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Reactions of potassium ethyl xanthate in aqueous solution Tipman, Norman Robert 1970

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THE REACTIONS OF POTASSIUM ETHYL XANTHATE IN AQUEOUS SOLUTION BY NORMAN ROBERT TIPMAN B.Sc. (Hons.) U n i v e r s i t y of A l b e r t a , 1962 A THESIS SUBMITTED IN PARTIAL';';FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of MINERAL ENGINEERING We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1970 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree t h a t permiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Depa rtment The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date ABSTRACT The r e a c t i o n s of potassium e t h y l xanthate (KEtX) and i t s o x i d a t i o n product d i e t h y l dixanthogen (EtX) ^ were stud i e d i n n e u t r a l and m i l d l y a l k a l i n e aqueous s o l u t i o n . D i s s o l v e d oxygen-was shown to be i n e f f e c t i v e i n o x i d i z i n g KEtX i n homogeneous systems ( s o l u t i o n ) , but (EtX)^ was produced e l e c t r o c h e m i c a l l y at a platinum e l e c t r o d e by a p p l i c a t i o n of a small anodic o v e r p o t e n t i a l . A slow h y d r o l y s i s r e a c t i o n was proposed f o r the a l k a l i n e decomposi-t i o n of aqueous e t h y l xanthate i n which carbon d i s u l f i d e and e t h y l a l c o h o l were the decomposition products. The f i r s t order r a t e constant -4 -1 k^ = 7.6 Z 1.0 x 10 hr (22°C) was independent of pH i n the re g i o n pH 7 to 11. -5 -7 -1 Low concentrations of (EtX)^ (10 to 10 moles l i t e r ) were determined by a new method based on e x t r a c t i o n of the aqueous ( E t X ^ by hexane followed by u l t r a v i o l e t spectrophotometric determination of the (EtX)^ i n the hexane e x t r a c t . A s e n s i t i v e technique f o r measuring s a t u r a t i o n of ( E t X ^ i n water using l i g h t s c a t t e r i n g photometry; was also developed. The s o l u b i l i t y of ( E t X ^ i n water was found to be 1.27 x 10~ 5 m o l e s / l i t e r at 22°C from pH 2 to 8.5. At pH > 8.5, aqueous ( E t X ) ^ was shown to react w i t h hydroxyl io n by a bimolecular displacement (S^2) mechanism which r e s u l t e d i n the formation of one mole of e t h y l xanthate and one mole of an i n t e r -mediate e t h y l xanthate s u l f e n i c a c i d . Decomposition of the intermediate compound r e s u l t e d i n carbon d i s u l f i d e and other r e a c t i o n products. The second order r a t e constant (k^ = 13.5 t 0.5 l i t e r mole "Snin ^ at 22°C) was determined. Rest p o t e n t i a l measurements using a platinum e l e c t r o d e i n potassium e t h y l x a n t h a t e - d i e t h y l dixanthogen s o l u t i o n s showed that the Nernst equation f o r a one e l e c t r o n r e a c t i o n was obeyed over a wide range of both KEtX and ( E t X ^ concentrations. D i s s o l v e d oxygen was found to generate mixed p o t e n t i a l s to which no o v e r a l l r e a c t i o n could be assigned. I t i s expected that the r a p i d o x i d a t i o n of e t h y l xanthate to d i e t h y l dixanthogen observed i n mineral systems proceeds by a c a t a l y t i c o x i d a t i o n or e l e c t r o c h e m i c a l r e a c t i o n at or near the mineral surface. TABLE OF CONTENTS CHAPTER 1. INTRODUCTION 1 1.1 Chemical Reactions of Xanthates 1 1.2 Reaction of Xanthates w i t h M i n e r a l Surfaces 4 1.3 Xanthate Adsorption Theories 4 1.4 Problems i n E x p l a i n i n g the Role of Oxygen i n F l o t a t i o n 6 1.4(a) Adsorbent System: The Reaction of Oxygen w i t h the M i n e r a l Surface 6 1.4(b). Adsorbate System: The Reaction of Oxygen w i t h the Di s s o l v e d Xanthate 7 1.5 The Unresolved P o i n t s 8 1.6 P l a n of the Present Work 11 CHAPTER 2. EXPERIMENTAL METHODS •.. 12 2.1 M a t e r i a l s 12 2.2 E t h y l Xanthate Decomposition Experiments 13 2.3 D i e t h y l Dixanthogen Decomposition Experiments .... 13 2.4 U l t r a v i o l e t Spectroscopic A n a l y s i s f o r Aqueous E t h y l Xanthate and Carbon D i s u l f i d e 14 2.5 U l t r a v i o l e t Spectroscopic A n a l y s i s f o r D i e t h y l Dixianthogen 18 2.6 L i g h t S c a t t e r i n g Method f o r Determining the S o l u b i l i t y of D i e t h y l Dixanthogen 19 2.7 I n f r a r e d Spectroscopic Methods 25 ..; 2.7Ca) The KBr P e l l e t Technique 25 2.7(b) I n f r a r e d C e l l f o r Aqueous S o l u t i o n s 25 2.7(c) Attenuated T o t a l Reflectance (ATR) Spectra of Platinum Surfaces 26 2.8 E l e c t r o c h e m i c a l Methods 28 2.8(a) C e l l f o r Rest P o t e n t i a l Measurements 28 2.8(b) C e l l f o r E l e c t r o d e P o l a r i z a t i o n Measurements 28 CHAPTER 3. RESULTS AND DISCUSSION 3 2 3.1 Decomposition of KEtX i n Aqueous A l k a l i n e S o l u t i o n s 32 3.1.1 Determination of Rate Law - 3 3 3.1.2 E f f e c t of Oxygen on the Decomposition Rate. 3& 3.1.3 E f f e c t of Hydroxyl Ion on the Decomposition Rate 41 3.1.4 E v a l u a t i o n of Reaction Products ^ 3 3.1.5 Q u a n t i t a t i v e Measurement of Carbon D i s u l f i d e 43 3.1.6 Mechanism of Xanthate Decomposition i n A l k a l i n e S o l u t i o n 4 6 3.1.7 Decomposition of KEtX i n the Presence of P o s s i b l e C a t a l y t i c Agents 50 3.1.7(a) Methylene Blue 51 3.1.7(b) P l a t i n i z e d Platinum 52 3.1.7(c) Ferrous S u l f a t e 53 3.2 Decomposition of D i e t h y l Dixanthogen i n Aqueous A l k a -l i n e S o l u t i o n 54 3.2.1 Stoichiometry of the Reaction of D i e t h y l D i -xanthogen w i t h Hydroxyl Ion;. 55 3.2.2 E l i m i n a t i o n of P o s s i b l e I n t e r f e r e n c e by Iodide . 5 5 3.2.3 E v a l u a t i o n of Reaction Products ... 57 3.2.4 C o n t r i b u t i o n of the Reaction Products to the 301 my Xanthate Absorption 58 3.2.5 Mechanism of the Decomposition of Dixanthogen 60 3.2.6 Reaction of Dixanthogen w i t h Other Nucleophiles ........ 61 3.3 Rate Studies on the Reaction Betweeen Dixanthogen and Hydroxyl Ion 5 9 3.3.1 Reaction Order w i t h Respect to D i e t h y l Dixanthogen Concentration 65 3.3.2 Reaction Order w i t h Respect to Hydroxyl Ion Concentration 6g 3.4 S o l u b i l i t y of D i e t h y l Dixanthogen i n Water 72 3.4.1 Measurement of S a t u r a t i o n by the E x t r a c t i o n Technique y2 3.4.2 Measurement of S a t u r a t i o n by the T u r b i d i -m e t r i c Technique 73 3.5 E l e c t r o c h e m i c a l Studies on Aqueous E t h y l Xanthate. 75 3.5.1 D e r i v a t i o n of the Nernst Equation f o r the Oxidation-^of E t h y l Xanthate to D i e t h y l Dixanthogen 76 3.5.2 Rest P o t e n t i a l Measurements w i t h a Platinum E l e c t r o d e .... 