r r e n t peak a t - 0 . 1 to - 0 . 5 V ( v e r s u s Hg/HgO|10.5 g . m o l / 1 K0H r e f e r e n c e e l e c t r o d e ) c o r r e l a t e d d i r e c t l y to the i n i t i a l f e r r a t e c o n -c e n t r a t i o n . Thus u s i n g a c a l i b r a t i o n c u r v e c o n s t r u c t e d i n t h i s way unknown f e r r a t e c o n c e n t r a t i o n s c o u l d be d e t e r m i n e d . Due to the speed and s i m p l i c i t y o f the s p e c t r o p h o t o -m e t r i c t e c h n i q u e the a n a l y s i s of weak f e r r a t e s o l u t i o n s by 184 c y c l i c v o l t a m e t r y was not q u a n t i t a t i v e l y e x a m i n e d . . B . 5 T o t a l I r o n A n a l y s i s T o t a l i r o n a n a l y s i s was c a r r i e d out by a t o m i c a b s o r p -t i o n s p e c t r o p h o t o m e t r y . A P e r k i n E lmer A tomic A b s o r p t i o n S p e c t r o p h o t o m e t e r (Model 306) was u s e d . The machine s e n s i t i v i t y i s 2 x 1 0 " ^ g .mol Fe/1 . The main p rob lem w i t h t h i s t e c h n i q u e i s the a c i d i f i c a -t i o n o f h i g h l y a l k a l i n e s o l u t i o n s (so as t o s o l u b l i z e a l i i the i r o n p r e s e n t ) meant t h a t t h e s o l u t i o n s have a h i g h s a l t l e v e l . To p r e v e n t n e b u l i z e r b l o c k a g e a l l s o l u t i o n s (and s t a n d a r d s ) were d i l u t e d t o about 4% s a l t l o a d ( t y p i c a l l y x 20 d i l u t i o n ) . The r e s u l t o f t h i s need f o r e x c e s s i v e d i l u t i o n was a l o s s of a c c u r a c y such t h a t r e a d i n g s t y p i c a l l y would have an e r r o r o f 4 t o 8% d e p e n d i n g on the f i n a l i r o n c o n -c e n t r a t i o n a f t e r d i l u t i o n . Append ix C TABLES OF RESULTS T86-T a b l e C . l t o C .4 The e f f e c t s o f F e r r a t e C o n c e n t r a t i o n on t h e F e r r a t e D e c o m p o s i t i o n R a t e [NaOH] • . 1 4 . 3 g . m o l s / 1 T e m p e r a t u r e • 333 K Table C . l Table C.2 Time (Seconds) (x 10 3 ) [NO*"] g.mol/1 (x 1 0 " 3 ) l og 1 0 [ Fe0 4 2 -3 1 " 2 l^g .moT 1 5 I<2~.,oo [ F e O p t - D (X) 0 1.797 - 2 . 7 4 6 23.6 100-0 0.61 1.481 - 2 . 8 2 9 26.0 8 2 . 4 . 1.15 1.336 -2 .874 27.4 74.3 1.76 1.236 - 2 . 9 0 8 28.4 68.8 2.36 1.173 -2 .931 29.2 65.3 3.02 1.030 - 2 . 9 8 7 31.1 57.3 4.86 0.823 - 3 . 0 8 5 34.9 45.8 5.53 0.761 - 3 . 1 1 9 36.2 42.3 6.72 0.658 - 3 . 1 8 2 39.0 33.6 8.6 0.537 - 3 . 2 7 0 43.1 29.9 11.2 0.409 - 3 . 3 8 9 49.4 22.8 0 1.336 ' - 2 . 8 7 4 27.4 100.0 0.49 0.914 -3 .094 33.1 68.4 1.01 0.847 - 3 . 0 7 2 34.4 63.4 1.62 0.765 - 3 . 1 1 6 36.2 57.3 2.22 0.702 -3 .154 37.7 52.5 2.88 0.616 - 3 . 2 1 0 40.3 46.1 4.69 0.507 -3 .295 44.4 37.9 5.39 0.453 -3 .344 47.0 33.9 6.68 0.359 •73.445 52.8 26.9 8.14 _ 0.319 -3 .496 56.0 23.9 0 0.921 -3 .036 33.0 100.0 0.37 0.637 - 3 . 1 9 6 39.6 69.2 0.89 0.529 -3 .277 43.5 57.4 1.47 . 0.476 - 3 . 2 2 3 45.8 51.7 2.08 0.415 - 3 . 3 8 2 49.1 45.1 2.73 0.373 - 3 . 4 2 8 51.8 40.5 4.54 0.254 -3 .594 62.7 2 7 . 6 . 5.21 0.239 - 3 . 6 2 2 64.7 26.0 6.38 0.198 -3 .704 71.1 21.5 7.96 0.134 -3 .871 86.4 14.5 0 0.46.1 -3 .336 46.6 100.0 0.26 0.339 - 3 . 4 7 0 54.3 73.5 0.79 0.268 - 3 . 5 7 3 61.1 58.1 1.37 0.219 - 3 . 6 6 0 67.6 47,5 1.97 0.198 -3 .704 71.1 43.0 2.59 0.149 -3 .827 81.9 32.3 3.24 0.137 - 3 . 8 6 3 85.4 29.7 4.43 0.101 - 3 . 9 9 5 99.5 21.9 5.06 0.094 -4 .026 103.0 20.4 1.87-T a b l e C . 5 T a b l e C . 6 T a b l e C . 7 [NaOH] • 1 4 . 3 g m o l / 1 Temperature K T i m e (Seconds) (x 1 0 3 ) [ F e O * - ] g . m o l s / l (x 1 0 " 3 ) l ° g 1 0 [ F e o | - ] t [FeOpt = t ' — " 1 0 0 (%) =0 2 7 3 . 3 0 1 7 5 3 - 2 7 5 6 1 0 0 . 0 8 6 . 4 0 0 1 674 - 2 7 7 6 9 5 . 5 2 6 6 . 4 0 0 1 5 4 2 - 2 8 1 2 8 8 . 0 4 3 2 . 0 0 0 1 309 - 2 8 8 3 7 4 . 7 6 0 4 . 8 0 0 1 106 - 2 9 5 6 6 3 . 1 7 7 7 . 6 0 0 1 0 3 9 - 2 9 8 3 59 3 9 6 8 . 4 0 0 0 8 3 2 - 3 0 8 0 4 7 . 5 1 6 4 5 . 2 0 0 0 4 5 3 - 3 344 2 5 . 8 2 9 4 . 3 0 3 0 4 5 - 2 5 1 6 1 0 0 0 1 . 4 0 0 2 9 6 6 - 2 528 9 7 . 4 5. 2 00 2 8 8 7 - 2 539 9 4 . 8 7 . 6 0 0 2 794 - 2 554 9 1 . 8 9 . 6 0 0 2 7 1 5 - 2 566 8 9 . 2 1 3 . 2 0 0 2 6 4 9 - 2 577 87 0 1 5 . 2 0 0 2 6 2 3 - 2 581 . 8 6 . 1 1 7 . 2 0 0 2 6 1 0 - 2 583 8 5 . 7 2 1 . 2 0 0 2 531 - 2 597 8 3 . 1 2 5 . 8 0 0 2 4 6 5 - 2 6 0 8 81 . 0 3 1 3 . 3 0 1 4 6 3 - 2 8 3 5 1 0 0 . 0 1 . 580 1 397 - 2 8 5 5 9 5 . 5 2 . 3 4 0 1 344 - 2 8 7 2 9 1 . 9 3 . 1 5 0 1 292 - 2 8 8 9 8 8 . 3 . 4 . 1 2 0 1 260 - 2 8 9 9 8 6 . 1 6. 800 1 135 - 2 945 i 7 7 - 6 8 . 9 0 0 1 057 - 2 976 7 2 . 2 1 1 . 5 6 0 0 9 1 5 - 3 0 3 9 6 2 . 5 1 4 . 0 5 0 0 8 2 6 - 3 0 8 3 5 6 . 5 ' • • 1 6 . 0 0 0 0 776 - 3 101 5 3 . 0 1 8 . 0 0 0 0 714 - 3 146 4 8 . 8 2 0 . 1 0 0 0 6 6 2 - 3 1 7 9 4 5 . 2 188 T a b l e s C. £ t o C. 9 The E f f e c t s o f T e m p e r a t u r e on t h e f e r r a t e D e c o m p o s i t i o n R a t e [NaOH] - .14.3 g . m o l / 1 Temperature T1 me (Seconds) g . m o l s / 1 (x 1 0 ' 3 ) 1 o 9 l 0 [FeoJ- 3 100 (X) K (x 10 3 ) [ F e O ^ T t . Q -3 2 3 . 