77 3.5.2Ca) E t h y l Xanthate S o l u t i o n s Free of D i e t h y l Dixanthogen 77 3.5.2(b) E t h y l Xanthate S o l u t i o n s Saturated w i t h D i e t h y l Dixanthogen ....... 79 3.5.2(c) E f f e c t of pH on the Rest P o t e n t i a l 86 3.5.2(d) E f f e c t of D i e t h y l Dixanthogen on the Rest P o t e n t i a l . 88 3.6 P o l a r i z a t i o n of the.Platinum E l e c t r o d e i n E t h y l Xanthate-Diethyl Dixanthogen S o l u t i o n s 91 3.7 I n f r a r e d I n v e s t i g a t i o n s on the Platinum Electrode Surfaces 97 3.8 I n t e r p r e t a t i o n of the E l e c t r o c h e m i c a l R esults .... 103 CHAPTER, 4. SUMMARY AND CONCLUSIONS . .... 1 0 7 APPENDIX I - Curve F i t t i n g and E r r o r A n a l y s i s I 1 2 APPENDIX I I - Decomposition of E t h y l Xanthate i n Ac i d S o l u t i o n 1 1 5 REFERENCES 120 LIST OF FIGURES Fig u r e Page 1 U l t r a v i o l e t absorption spectra of aqueous e t h y l x a n t h ate, hydroxyl i o n and carbon d i s u l f i d e 16 2 C a l i b r a t i o n curve f o r potassium e t h y l xanthate and carbon d i s u l f i d e i n water 17 3 U l t r a v i o l e t absorption s p e c t r a of d i e t h y l dixanthogen i n hexane 20 4 C a l i b r a t i o n curve f o r d i e t h y l dixanthogen i n hexane vs. hexane reference .. 21 5 M a t e r i a l balance between the concentrations of e t h y l xanthate t i t r a t e d w i t h i o d i n e and d i e t h y l dixanthogen determined by hexane e x t r a c t i o n 22 6 E f f e c t of i o d i d e on the s o l u b i l i t y of d i e t h y l dixantho-gen as determined by l i g h t s c a t t e r i n g , pH 7.5-7.9 ... 24 7 M u l t i p l e i n t e r n a l r e f l e c t i o n e f f e c t i l l u s t r a t i n g the pe n e t r a t i o n of the IR beam through the KRS-5 c r y s t a l surface . . 27 8 Apparatus f o r measuring pH and r e s t p o t e n t i a l 30 9 Apparatus f o r p o l a r i z a t i o n s t u d i e s 31 10 Decomposition of aqueous e t h y l xanthate i n (a) s o l u t i o n s saturated w i t h oxygen (b) deoxygenated s o l u t i o n s under argon atmosphere 35 11 Log-log p l o t of the decomposition rat e of aqueous e t h y l xanthate vs. i n i t i a l e t h y l xanthate co n c e n t r a t i o n f o r deoxygenated s o l u t i o n s under an argon atmosphere 38 12 Log-log p l o t of the decomposition r a t e of aqueous e t h y l xanthate vs. i n i t i a l e t h y l xanthate concentra-t i o n f o r s o l u t i o n s saturated w i t h oxygen 40 13 pH. s h i f t of e t h y l xanthate decomposition runs i n deoxygenated water under argon atmosphere 42 14 Decomposition of e t h y l xanthate i n deoxygenated water i n a sealed c e l l , pH 7.4 . • . •_ - _ j :._ (a) r a t e of xanthate decomposition, (b) r a t e of CS2 formation..... ;. . 47 Figure 15 U l t r a v i o l e t spectra of s u l f u r c o n t a i n i n g compounds i n the system CS2~NaOH-alcohol. Cl) d i t h i o c a r b o n a t e i n 5 N NaOH; (2) CS 2; C3) Na 2S i n 0.1 N NaOH; C4) e t h y l monothiocarbonate (C 2H 5 0C0SK) ; C5) t r i -thiocarbonate CCS3 =); (6) potassium e t h y l xanthate CC2H5OCSSK); (7) tet r a t h i o p e r c a r b o n a t e ( C S 4 = ) ; (8) t r i t h i o p e r c a r b o n a t e (CS30 =); ( a f t e r Hovenkamp (57)) 16 F i r s t order r a t e p l o t f o r the r e a c t i o n of aqueous d i e t h y l dixanthogen w i t h hydroxyl i o n , S e r i e s 4 17 E y a l u a t i o n of order f o r hydroxyl i o n i n the r e a c t i o n of d i e t h y l dixanthogen w i t h hydroxyl i o n 18 E f f e c t of pH on the s o l u b i l i t y of d i e t h y l dixanthogen i n water 19 E f f e c t of oxygen on the r e s t p o t e n t i a l of e t h y l xanthate s o l u t i o n s f r e e from d i e t h y l dixanthogen, pH 7.0-8.0 20 E f f e c t of oxygen on the r e s t p o t e n t i a l of e t h y l xanthate s o l u t i o n s saturated w i t h d i e t h y l dixanthogen, pH 7.0-8.0 21 Eh as a f u n c t i o n of d i s s o l v e d oxygen concentration (moles l i t e r - - * - ) i n d i s t i l l e d water, pH 6.8 ( a f t e r Natarajan and Iwasaki (66)) 22 Rest P o t e n t i a l s of a platinum e l e c t r o d e i n s o l u t i o n s saturated w i t h d i e t h y l dixanthogen and under an argon atmosphere 23 E f f e c t of pH on the r e s t p o t e n t i a l of aqueous e t h y l xanthate saturated w i t h d i e t h y l dixanthogen 24 E f f e c t of d i e t h y l dixanthogen on the r e s t p o t e n t i a l of a platinum e l e c t r o d e under argon atmosphere and constant e t h y l xanthate c o n c e n t r a t i o n , pH 6.9-7.7 ... 25 C o r r e l a t i o n of the r e s t p o t e n t i a l s of e t h y l xanthate and d i e t h y l dixanthogen s o l u t i o n s w i t h the Nernst equation - 3 26 P o l a r i z a t i o n curves f o r a platinum e l e c t r o d e i n 10 M e t h y l xanthate i n 1 N KC1 e l e c t r o l y t e - 3 27 P o l a r i z a t i o n diagram f o r a platinum e l e c t r o d e i n 10 M e t h y l xanthate, 1 M KC1 e l e c t r o l y t e Ca) p o l a r i z a t i o n over a one v o l t range (b) m a g n i f i c a t i o n of (a) to show T a f e l behavior. Figure Page 28 I n f r a r e d s p e c t r a of e t h y l xanthate compounds Ca) KEtX, s o l i d , KBr p e l l e t (b) d i e t h y l dixanthogen, c a p i l l a r y f i l m (c) e t h y l xanthate i o n s , aqueous s o l u t i o n (d) platinum xanthate, KBr p e l l e t 99 29 I n f r a r e d (ATR) spectra of platinum f o i l a f t e r v a r y i n g treatments 100 30 Rates and r e a c t i o n s of e t h y l xanthate and d i e t h y l dixanthogen i n aqueous s o l u t i o n at 22°C 109 LIST OF TABLES Table Page 1 Decomposition of E t h y l Xanthate Under Argon Atmosphere at 22° C, S e r i e s 1 #2 34 2 Decomposition Rates f o r Aqueous E t h y l Xanthate Under Argon Atmosphere at 22°C 37 3 Decomposition Rates f o r Aqueous E t h y l Xanthate Under Oxygen Atmosphere at 22° C 39 4 E f f e c t of pH on the Decomposition of E t h y l Xanthate.. 44 5 R e l a t i o n s h i p between E t h y l Xanthate Decomposition and ptt Increase 45 6 M a t e r i a l Balance Between E t h y l Xanthate Decomposed and CS2 Evolved from Sealed C e l l Measurements 48 7 Decomposition of Aqueous E t h y l Xanthate i n the Presence of P o s s i b l e C a t a l y t i c Agents 51 8 Reaction of D i e t h y l Dixanthogen w i t h Hydroxyl Ion at 22°C .. 