3 • 0 1 710 - 2 767 1 0 0 . 0 0 . 4 0 5 1 614 - 2 . 792 9 4 . 4 0 . 8 4 0 1 . 529 - 2 . 816 8 9 . 4 1 . 3 5 0 1 . 4 54 - 2 . 837 8 5 . 0 1 . 9 3 0 1 . 390 - 2 . 857 81 . 3 2 . 890 1 . 315 .2. 8*1 7 6 . 9 3 . 4?n 1 . 251 - 2 . 903 7 3 . 2 4 . 490 1 . 176 - 2 . 929 6 6 . 8 5 . 6 6 0 1 . 1 33 - 2 . 946 6 6 . 3' 7 . 0 9 0 0 . 989 - 3 . 005 , 5 7 - 8 9 . 460 0 . 861 -•3. 065 50- & •' 1 0 . 5 6 0 0. 828 - 3 . 082 4 8 . 4 Temperature K T ime (Seconds) [ F e O * " ] g . m o l s / 1 (x 1 0 ' 3 ) l o g 1 0 [FeO*- ] [Fel>;-3t [ F e 0 < - ] t • t 100 . (5 ) 3 3 3 . 3 See T a b l t C . l .'• 3 S 3 . 3 0 1 . 3 5 8 - 2 867 1 0 0 . 0 £9 1 243 - 2 906 91 . 5 132 1 160 - 2 936 8 5 . 4 224 1 . 0 7 3 - 2 . 969 . 7 9 0; 309 1 . 0 0 0 ' - 3 . 0 0 0 -. 73 6 424 0 . 924 - 3 . 0 3 4 68 0 5*4 . 0 . 8 7 1 3 - 3 . 0 6 0 64 2 ; 724 0 . 7 5 0 . . - 3 . 1 2 5 2 944" . 0 . 6 1 6 - 3 . 2 1 1 45 4 1224 0 . 558 - 3 . 254 41 1 1392 0 . 409 - - 3 . 389 30 1 1584 0 . 3 3 6 - 3 . 4 7 4 24 7 1994 0 . 1 9 2 - 3 . 7 1 6 14 1 189 T a b l e s C I O t o C. 1 2 The E f f e c t s o f Sodium H y d r o x i d e C o n c e n t r a t i o n on the Rate F e r r a t e  Decompos i t i on T e m p e r a t u r e = 323 K [NaOH] g . m o l / 1 Time ( s ) [ F e O * ' ] g . m o l / 1 (x 1 0 - 3 ) l o 9 l 0 [FeO*"] [ F e 0 * - ] t = [ F e O J j - ] f t • - - 1 0 0 {%) 0 -5 0 1 048 - 2 980 1 0 0 . 0 50 .0 919 - 3 037 8 7 . 7 .90 0 743 - 3 129 7 0 . 9 130 0 620 - 3 208 5 9 . 2 10 0 1 .568 - 2 805 100 0 150 1 539 - 2 813 98 2 310 1 465 - 2 834 93 4 480 1 411 • - 2 850 90 0 690 1 336 - 2 874 85 2 910 1 245 - 2 905 79 4 1 190 1 177 - 2 929 75 1 1490 1 122 - 2 950 71 6 1790 1 042 - 2 982 66 5 2090 0 983 - 3 007 62 7 2640 0 877 - 3 057 55 9 2980 0 802 - 3 096 51 1 3570 0 716 - 3 145 45 7 4220 0 622 - 3 206 39 7 4940 0 526 . - 3 279 33 5 5870 0 438 - 3 358 2 7 . 9 . 7030 0 347 - 3 460 22 1 12 0 1 723 - 2 764 100 0 310 1 577 - 2 802 91 5 750 1 443 - 2 841 83 7 1190 1 358 - 2 867 7 8 . 8 1460 1 240 - 2 907 7 2 . 0 2210 1 176 - 2 930 6 8 . 3 2840 1 048 - 2 980 6 0 . 8 3390 0 . 957 - 3 . 019 5 5 . 5 3960 0 877 . - 3 057 5 0 . 9 4910 0 802 - 3 096 4 6 . 5 5980 0 . 7 06 - 3 . 1 51 41 . 0 7420 0 . 609 - 3 . 215 3 5 . 3 9790 0 465 - 3 . 333 2 7 . 0 1 4 . 2 9 5 See T a b l e C . 8 190 Tables C.13 to C.18 Continuous Flow Anolyte Experiments (Aroco Iron Anodes) Temperature » 50CC Anode Area * 3 cm^ Sample Volume * SO ml Approximate Flow Rate * 3.1 ml Couloros Passed ?