56 9 Stoichiometry of the Reaction Between Various Nucleophiles and D i e t h y l Dixanthogen 63 10 H y d r o l y s i s of Dixanthogen w i t h Hydroxyl Ion (Seri e s 4, #3) 67 11 E v a l u a t i o n of Second Order Rate Constants f o r H y d r o l y s i s of D i e t h y l Dixanthogen 71 12 Standard Reduction P o t e n t i a l s of the Xanthate-Dixanthogen Couples 85 13 V i b r a t i o n a l Band Assignments f o r the I n f r a r e d Spectrum of Platinum E t h y l Xanthate 102 14 C o r r e l a t i o n C o e f f i c i e n t and E r r o r i n Rate Constants f o r the O x i d a t i o n of KEtX S o l u t i o n s 114 15 D i s s o c i a t i o n Constants and Decomposition Rate Constants f o r E t h y l Xanthic Acid 119 LIST OF SYMBOLS S KEtX = potassium e t h y l xanthate (CILj-CK^O-C-S K ) S S II II ( E t X ) 2 E d i e t h y l dixanthogen (CH3-CR2-0-C-S-S-C-0-CH2-CH3 S II _ EtX = e t h y l xanthate i o n (CH -CH -0-C-S ) X = xanthate E a l k a l i a l k y l xanthate CR-O-C-S Na (K )) S S ll II X = dixanthogen (R-0-C-S-S-C-O-R) R = short hydrocarbon chain M = molar c o n c e n t r a t i o n - moles l i t e r ^ ACKNOWLEDGEMENTS The h e l p f u l guidance of my research supervisor Dr. J . L e j a throughout the course of t h i s study i s g r a t e f u l l y acknowledged. I wish to thank the v a r i o u s members of the Department of M i n e r a l Engineering, Dr. L.G. Ha r r i s o n of the Department of Chemistry, and -my colleagues f o r -many h e l p f u l d i s c u s s i o n s . The f o l l o w i n g s c h o l a r s h i p s are g r a t e f u l l y acknowledged, Sherritt-Gordon Graduate Fellowship (1965) Giant Y e l l o w k n i f e Mines Graduate Scholarship (1966) U.B.C. Graduate Fellowship (1967) U.B.C. Graduate Fellowship (1968). CHAPTER I INTRODUCTION Xanthates, or a l k y l d i t h i o c a r b o n a t e s , are a s p e c i a l c l a s s of organic thiocarbonate s a l t s that have important i n d u s t r i a l uses. Since the t u r n of the century, xanthates have been used e x t e n s i v e l y i n the manufacture of v i s c o s e rayon, and to a small extent, as p e s t i c i d e s . In 1924, C H . K e l l e r discovered that .xanthates could be used to separate s e l e c t i v e l y s u l f i d e minerals from t h e i r ores. C u r r e n t l y , the widest use of xanthates i s i n the mi n e r a l i n d u s t r y , where they c o n s t i t u t e the most u s e f u l chemical f o r the s e l e c t i v e s e paration of s u l f i d e s from complex ores. Considerable research has shown that the chemical behavior and surface chemical p r o p e r t i e s of aqueous xanthates are very complex. 1.1 Chemical Reaction of Xanthates S ii - + + A l k a l i metal xanthates of formula R-O-C-S K (Na ) where R r e f e r s to a short hydrocarbon c h a i n , are e a s i l y s o l u b i l i z e d i n water ( i . e . 120 gms/1. at 20°C f o r potassium e t h y l xanthate) and r e a d i l y i o n i z e to form completely d i s s o c i a t e d xanthate i o n s . Under a c i d i c c o n d i t i o n s the xanthate i o n i s protonated, forming x a n t h i c a c i d which i s unstable and decomposes r a p i d l y i n t o carbon d i s u l f i d e and a l c o h o l . S II R-O-C-SH y ROH + CS Xanthic a c i d has been i s o l a t e d by e x t r a c t i o n i n t o non-aqueous solv e n t s such as benzene or carbon t e t r a c h l o r i d e and upon p u r i f i c a t i o n was found to have a me l t i n g p o i n t of ca. -53°C and d i s p r o p o r t i o n a t e d at 25°C (1). Under n e u t r a l and b a s i c c o n d i t i o n s , xanthates decompose s l o w l y , but the mechanism and products of these r e a c t i o n s have not yet been c l e a r l y e s t a b l i s h e d . Xanthates undergo e s t e r i f i c a t i o n when reacted w i t h a l k y l h a l i d e s or a l k y l s u l f a t e s (2). Both r e a c t i o n s proceed r a p i d l y at room tempera-tur e i n ethanol s o l v e n t . R^ and represent d i f f e r e n t a l k y l hydro-carbon chains. " _ + II Rj-O-C-S K + R 2C1 >• R 1-0-C-SR 2 + KC1 S S II _ + II R..-0-C-S K + KR.SO. y R-0-C-SRo + K.SO. 1 2 4 2 2 4 The e s t e r s can be hydrolysed by r e f l u x i n a l c o h o l i c KOH but d e s u l f u r i z a t i o n takes place during the r e a c t i o n ( 2 ) , r e s u l t i n g i n the formation of a l k y l monothiocarbonates. ' I I I _ + R-0-G-SR2 + KOH y R-O-C-0 K + R SH An important chemical property of xanthates i s t h e i r a b i l i t y to under-go r e a c t i o n w i t h a number of o x i d i z i n g agents to form dixanthogens. Examples of the r e a c t i o n of aqueous xanthates w i t h i o d i n e , copper s u l f a t e and potassium t e t r a t h i o n a t e have been given by Cambron and Whitby (3). - + S S H II 2R-0-C-S K + KI > R-0-C-S-S-C-O-R + 3KI. S S S S II _ + II » H 4R-0-C-S K + 2CuS0. > R-O-C-S-S-C-O-R + 2R-0-C-S-Cu 4 + 2K oS0. 2 4 S S II II 2R-0-C-S K + K oS.0, y R-O-C-S-S-C-O-R + 2K oS o0„ 2 4 6 I I 3 D i e t h y l dixanthogen i s a l i g h t y e l l o w s o l i d , m e l t i n g p o i n t 31.5-32°C. I t i s s p a r i n g l y s o l u b l e i n water (ca. 1.3 x 10 ^ moles/ l i t e r ) , but completely s o l u b l e i n organic s o l v e n t s and can be p u r i f i e d by vacuum d i s t i l l a t i o n (b.p. 107-9°C at 0.05 mm. Hg) (1). Diethyl dixanthogens can be d e s u l f u r i z e d by treatment w i t h potassium cyanide or sodium arsenate i n r e f l u x i n g ethanol ( 3 ) , r e s u l t i n g i n the formation of xanthogen monosulfides. S S S II II II (R-0-C-S) 2 + KCN y R-O-C-S-C-O-R + KSCN S S S II II II (R-O-C-S) + Na As0 3 • R-O-C-S-C-O-R + Na AsSO Xanthates a l s o react w i t h m e t a l l i c ions to form r e l a t i v e l y i n s o l u b l e heavy metal xanthates. The bulk p r o p e r t i e s of heavy metal xanthates have been given i n d e t a i l by Sheka and K r i s s (4). Poling;-, (5) has shown that the aqueous s o l u b i l i t i e s are r e l a t e d to the degree of covalency of the metal s u l f u r bond. For example, c o v a l e n t l y bonded -30 -20 gold and copper xanthates are l e a s t s o l u b l e (Ksp = 10 to 10 ) w h i l e i o n i c metal xanthates have s o l u b i l i t i e s i n the order of 2-5 M. Extensive s t u d i e s on the i n f r a r e d s p e c t r a of metal xanthates have been c a r r i e d out by L e j a , e t . a l . (6) and by Watt and McCormick (7). X-ray c r y s t a l l o g r a p h i c s t u d i e s on the s t r u c t u r e of Pb, Zn, Sb, As, and N i xanthates have been reported (8,9,10,11,12). 1.2 Reaction of Xanthates w i t h M i n e r a l Surfaces The f l o t a t i o n of minerals has been shown to occur because of a chemical r e a c t i o n between the xanthate and.the m i n e r a l surface. The r e a c t i o n r e s u l t s i n a change of the surface c o n d i t i o n from h y d r o p h i l i c to hydrophobic once a s u f f i c i e n t c o a t i n g ( u s u a l l y l e s s than a monolayer) of heavy metal xanthate has been formed. The chemisorption of xanthates onto mineral surfaces has been widely stu d i e d and a number of t h e o r i e s have been proposed to e x p l a i n t h i s phenomenon. 