_ Interval FeO, 4 7.Ml " (Total) Coulombs Passed For Hean Rate of Ferrate Produced Coulombs cm2 (x 103) Mean Rate of Ferrate Production o.mols Coulomb (x 10* S) Coulombs Cm2 of l> anode 103> Unit Area of Anode o.mols e«? (x 10" 5) Unit Area of Anooe g.mols a? (x 10' 6 ) Table C.13 Ferret e Formation at 10 KA/m2 Superficial Current Density • 10 KA/m2 fNaOHl 14.3 o.mol/1 1.16 2 19 . 2 .19 0.58 18.91 2.27 4 24 6 43 1.72 38.23 3.45 3 86 10 29 2.86 32.74 4.6 3 75 14 .04 4.03 32.57 5.7 3 64 17 .66 5.15 33.07 6.9 3 66 21 28 6.3 30.64 • 8.1 3 18 24 46 7.5 26.53 9.25 2 66 27 32 8.6e 24.69 10.4 2 56 29 86 9.63 22.24 11.5 2 32 32 2 10.95 21.06 12.65 1 97 34 17 12.06 17.09 13.6 1 74 35 91 13.23 15.10 15.0 1 54 37 45 14.4 12.81 16.2 1 25 38 7 15.6 10.44 17.3 0 80 39 5 16.75 7.30 16.45 • 0 56 40 06 17.88 4.64 15.6 0 42 40 48 19.03 3.63 20.75 0 26 40 74 20.16 2.78 21.9 0 20 40 94 21.33 1.72 23.1 0 13 41 07 22.5 1.11 24.3 0 10 41 17 23.7 . 0.8E 25.35 0 12 41 29 24.8 0.12 lable C.14 Ferrate Format ion at 10 KA/m2 Superficial Current Density " 10 KA/m2 [ NaOH"] 14.3 o.mol/1 1.1 4 36 ' 4 38 0.55 35.78 1.95 3 99 8 37 1.53 46.89 3.1 4 65 13 02 2.53 40.48 4.25 4 47 17 49 3.68 38.85 5.4 4 32 21 81 4.83 37.59 6.65 3 85 25 66 5.98 33.50 7.65 3 65 29 31 7.1 33.17 6.6 3 36 32 67 B.23 29.22 9.95 2 96 35 63 9.38 25.73 11.1 2 80 38 43 10.53 24.35 12.25 2 32 40 75 11.68 20.16 13.4 2 14 42 89 12.83 18.64 14.55 1 B4 44 73 13.98 15.97 15.7 1 51 46 24 15.13 13.09 16.65 1 18 47 42 16.20 17.96 0 86 48 26 17.42 7.62 19.1 0 54 48 82 18.54 4.86 20.25 0 33 49 15 19.68 2.84 21.36 0 23 45 3C 20.61 2.05 22.5 0 15 49. 53 21.94 1.30 23.63 0 10 49. 63 23.07 0.85 191 Tables C.13 to C I S Continuous Flow Anolyte Experiments (Anneo Iron Anodes) Temperature • SO'C 2 Anode Area " 3 cm Simple Volume « SO nl Approximate Flo* Rate • 3.1 ml Coulorrbs Passed Coulombs cm2 of (* 10 ) anode Interval FeO, Unit Area of Anode q.mols on 5 (x 10" 6) ]Pf>0, (Total) Unit Area of Anode q.mols o ? (x 10" 6) Coulombs Passed For Mean Rate of Ferrate Produced Coulombs a? (x i o 3 ) Hean Rate of Ferrate Production q.mols . Coulomb (x 10" Table C IS Ferrate Formation at 9 KA/m- Superficial Current Density • S KA/m fWaDHl » 14.3 o.mol/1 0.94 1.98 2.97 4.05 S.085 6.075 7.155 8.19 9.27 10.35 11.34 12.33 13.365 14.445 15.46 16.52 17.55 18.54 19.67 20.7 21.78 22.86 23.99 2.54 3.71 3.56 3.73 3.52 3.40 3.044 2.e7 2.31 2.30 1.94 1.83 1.62 1.49 1.40 1.17 1.06 0.B6 0.71 0.60 0.55 0.46 0.37 2.54 6.25 9.81 13.54 17.06 20.46 23.50 26.37 .28.68 30.98 32.92 34.75 36.37 37.86 39.26 40.43 41.49 42.35 43.06 43.66 44.21 44.67 45.04 0.47 1.46 2.48 3.51 4.57 5.58 6.615 7.67 8.73 9.81 10.85 11.84 12.85 13.90 14.96 16.00 17.03 18.05 19.10 20.16 21.24 22.32 23.4225 27.02 35.67 36.00 34.54 34.00 34.34 28.19 27.68 21.40 21.31 19.55 18.43 15.66 13.76 13.49 11.32 10.23 B.66 6.35 6.82 5.04 4.24 3.30 Table C.16 Ferrate Formation at 5 KA/ir, 2 Superficial Current Density " 5 KA/m [NaOH] • 14.3 q.mol/1 Approximate Anolyte Flow Rate * 4.4 ml/min 0.65 1.68 2.45 3.3 4.13 4.98 5.78 6.6 7.43 6.26 9.08 9.9 10.75 11.58 12.4 13.23 14.05 14.68 15.7 16.53 17.36 18.23 19.05 2.33 3.71 2.62 2.59 2.36 2.05 2.05 1.92 1.47 1.44 1.30 1.06 0.80 0.64 0.31 0.13 2.33 6.04 9.89 13.62 17.2 20.65 23.94 27.28 30.12 32.74 35.33 37.69 39.72 41.79 43.71 45.18 46.62 47.92 4B.98 49.78 50.32 50.63 50.76 0.43 1.26 5 6 7 7.84 6.66 9.49 10.33 11.16 11.99 12.81 13.64 14.46 15.29 16.11 16.96 17.8 16.64 27.40 45.00 45.7 43.87 43.41 40.64 41.05 40.53 34.44 31.78 31.40 28.62 24.14 24.80 23.30 17.81 .17.51 15.7 12.89 9.76 6.29 3.67 1.62 192 Tables C.13 to C I S Continuous Flow Anolyte Experiments (Armco Iron Anodes) Temperature « 50°C Anode Area • 3 cm 2 Sample Volume • 50 ml Approximate Flow Rate * 3.1 ml Coulombs Passed Interval FeO^ J V e O * ' (Total) Coulombs Passed For Mean Rate of Ferrate Produced Coulombs cm 2 (x 10 3 ) Mean Rate of Fer ra te Coulombs , cm 2 of (x 10 ) anode Unit Area of Anode fl.mols em2 (x 1 0 ' 5 ) Unit Area of Anode g.mols cm 2 (x 1 0 ' 5 ) Product ion g.mols Coulomb (x 10* 9 ) Table C.17 Ferrate Formation at 18 KA/m2 S u p e r f i c i a l Current Density • fNaOHl 18 KA/m Approximate Anolyte Flow Rate « 4.4 ml/m1n 14.3 o.mol/1 1.44 3.81 3.81 0.72 26.44 2.88 4.42 8.23 2.16 30.68 4.32 4.24 12.47 3.f i ?9 .45 5.76 3.94 16.41 5.04 27.34 7.2 3.38 19.79 6.48 23.49 8.64 2.62 22.41 7.92 16.21 10.08 1.95 24.36 9.36 13.54 11.52 1.60 25.96 10.8 11.14 12.96 1.22 27.18 12.24 . 8.47 14.4 1.43 23.61 13.68 9.91 15.48 0.99 29.6 15.12 6.85 Table C.18 Ferrate Formation at TNaOH] » 10 q.mol/1 S u p e r f i c i a l Current Density * 10 KA/m2 TNaOH] K 10 Q.mol/1 1.125 1.74 1.74 0.56 15.44 2-3 2.40 4.14 1.71 20.44 3.45 2.35 6.49 2.88 20.42 4.6 2.42 8.91 4.03 . 