1.3 Xanthate Adsorption Theories Taggart, e t . a l . (13) o r i g i n a l l y proposed that metal xanthates are formed by a chemical exchange mechanism whereby the l e s s s o l u b l e metal xanthate i s p r e c i p i t a t e d on the more s o l u b l e mineral surface by w e l l known chemical r e a c t i o n s . However, when s o l u b i l i t y data were obtained f o r lead xanthate and were a p p l i e d to t h i s theory, i t became apparent that PbS could not r e a c t w i t h any appreciable amount of xanthate. The disco v e r y that oxygen played an e s s e n t i a l r o l e i n f l o t a t i o n complicated many t h e o r i e s . Sutherland and Wark (14) and Gaudin (15) then proposed that xanthate reacted by displacement of ions p r e v i o u s l y formed on the mineral surface. This included adsorbed ions such as hydroxide or carbonate, and ions formed by o x i d a t i o n of the s u l f i d e m i n e r a l , such as s u l f a t e or t h i o s u l f a t e . This theory enjoys the l a r g e s t p o p u l a r i t y among workers i n the f i e l d . I n c o n s i d e r i n g that o x i d a t i o n of the mineral surface r e s u l t e d i n a l a r g e negative e l e c t r i c a l double l a y e r p o t e n t i a l , Cook, e t . a l . (16,17) reasoned that adsorption of the n e g a t i v e l y charged ions of xanthate through t h i s p o t e n t i a l b a r r i e r would be impossible. A new theory was proposed whereby an uncharged molecule such as x a n t h i c a c i d was considered to be the e f f e c t i v e adsorbing s p e c i e s . Noting that the con c e n t r a t i o n of f r e e x a n t h i c a c i d i n n e u t r a l to b a s i c s o l u t i o n s would be almost n e g l i g i b l e , L e j a (18) suggested that dixanthogen, formed by o x i d a t i o n of xanthate i n s o l u t i o n , would be a s u i t a b l e uncharged molecule that could penetrate the e l e c t r i c a l double l a y e r and be e f f e c t i v e i n at l e a s t the f i r s t l a y e r s of adso r p t i o n . This theory, c a l l e d the " h y d r o l y t i c " adsorption mechanism i s the most recent and c o n t r o v e r s i a l of the three t h e o r i e s . 1.4 Problems i n E x p l a i n i n g the Role of Oxygen i n F l o t a t i o n I t i s w e l l known that d i s s o l v e d oxygen i s a.necessary reagent f o r the f l o t a t i o n of most s u l f i d e m i n e r a l s S t u d i e s on.the e f f e c t of d i s s o l v e d oxygen have considered i t s i n t e r a c t i o n w i t h the mineral surface (adsorbent) and i t s i n t e r a c t i o n w i t h the xanthate (adsorbate). 1.4.(a) Adsorbent system: The r e a c t i o n of oxygen w i t h the mineral surface Considerable research has been c a r r i e d out to i d e n t i f y the r e a c t i o n products formed by o x i d a t i o n of the mineral surface. In the case of galena (PbS) , Hagihara (19) i d e n t i f i e d PbSO^ as an o x i d a t i o n product. P o l i n g and L e j a (21) and Greenler (20), using i n f r a r e d s p e c t r o s c o p i c methods, showed that PbS20^ was formed during the i n i t i a l stages of o x i d a t i o n . Recently, Eadington and Prosser (22), using chemical methods, ' found that SO^ , S^O^ a n ^ S^O^ were present a f t e r exposure of the m i n e r a l surface to aqueous oxygen. P l a k s i n (23) and Tolun and Kitchener (39) showed that 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 of galena i s s i g n i f i -c a n t l y a l t e r e d by the o x i d a t i o n products formed on the surface. Studies on the r a t e of absorption of oxygen by galena and c h a l c o p y r i t e have been c a r r i e d out by Cusack (25). The r e s u l t of t h i s research has shown that a number of o x i d a t i o n J products are formed on mineral surfaces at d i f f e r e n t r a t e s . Further s t u d i e s on the chemisorption of xanthate have been complicated by the f a c t that not a l l the o x i d a t i o n products r e a c t w i t h the d i s s o l v e d xanthate at the same r a t e . For example, xanthate has been shown to reacted more r a p i d l y w i t h PbS„0„ than PbSO. formed on galena surfaces (21). 1.4.(b) Adsorbate System: The r e a c t i o n of oxygen w i t h the d i s s o l v e d xanthate. Gaudin, e t . a l . (26) f i r s t recognized that o x i d a t i o n of xanthate to dixanthogen i n aqueous s o l u t i o n may pla y an important r o l e i n the r e a c t i o n between the mineral surface and xanthate. D i b u t y l dixanthogen has been shown to be a more e f f e c t i v e c o l l e c t o r f o r the f l o t a t i o n of cement copper than K-n-butyl xanthate (27) . P o l i n g and L e j a (21) showed that m u l t i l a y e r s of le a d xanthate r e s u l t e d from the r e a c t i o n between o x i d i z e d galena surfaces and deoxygenated aqueous s o l u t i o n s c o n t a i n i n g d i e t h y l dixanthogen. Considering that oxygen alone may be e f f e c t i v e i n converting xanthate to dixanthogen, PomianowskL and L e j a (18) p o s t u l a t e d the r e a c t i o n i n n e u t r a l and m i l d l y a l k a l i n e s o l t u i o n s S S S II _ + II l| 4R-0-C-S K + C>2 + 2H 20 +- 2R-0-C-S-S-C-0-R + 40H xanthate dixanthogen The r e a c t i o n model of Pomianowski and L e j a proposed that xanthate, dixanthogen, x a n t h i c a c i d , carbon d i s u l f i d e and a l c o h o l would reach complex e q u i l i b r i u m depending on the r e l a t i v e concentrations of a l l of the species present. Rao and P a t e l published a s e r i e s of papers (28, 29,30,31) i n which they s t u d i e d the decomposition of aqueous xanthates i n n e u t r a l and a l k a l i n e c o n d i t i o n s w i t h and without the presence of m e t a l l i c s a l t s and under oxygen and carbon d i o x i d e atmospheres. In the presence of oxygen, xanthate s o l u t i o n s (28) were p o s t u l a t e d to undergo two types of r e a c t i o n s : o x i d a t i o n to dixanthogen and h y d r o l y t i c decomp-o s i t i o n to K„S, K CO., and a l c o h o l . In the presence of f e r r i c (29) or lead (31) s a l t s , the o x i d a t i o n of aqueous b u t y l and amyl xanthates were increased but e t h y l xanthate was decreased. Copper s a l t s (30) were shown to enhance the o x i d a t i o n of a l l xanthate homologues at n e u t r a l pH. The decomposition of xanthate s o l u t i o n s i n strong a l k a l i n e medium (0.1 to 10 M) was studi e d by P h i l i p p and F i c h t e (32) w h i l e i n v e s t i -g a t i n g the r i p e n i n g of v i s c o s e rayon ( c e l l u l o s e xanthate). Subsequent work by Hovenkamp (33), Dautzenburg and P h i l i p p (34) and others (35,36) have p o s t u l a t e d that the decomposition of xanthate would