21.07 5.75 2.28 11.19 5.18 19.82 6.9 2.21 13.4 6.33 19.20 8.1 2.13 16.53 7 .5 17.77 9.4 2.35 17.88 8.75 18.11 10.59 • 2.12 20.00 10.00 17.83 11.74 1.79 21.79 11.17 15.58 12.85 1.69 23.48 12.30 15.23 14.0 1.56 25.04 13.43 13.52 15.13 .1.39 26.43 14.57 12.28 16.26 1.20 27.63 15.70 10.61 17.37 1.00 28.63 16.82 8.98 18.55 0.81 29.44 17.96 6.88 19.66 0.55 29.99 19.11 4.95 20.81 0.41 30.4 20.24 3.61 21.95 0.29 30.69 21.38 2.56 23.2 0.19 30.88 22.58 1.55 24.45 0.13 31.01 23.83 1.03 25.6 0.09 31.10 25.03 0.76 193 Tables C.19 to C.22 Beaker Experiments - Type I Anode Material = Armco Iron Temperature = 50°C [NaOH] =14.3 g.mol/1 Coulombs Total Iron Removed Total Iron Removed of Anode Area from the From the Anode Charge Anode Per Unit Area Passed of Anode Coulombs q.mol cr/ of anode(x 10' 1 ) (cm2) g.mols (x 10~6) cm2 (x l0~ 6 ) " Table C.19 Iron Removal from Iron Anodes at 5 KA/m2 Superficial Current Density = 5 KA/m2 5 1.395 313 224 1.80 416 231 10 1.40 494 353 11 1.215 569 468 20 1.30 616 474 1.748 1290 738 2.10 1147 546 30 1.40 631 450 1.33 593 446 Table C.20 Iron Removal from Iron Anodes at 10 KA/m2 Superf ic ial Current Density = 10 KA/m2 5 0.60 121 202 1.575 348 198 10 0.60 170 284 1.80 6.7 343 20 0.60 197 328.8 1.90 1174 618 2.25 1690 751 30 0.5775 199 345 0.60 ' 215 358 Tables C. 19 to C.22 Beaker Experiments - Type I Anode Material = Armco Iron Temperature = 50°C [NaOH] = 14.3 g.mol/1 Coulombs Total Iron Removed Total Iron Removed of . Anode Area ... from the From the Anode Charge Anode Per Jnit Area Passed . . . of Anode Coulombs (cm2) q.mol cm of anode(x 10* ) g.mols (x 10"^) 2 cm (x lO" 6 ) Table C.21 2 Iron Removal from Iron Anodes at 60 KA/m 2 Superf ic ial Current Density = 60 KA/m 6 0.998 218 218 0.5497 158 243 10 0.855 294 344 0.5514 175 265 20 0 . 8 8 " ; 408 464 0.6131 218 356 30 0.96 419 437 0.67 309 461 0.621 396 - 638 Table C 22 Iron Removal from Magnetite Anodes at 10 KA/m Anode Material = Massive Magnetite Temperature = 50°C Superf icial Current Density = 10 KA/m [NaOH] = 14.3 g.mols/1 5 1.8 37.8 21 1.8 21.6 12 10 1.8 32.4 18 1.8 54 30 20 1.8 46.8 26 1.8 37.8 21 30 • 1.8 48.6 27 1.8 63 35 Tab le s C 2 3 t o C.33 Beaker Expe r imen t s - Type 2 (Armco I r on Anodes) Temperature - 50°C C u r r e n t Passed Coulombs I n t e r v a l T o t a l I r on g.mol VT o t a l I r o n q.mol Mean Rate o f q.mol 2 cm (x 1 0 3 ) U n i t A rea (x 1 0 " 4 c m 2 ) U n i t Area cm (x 1 0 * 4 ) I r on L e a v i n g . . , t he Anode C o u l o m b (x 1 0 " 9 ) T a b l e C.23 2 I r on Removal a t 50 KA/m S u p e r f i c i a l C D . = 50 KA/m 2 [NaOH] = 14.3 g mol/1 Anode Area = 4 .155 c m ' 18 2.931 2.931 16.3 36 2.35 5.28 13.1 54 1.19 6.47 6.6 72 1.72 8.19 9.6 90 1.36 9.55 7.6 108 1.73 11.28 9.6 144 3.40 14.68 9.4 180 3.60 18.28 10 .0 T a b l e C.24 I r o n Removal a t 25 KA/m 2 S u p e r f i c i a l C D . = 25 KA/m 2 [NaOH] = 14.3 g.mol/1 Anode Area = 3.8 cm 9 1.92 1.92 21.3 18 1.26 3.18 14.0 27 1.27 4.45 14.1 36 1.13 5.57 12.6 45 0.91 6.48 10.1 54 0.94 7.42 10.4 72 1.29 8.71 7.2 90 1.24 9.95 6.-9 T a b l e C.25 I r o n Removal a t 10 KA/m 2 S u p e r f i c i a l C D . = 10 KA/m 2 [NaOH] = 14.3 g.mol/1 Anode Area • .3.8 cm' 3.6 0.9 0.90 25 .0 7.2 0.70 1.60 19.4 10.8 1.08 2.68 30.0 14.4 0.84 3.52 23 .3 18.0 0.63 4.16 17 .5 21.6 0.35 4.51 9.7 25 .2 0.40 4.91 11.1 28.8 0.26 5.18 7.2 32.4 . 0.41 5.59 11.4 36.0 , 0.51 6.1 14.2 Tab l e s C.23 t o C.33 Beaker Expe r imen t s - Type 2 (Armco I r on Anodes) • 196 Temperature = 50°C C u r r e n t Passed Coulombs I n t e r v a l T o t a l I r on g.mol ^ T o t a l I r on q.mol Mean Rate o f q.mol 2 cm (x 1 0 3 ) U n i t A rea (x 1 0 - " cm 2 ) • f U n i t Area cm (x l O " 4 ) I r on L e a v i n g •the Anode Coulomb (x 1 0 ' 9 ) T a b l e C.26 I r o n Removal a t 5 KA/m 2 S u p e r f i c i a l C D . = 5 KA/m 2 [NaOH] « 14 .3 g.mol/1 Anode Area = 3.8 cm c 2.5 0.91 0.91 36 4 5.0 0.55 1.46 22 0 7.5 0.37 1.83 14 9 10-0 0.40 2.23 15 9 12.5 0.35 2.57 13 9 17 .5 0.50 3.07 9 9 22 .5 0.52 3.59 10 5 27.5 0.56 4.15 11 1 32.5 0.61 4.75 12 11 40 .0 0.61 5.36 8 1 47 .5 0.76 6.12 10.1 T a b l e C.27 I r on Removal a t fNaOH] = 1 g.mol/1 S u p e r f i c i a l C D . = 10 KA/m [NaO H] * 1 g.mol/1 Anode A rea = 3.8 cm 5.0 0.12 0.12 2 5 15.3 0.19 0.31 1 8 25 .5 0.20 0.51 1 7 30.6 0.09 0.60 1 7 40.2 0.13 0.73 1 3 • 51.5 0.10 0.82 0 8 61 .5 0.07 0.89 0 7 71.5 0.05 0.94 0 55 81.5 0.06 1.00 . 0 57 T a b l e C.28 I r on Removal a t [NaOH] = 5 g.mol/1 S u p e r f i c i a l C D . = 10 KA/m 2 • [NaOH] = 5 g.mol/1 = 3.8 cm 2 Anode Area 5.0 0.49 0.49 9. 8 10.0 0.45 .0.94 9. 1 15 .0 0.30 1.24 6. 0 20 .0 0.25 1.49 5 1 25 .0 0.34 1.83 6. 8 30 .0 0.45 2.28 8.9 j 35.0 0.32 2.60 6. 5 . 40 !0 0.41 3.01 8. 1 45.4 0.33 3.35 • 6. 2 50.4 0.28 3.63 •5. 7 Tab l e s C.23 t o C.33 Beaker Expe r imen t s - Type 2 (Armco I r on Anodes) 1 97 Temperature = 50°C C u r r e n t Passed Coulombs I n t e r v a l T o t a l I r on g.mol V T o t a l I r o n q.mol Mean Rate o f q.mol c m 2 (x 1 0 3 ) U n i t A rea (x 1 0 ' cm 4 ) U n i t A rea cm (x 1 0 ' 4 ) l r ° " L ? a v ^ n 9 Coulomb the Anode (x 1 0 ~ 9 ) T a b l e C.29 I r o n Removal a t [NaOH] = 10 q.mol/1 S u p e r f i c i a l C D . = 10 KA/m' [NaOH] - 10 g.mol/1 Anode Area = 3.8 c m ' 5.0 0.80 