proceed by three p o s s i b l e mechanisms. S II (1) ROCS S II (2) ROCS S II (3) ROCS The c o n d i t i o n s under which each r e a c t i o n would be e f f e c t i v e was pos t u l a t e d to be dependent upon [OH ], but considerable overlapping of r a t e s was expected, p a r t i c u l a r i l y at [OH ] > 1.0 M. The problem of xanthate o x i d a t i o n i n aqueous s o l u t i o n i s c l e a r l y shown to be complex and no systematic conclusions appear to be a v a i l a b l e . + H 20 — R O H + CS 2 + OH + OH * ROH + CS 20 S II _ + OH > ROCO + SH - OH + ROH + C0 3 + 2SH 1.5 The Unresolved P o i n t s Pomianowski and L e j a (18) developed a r e a c t i o n model f o r the decomposition of aqueous e t h y l xanthates. They noted that the concentrations of e t h y l xanthate, d i e t h y l dixanthogen and carbon d i s u l f i d e (as determined s p e c t r o s c o p i c a l l y ) reached a s t a t i o n a r y s t a t e a f t e r 15 to 20 days of r e a c t i o n i n a closed v e s s e l at pH 6.5-7.1. These workers concluded that an e q u i l i b r i u m had been reached between the va r i o u s species and the d i s s o l v e d oxygen p a r t i c i p a t e d as the o x i d i z i n g agent i n the r e a c t i o n s . This r e a c t i o n model was quesioned by F i n k e l s t e i n (37) who demonstrated that oxygen had no e f f e c t on the decomposition r a t e of aqueous e t h y l xanthate. He p o s t u l a t e d that the aqueous e t h y l xanthate decomposed by a complex h y d r o l y t i c mechanism that was based on the work of P h i l i p p and F i c h t e (32) and has been summarized i n the previous review s e c t i o n (1.4.(b)). Since F i n k e l s t e i n had not undertaken any measurements f o r the co n c e n t r a t i o n of d i s s o l v e d d i e t h y l dixanthogen during the course of h i s experiment, he had not confirmed whether d i e t h y l dixanthogen was or was not a r e a c t i o n product. Consequently, there was no b a s i s f o r r e j e c t i n g e i t h e r the approach of Pomianowski and L e j a , or that of F i n k e l s t e i n . E l e c t r o c h e m i c a l i n v e s t i g a t i o n s on the o x i d a t i o n of xanthate have shown that dixanthogen was a product of the r e a c t i o n , which has been w r i t t e n S n S S H II ROCSSCOR 2R0CS + 2e Rest p o t e n t i a l measurements have been c a r r i e d out usi n g a platinum e l e c t r o d e (38,39,40), s i l v e r e l e c t r o d e (41,42), and mercury el e c t r o d e (43). The m a j o r i t y of r e s u l t s are i n general agreement that the standard e l e c t r o d e p o t e n t i a l f o r the e t h y l x a n t h a t e - e t h y l dixanthogen couple i s approximately -0.07 v o l t s . F i n k e l s t e i n (37), using t h i s value and combining i t w i t h the oxygen-hydroxyl i o n h a l f c e l l r e a c t i o n ( v i z . 0 2 + 2H 20 + 2e~ -—r 40H~ E° = +0.401 V) c a l c u l a t e d that the e q u i l i b r i u m constant obtained from spectroscopic data by Pomianowski and L e j a (18) was i n e r r o r by -0.17 V. F i n k e l s t e i n (37) attempted to r e s o l v e t h i s discrepancy by employing redox i n d i c a t o r s i n e t h y l xanthate s o l u t i o n s and measuring the of the e t h y l x a n t h a t e - d i e t h y l dixanthogen couple without the i n t r o d u c t i o n of an e l e c t r o d e surface. The method could be questioned s i n c e the p o s s i b i l i t y of a chemical r e a c t i o n between the e t h y l xanthate and the i n d i c a t o r would lead to erroneous r e s u l t s . A number of questions regarding the chemistry of aqueous xanthate s o l u t i o n s have become apparent from the s t u d i e s conducted by the v a r i o u s workers. (1) What i s the mechanism of the decomposition r e a c t i o n of e t h y l xanthate under n e u t r a l and s l i g h t l y b a s i c c o n d i t i o n s ? Does i t f o l l o w an o x i d a t i o n mechanism to dixanthogen as suggested by Pomianowski and L e j a (18), or a h y d r o l y t i c decomposition mechanism as p o s t u l a t e d by P h i l l p p and F i c h t e (32)? (2) What are the r e a c t i o n s of dixanthogen under b a s i c c o n d i t i o n s and are they d i f f e r e n t from the r e a c t i o n of xanthate? (3) Are the e l e c t r o c h e m i c a l measurements i n d i c a t i v e of a r e v e r s i b l e e l e c t r o d e p o t e n t i a l between xanthate and dixanthogen as has been suggested or are the measurements subject to i n t e r f e r e n c e by the formation of other compounds such as m e t a l l i c xanthates? The s t u d i e s on the base decomposition of (EtX)^ have shown that the c o n c e n t r a t i o n of ( E t X ) 2 i s r a p i d l y reduced when the aqueous s o l u t i o n contained s mall amounts of n u c l e o p h i l i c reagents. This o b s e r v a t i o n appears to be i n c o n s i s t e n t w i t h other observations that ( E t X ) 2 i s present i n m i n e r a l systems i n d e t e c t a b l e q u a n t i t i e s . Such evidences have been obtained by Fuerstenau, et a l . (71), Gaudin, et a l . (72) and Majima and Takeda (40) who have shown that the formation of dixanthogen i s necessary f o r the f l o t a t i o n of p y r i t e . In order to e x p l a i n the formation of 1:1 metal:xanthate surface complexes f o r ads o r p t i o n of e t h y l xanthate on galena, L e j a (73) considered that dixanthogen was necessary f o r attachment to the mineral surface i n at l e a s t the f i r s t adsorbed l a y e r . Studies on adsorption of xanthate and dixanthogen on galena ( 5 ) , copper (27) and mercury (90) have shown that dixanthogen i s an e f f e c t i v e reagent i n the absence of oxygen. Obviously, the o x i d a t i o n of xanthate to dixanthogen at the mineral surface i s an important process i n the f l o t a t i o n of m i n e r a l s . 