0.80 23.2 10.0 1.08 1.88 21.6 15.2 0.32 2.20 6.2 20.2 0.48 2.69 9.6 25 .2 0.29 2.98 5.8 30.2 0.43 3.41 8.6 35.2 0.36 3.77 7-2 40 .3 0 .20 3.97 4 .0 45 .3 0.47 4.44 9.4 50.3 0 .25 4.69 4 .9 T a b l e C.30 I r on Removal a t [KOH] = 10 g.mol/1 S u p e r f i c i a l C D . = 10 KA/ir 2 [KOH] = 10 g.mol/1 Anode Area * 3.8 i. cm 5.0 0.92 0.92 18.3 10.0 0.63 1.54 12.6 15.0 0 .43 1.97 8.6 20 .0 0.62 2.59 12.3 25 .3 0.47 3.06 8.9 30.3 0.27 3.33 5.3 35 .3 0.26 3.58 5.1 41 .5 0.22 3.80 3.5 46 .5 0.22 4.01 4 .3 51.6 0 .28 4.30 5.6 198 Tab !e € . 3 1 T o t a l I ron Removed f rom I ron Anodes a f t e r 30 Ks 2 9 10 KA/m f o r V a r i o u s Sodium H y d r o x i d e C o n - c e n t r a t i o n s . Temperature = 50°C Type 2 - Beaker E x p e r i m e n t s Sodium H y d r o x i d e C o n c e n t r a t i o n g . m o l / 1 T o t a l Removed f rom Anode a f t e r 30 Ks @ 1 0 KA/m 2 g . m o l s / l (x 1 0 " 6 ) 1 60 5 228 10 341 1 4 . 3 525 199 Table C.32 Iron Removal from Iron Anodes et [NaOH] » 14.3 c.mol/1 with 0.05 K Chloride Additions Type 2 - Beaker Experiments Temperature • 50°C ? Superficial Current Density • 10 KA/m 2 Anode Area * 3.8 cm [NaOH] « 14.3 g.mol/1 [Nacd ' 0-05 g.mol/1 Current Passed Interval Mean Rate of Q.mol Coulombs Total Iron Q.mol V Total Iron g.mol cm Iron Leaving coulomb J <x 103) Unit Area cm2 (x 10"*) Unit Area (x 10" ' the Anode (x 10"') 3.6 1.02 1.02 28.3 . '-2 0.83 1.85 23.1 10.6 0.67 2.42 15.7 0.34 2.76 9.4 18.0 0.25 3.05 6.0 Table C.33 Iron Removal From Iron Anodes Undergoing Periodic Current Reversal. Type 2 - Beaker Experiment Temperature Sjperf ic ia l Current Density Electrode Area [NaO.i] Current Cycle 1) Coulombs passed with Iron Electrode as the Anode « 5 x TO3 Coulombs. 2) Coulombs passed with Iron Electrode as the Cathode • 5 « 10 Coulombs. 50°C 10 KA/m* 3.8 cm2 14.3 g.mol/1 Coulombs AnodiCfllly Coulombs Cathodical ly Total Coulombs Interval Total Iron q. mol cm V Total Iron o.mols Passed Coulombs cm2 {% 103) Passed Coulombs cm' (x 103) Passed Coulombs cm2 (x 103) Unit Area of Electrode <x 10" 4) *~ Unit Area 2 cm of Electrode. O0-") 5 0.5 5.5 1.26 1.26 5 0-E 10.5 11.0 1.03 ) ) 0.22 ) . 1.25 2.28 2.51 0.5 16.5 1.04 3.54 5 5 0.5 21.5 22.0 0.94 ) . ) 0.19 ) 1.13 4.49 4.68 0.6 27.5 1.06 5.74 5 0.5 32.5 33.0 . 0.89 ) ) 0.17 ) 1.06 6.63 6.79 5 0.5 38.5 1.13 7.93 5 0.5 . 43.5 44.0 1.00 ) ) 0.17 ) 1.17 8.93 5.10 Av.lron from Cathodl2at1on • 0.22 * 0.19 • 0.17 • 0.17 

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