1.6 P l a n of the Present Work The plan of work covered i n t h i s t h e s i s was a study of the problems of homogeneous r e a c t i o n s of aqueous e t h y l xanthate faced by the previous workers i n the f i e l d using modern a n a l y t i c a l techniques. I t was hoped that the l a c k of agreement between these workers could be r e s o l v e d by e v a l u a t i n g each chemical system f o r the mechanism of i t s r e a c t i o n . The o b j e c t i v e s can be given as f o l l o w s : (a) to evaluate the decomposition of aqueous e t h y l xanthate i n argon and oxygen atmospheres and determine the mechanism from the r a t e and products of the r e a c t i o n . (b) to evaluate the r e a c t i o n between d i e t h y l dixanthogen and hydroxyl i o n and i f p o s s i b l e , to e s t a b l i s h a mechanism f o r the r e d u c t i o n r e a c t i o n . (c) to review the r e s u l t s of the e l e c t r o c h e m i c a l i n v e s t i g a t o r s and determine i f 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 of e t h y l xanthate and the aqueous o x i d a t i o n of e t h y l xanthate are two compatible systems. CHAPTER 2 EXPERIMENTAL METHODS 2.1 Materials The potassium e t h y l xanthate (KEtX) was prepared by a method described by L i t t l e and Leja (44) and Bulmer and Mann (45). Sodium ethoxide was;prepared by d i s s o l v i n g one mole (40 gms) NaOH i n 500 mis ethanol. The product was cooled i n an i c e bath and one mole (76 gms) CS^ was added dropwise with s t i r r i n g while maintaining the temperature at l e s s than 10°C. The s o l i d ethyl xanthate product was f i l t e r e d , r e c r y s t a l l i z e d three times from acetone, then washed with d i e t h y l ether and dried i n vacuum. The f i n a l product analysed 98+% p u r i t y by iodine t i t r a t i o n . Diethyl dixanthogen ((EtX^) was prepared by the addition of aqueous iodine to neutral aqueous KEtX sol u t i o n s , and the milky suspension was extracted with either hexane or d i e t h y l ether. The organic layer was washed several times with d i s t i l l e d water and the f i n a l product was prepared by vacuum evaporation of the solvent. The melting point of the s o l i d yellow diethyl dixanthogen product was 32°C which agreed with the value reported i n the l i t e r a t u r e (1). Double d i s t i l l e d water was deoxygenated by b o i l i n g i n a narrow necked f l a s k and bubbling argon through the so l u t i o n . The f l a s k was sealed with a rubber septum and allowed to cool under a p o s i t i v e argon pressure. The argon, "Linde" high purity grade, with specified oxygen content (< 5 ppm) was further purified by passing through a gas train consisting of s i l i c a gel (H^ O removal), ascarite (CO^ removal), copper gauze heated to 300°C (0^ removal) and drierite (I^O removal). Oxygen saturated water was prepared from deoxygenated double d i s t i l l e d water by bubbling 1"Linde" high purity oxygen through the solution for 2 to 3 hours. The "Linde" high purity oxygen was further purified by an ascarite (CC^ removal) and drierite gas train. 2.2 Ethyl Xanthate Decomposition Experiments Decomposition runs were carried out in 1000 ml volumetric flasks that had been f i l l e d with deoxygenated double d i s t i l l e d water. The flask was sealed with a rubber septum and samples were removed with a 50 ml hypodermic syringe. Argon or oxygen was admitted into the flask, also by hypodermic needle, to maintain a positive gas pressure inside the flask during sampling. The flasks were stored in a covered constant temperature bath to exclude light from the samples. 2.3 Diethyl Dixanthogen Decomposition Experiments 200 ml aliquots of stock xanthate solution in deoxygenated double d i s t i l l e d water at 22°C and neutral pH were poured into a 250 ml flask in an argon glove bag. Known concentrations of (EtX)^ were then prepared by injecting small volumes (< 1 ml) of 0.1 M KI^ into the KEtX solution. pH adjustment of the KEtX solution to basic conditions was also made by injection of small volumes of strong KOH with a 1 ml syringe. The solution was stirred vigorously with a magnetic stir r e r and samples were removed as r e q u i r e d f o r u l t r a v i o l e t s pectroscopic a n a l y s i s or f o r the determination of ( E t X ) 2 c o n c e n t r a t i o n . The blank f o r the reference beam of the u l t r a v i o l e t spectrophoto-meter contained K l i n a concentration equal to the c o n c e n t r a t i o n of K I ^ used f o r preparing the ( E t X ^ . By t h i s method, the i o d i d e absorbances at 225 my and 192 my were blanked out. 2.4 U l t r a v i o l e t Spectroscopic A n a l y s i s f o r Aqueous E t h y l Xanthate and Carbon D i s u l f i d e Stock s o l u t i o n s of KEtX were prepared by a d d i t i o n of weighed q u a n t i t i e s of the f i n e c r y s t a l l i n e powder to s l i g h t l y b a s i c (pH 7.5-8.5) deoxygenated double d i s t i l l e d water. The absorption spectrum obtained on a Perkin-Elmer Model 450 U l t r a v i o l e t Spectrophotometer showed two strong peaks at 301 my and 226 my (Figure 1). C a l i b r a t i o n of the two peaks showed that Beer's law was obeyed f o r these s o l u t i o n s the 301 my absorption (e^g-^ = 17,500 l i t e r mole "^cm "*") and the 226 my (Figure 2). The molar e x t i n c t i o n c o e f f i c i e n t s were determined f o r DO l i t e i -1 " I absorption (^26 = ^»^-^ l i t e r mole cm ). These values compared w i t h £301 = 17,750 l i t e r mole "*"cm and ^ 2 6 = ^ > ^ ^ ® l i t e r mole "*"cm ^ determined by Pomianowski and L e j a (18) and e^oi = 17,460 l i t e r mole "*"cm ^ and ^226 = ^ > 7 ^ l i t e r mole "'"cm ^ as determined by Ma j ima (46). The xanthate absorption i n the u l t r a v i o l e t r e gion has been a t t r i -buted to e l e c t r o n i c t r a n s i t i o n s i n the f u n c t i o n a l group of the xanthate molecule. Shankaranarayana and P a t e l (89) have t e n t a t i v e l y assigned the 301 my a b s o r p t i o n to a TT—TT t r a n s i t i o n and the 226 my absorption to a n-a t r a n s i t i o n based on an a n a l y s i s of the behavior of each band to changes i n the p o l a r i t y of the sol v e n t . In t h i s r e p r e s e n t a t i o n 7 7 denotes a bonding and T T and antibonding T T o r b i t a l , n denotes a non-bonding e l e c t r o n l o c a l i z e d on the s u l f u r of the d i t h i o c a r b o n a t e group * 1 and a denotes an antibonding a o r b i t a l . A n a l y s i s f o r carbon d i s u l f i d e was a l s o c a r r i e d out spectrophoto-m e t r i c a l l y by measurement of the C=S absorption at 206.5 my (Figure 1). C a l i b r a t i o n of the peak was obtained by i n j e c t i o n of small volumes (.5, .7, 1.0 y l ) of l i q u i d CS^ from a c a l i b r a t e d 5 y l syringe i n t o l i t e r f l a s k s f i l l e d w i t h d i s t i l l e d water. The f l a s k s were completely f i l l e d and sealed. A f t e r the sm a l l d r o p l e t s had d i s s o l v e d (3-5 hours) the f l a s k s were s t i r r e d and 1 cm cuvettes were f i l l e d f o r U.V. determination The assignment of e l e c t r o n i c t r a n s i t i o n s f o r the U.V. spectrum of xanthates using the method of Shankaranarayana and P a t e l i s not d e f i n i t i v e i n g i v i n g the c o r r e c t t r a n s i t i o n s . Dr. Bree, of the Department of Chemistry, U.B.C. recommended that more exact methods must be used to a s c e r t a i n the nature of the band absorptions. The a d d i t i o n a l i n f o r m a t i o n might be obtained by the f o l l o w i n g methods: (1) experiments i n which p o l a r i z e d u l t r a v i o l e t l i g h t i s d i r e c t e d on e i t h e r s i n g l e c r y s t a l s of xanthate or frozen aqueous xanthate s o l u -t i o n s . From the p o l a r i z a t i o n angle of the emitted r a d i a t i o n , a s s i g n -ments of e i t h e r a or TT bond absorption f o r each of the 301 my and 226 my frequencies can be made. (2) Hiickel Molecular O r b i t a l c a l c u l a t i o n s would be re q u i r e d to define the probable e l e c t r o n i c t r a n s i t i o n s and the r e l a t i v e energies of the T T — T T and n-a t r a n s i t i o n s . Comparison of the r e s u l t s of these c a l c u -l a t i o n s w i t h experimental data would e s t a b l i s h the c o r r e c t band assignments. Wavelength (my) Figure 1. U l t r a v i o l e t absorption spectra of aqueous ( ^ e t h y l xanthate, hydroxyl i o n and carbon d i s u l f i d e . of the CS^ absorbance. The C=S molar e x t i n c t i o n c o e f f i c i e n t was found to be 72,000 l i t e r mole ^cm ^. Previous e v a l u a t i o n by Pomianowski and L e j a (18) had placed e between 60,000 and 70,000 l i t e r mole ^cm CS 2 2.5 U l t r a v i o l e t Spectroscopic A n a l y s i s f o r Dixanthogen Rapid and accurate methods f o r the determination of small q u a n t i t i e s of d i s s o l v e d dixanthogen d i d not appear to e x i s t i n the l i t e r a t u r e . A method reported by Shankaranarayana and P a t e l (47) i n v o l v e d the re d u c t i o n of d i s s o l v e d dixanthogen by heating the t e s t s o l u t i o n w i t h NH^NO^ and KCN, then o x i d i z i n g the r e s u l t a n t SCN w i t h and back t i t r a t i o n of the i o d i d e produced w i t h Na2S20.^ Recently, Mamiya and Majima (48) have reported an a n a l y t i c a l method wherein the dixanthogen i s e x t r a c t e d from the aqueous s o l u t i o n w i t h p y r i d i n e , and the o p t i c a l d e n s i t y of the pyridine-dixanthogen e x t r a c t i s determined at 420 my 4 - 1 - 1 (e = 1.59 x 10 l i t e r mole cm ). Unfo r t u n a t e l y , both a n a l y t i c a l methods are subject to i n t e r f e r e n c e by the presence of m e t a l l i c ions and were consequently not s u i t a b l e for the bulk of the present testwork. A new a n a l y t i c a l method was developed based on the e x t r a c t i o n of dixanthogen by hexane combined w i t h u l t r a v i o l e t s pectroscopic a n a l y s i s of the organic l a y e r . Numerous organic s o l v e n t s , such as chloroform, benzene, toluene, carbon t e t r a c h l o r i d e , hexane and d i e t h y l ether were t e s t e d , but only s p e c t r a l grade hexane (Baker "Analysed", b o i l i n g p o i n t range 0.2°C) was found to have a s u i t a b l e s p e c t r a l window, a high e x t r a c t i o n c o e f f i c i e n t and d i d not a f f e c t the d i s s o l v e d xanthates or metal xanthates that were present. A t y p i c a l U.V. spectrum f o r (EtX)„ in hexane is given in Figure 3. The "Baker" hexane was found to have a useful window region to 215 mp. -4 -1 Beer's Law was obeyed up to concentrations of 10 mole l i t e r (EtX) 2 in hexane (Figure 4). The molar extinction coefficient of the two major peaks were also determined, e O Q O = 8,600 l i t e r mole ''"cm ^ and Zoo £238 = ^-7,800 l i t e r mole ''"cm "*". The accuracy and reproducibility of the method was tested by extraction of known quantities of (EtX)^ from KEtX solutions. (EtX) 2 was produced stoichiometrically by the reaction between iodine and stock KEtX solutions as given by the equation 2EtX + I 3 > (EtX) 2 + 31 (1) A material balance between the quantity of iodine added, the moles of EtX consumed and the moles of (EtX) 2 extracted i s given in Figure 5. —6 The limit of detectability of the method was estimated to be 10 mole l i t e r ^ (EtX) 2 i n hexane, but the sensitivity could be increased by using a hexane:aqueous sample ratio less than one, with the result that aqueous (EtX>2 concentrations as low as 1-x 10 ^ moles l i t e r "*" could be measured. 2.6 Light Scattering Method for Determining the Solubility of Diethyl Dixanthogen Turbidity measurements were used by Kakovskii, et.al. (49) to determine the solubility of bis-dithiophosphate esters in aqueous solution. Unfortunately, the experimental details of their method have not been reported. Concentration (moles l i t e r x 10 ). Figure 4. C a l i b r a t i o n curve f o r d i e t h y l dixanthogen i n hexane vs. hexane reference. 10.0 8.0 . T (fl d)s rH O e CO/v C o o 6.0 " 4.0 -2.0 -1.0 O (EtX>2 determined from a d d i t i o n (EtX)^ determined from hexane e x t r a c t i o n — i — 2.0 3.0 4.0 5.0 6.0 7.0 ( E t X ) 2 (moles x 10 ) Figure 5. M a t e r i a l balance between the concentrations of e t h y l xanthate t i t r a t e d w i t h i o d i n e and d i e t h y l dixanthogen determined by hexane e x t r a c t i o n . The p r i n c i p l e advanced by these workers was developed f o r measur-i n g the s o l u b i l i t y of d i e t h y l dixanthogen i n water. The method i n v o l v e d measurement of the t u r b i d i t y of (EtX) s o l u t i o n s as the conce n t r a t i o n of (EtX)2 reached s a t u r a t i o n and then became supersaturated or t u r b i d . The various concentrations of (EtX)^ were produced by a d d i t i o n of known q u a n t i t i e s of i o d i n e to standard KEtX s o l u t i o n as given by equation 1. The maximum s o l u b i l i t y was determined by e x t r a p o l a t i n g the t u r b i d i t y (T) readings to zero t u r b i d i t y (see Figure 6). The technique was found to be very s e n s i t i v e to small changes i n concentra-t i o n at the s o l u b i l i t y l i m i t , and the experimental e r r o r was estimated as ± 0.5 x 10 ^ moles l i t e r The measurements were made w i t h a Brice-Phoenix Model 2000 L i g h t -S c a t t e r i n g Photometer using a 3 cm c e l l . A f t e r a d d i t i o n of i o d i n e to the stock KEtX s o l u t i o n , the f l a s k was a g i t a t e d and a p o r t i o n of the s o l u t i o n was immediately t r a n s f e r r e d to the 3 cm c e l l . Readings f o r G(s) and G(w) were obtained from measurements of the galvanometer d e f l e c t i o n at 0° and 90° to the i n c i d e n t l i g h t (436 mp). The absolute t u r b i d i t y (T) was c a l c u l a t e d from the equation T = k(G(s)/G(w)) The constant (k) inc l u d e d terms r e l a t e d to the instrument and operating c o n d i t i o n s , such as type of c e l l , s o l v e n t , r e f r a c t i o n e f f e c t s and f i l t e r s used. Figure 6 i l l u s t r a t e s three runs i n which v a r i o u s concentrations of K l were added to determine the e f f e c t of the r e s i d u a l i o d i n e on the •H T3 •H X> S-i 4 J s 100 J 90 80 70 60 ' 50 -| 40 30 20 -10 -0.5 1.0 1.5 2.0 2.5 No- I added O 10 4 M KI A 5 x 10 M KI 3.0 3.5 Concentration of (EtX) (moles l i t e r 1 x 10 5) Figure 6. E f f e c t of iodid e on the s o l u b i l i t y of d i e t h y l dixanthogen as determined by l i g h t s c a t t e r i n g , pH 7.5-7.9. s o l u b i l i t y of ( E t X ^ . Accurate measure of the concentrations of (EtX)2 were obtained using the hexane e x t r a c t i o n method (Se c t i o n 2.5) and these r e s u l t s were compared w i t h the concentrations of i o d i n e added. The [ ( E t X ) 2 ] i n the s o l u t i o n measured from the two methods were i n agreement to T 0.1 x 10 ^ mole l i t e r . : " ^ . 2.7 I n f r a r e d Spectroscopic Methods 2.7(a) The KBr P e l l e t Technique The KBr p e l l e t method, developed by Stimson and O'Donnell (50) i s now a standard method f o r preparing s o l i d samples f o r i n f r a r e d s p e c t r o s c o p i c study. The KBr p e l l e t d i e (Perkin-Elmer Model 186 - 0025) was loaded w i t h a powder mix of 0.5 gm KBr co n t a i n i n g 2.0 to 3.0 mg of sample. The die was then evacuated f o r f i v e minutes and pressed at 30,000 l b . load f o r one minute. The p e l l e t s were removed from the d i e and suspended i n the i n f r a r e d beam w i t h a s p e c i a l holder. 2.7(b) I n f r a r e d C e l l f o r Aqueous S o l u t i o n s Ordinary i n f r a r e d c e l l s (NaCl) were not s u i t a b l e f o r o b t a i n i n g s p e c t r a of aqueous xanthate s o l u t i o n s because of t h e i r s o l u b i l i t y i n water. KRS-5 ( t h a l l i u m bromide-iodide) was not s u i t a b l e because of the r e a c t i o n between KRS-5 and xanthate to form t h a l l i u m xanthate. A s p e c i a l i n f r a r e d c e l l was constructed from a bag of "Handi-Wrap" p o l y -ethylene c o n t a i n i n g a 0.2 mm IR window spacer. The t e s t s o l u t i o n was i n j e c t e d i n t o the bag which was then sealed to NaCl windows w i t h "Nuj;ol" hydrocarbon o i l . This procedure e l i m i n a t e d the problems produced by i n t e r f e r e n c e f r i n g e s when the bag alone was placed i n t o the i n f r a r e d beam. The c e l l had a window region from 1400 cm ^ to 700 cm which was s u i t a b l e f o r the d e t e c t i o n of xanthate species. 2.7(c) Attenuated T o t a l Reflectance (ATR) Spectra of Platinum Surfaces The development of the ATR technique as a t o o l f o r use i n i n f r a r e d s p e c troscopic s t u d i e s was reported independently by Fahrenfort (51) and H a r r i c k (52). The l a t t e r has r e c e n t l y published a book (53) which contains an exhaustive treatment of the theory and i n c l u d e s examples i l l u s t r a t i n g the wide use of ATR i n the study of powders, f i b e r s , p a i n t s and chemisorbed m a t e r i a l s on surf a c e s . Tipman and Le j a (54) employed t h i s method i n a study of xanthate adsorption on evaporated copper s u b s t r a t e s . The i n f r a r e d s p e c t r a were obtained on a Perkin-Elmer model 521 Grating I n f r a r e d Spectrophotometer using a "Wilks Model 9" m u l t i p l e ATR attachment. The beam entered the KRS-5 prism (Figure 7) from the 45° b e v e l l e d face and was t o t a l l y i n t e r n a l l y r e f l e c t e d s i n c e the angle of incidence of the beam on the prism face was greater than the c r i t i c a l angle f o r r e f r a c t i o n . The s i z e of the c r y s t a l was 5 cm x 2 cm and approximately 25 r e f l e c t i o n s were re q u i r e d before the beam l e f t the c r y s t a l and was refocused i n t o the IR spectrophotometer. Since the beam could penetrate through the c r y s t a l surface a distance of about 1/10 of the wavelength of the l i g h t used, i t was p o s s i b l e to study the deposits on p o o r l y r e f l e c t i n g metal surfaces such as p l a t i n i z e d platinum by var y i n g the pressure a p p l i e d to the metal f o i l , and hence the pr o x i m i t y of the f o i l to the metal surface. The r e a c t i v i t y of ( E t X ) 2 toward TIBr (KRS-5 c r y s t a l matrix) was te s t e d i n a separate experiment where the TIBr was contacted f o r 20 minutes w i t h saturated aqueous (EtX) ^ s o l u t i o n . No change i n the conce n t r a t i o n of ( E t X ) 2 was noted before and a f t e r the experiment. An i n f r a r e d p e l l e t of 0.5 gm of the reacted TIBr powder showed no i n f r a r e d a bsorption bands. I n f r a r e d s p e c t r a of TIBr p e l l e t s c o n t a i n i n g 2.5 mg (E t X ) ^ d i d not show any formation of t h a l l i u m xanthate. These r e s u l t s e l i m i n a t e d the p o s s i b i l i t y that the KRS-5 c r y s t a l surface may have reacted w i t h the (EtX) ^ deposited on the surface of the platinum e l e c t r o d e s . The platinum f o i l specimens were cut to s u i t the a v a i l a b l e c r y s t a l area and were clamped i n t o the c e l l immediately a f t e r removal from the r e a c t i o n v e s s e l . No s i g n i f i c a n t d i f f e r e n c e s were noted between the sp e c t r a obtained by t h i s technique and those obtained from KBr p e l l e t s or " n u j o l " m u l l s . IR beam c r y s t a l Figure 7. M u l t i p l e i n t e r n a l r e f l e c t i o n e f f e c t i l l u s t r a t i n g the penetra-t i o n of the IR beam through the KRS-5 c r y s t a l s u r f ace. 2. 8 E l e c t r o c h e m i c a l Methods 2.8(a) C e l l f o r Rest P o t e n t i a l Measurements The e l e c t r o c h e m i c a l c e l l used f o r the el e c t r o d e p o t e n t i a l measure-ments shown i n Figure 8, followed a design given by Gardiner (58). The e l e c t r i c a l connection between the removable platinum e l e c t r o d e and the Ag/AgCl/4M KC1 reference e l e c t r o d e was made through a Luggin c a p i l l a r y c o n t a i n i n g 1 M KC1. The c a p i l l a r y minimized the e f f e c t of the p o s s i b l e IR drop through the s o l u t i o n . P o t e n t i a l measurements were made using a K e i t h l e y Model 610 B electrometer. The c e l l a l s o included pH electr o d e s which were connected to a Corning Model 10 pH meter. A s i m i l a r c a p i l l a r y connection between the s o l u t i o n and the Ag/AgCl/4 M KC1 reference e l e c t r o d e was used to e l i m i n a t e the p o s s i b i l i t y of any contamination of the reference e l e c t r o d e by the xanthate s o l u t i o n . A l l p o t e n t i a l measurements were co r r e c t e d to the normal hydrogen e l e c t r o d e (E. ,. „^  .. „„. = + 0.1872V vs SHE). J Ag/AgCl/4 M KC1 Test s o l u t i o n s were added to the c e l l w i t h vigorous s t i r r i n g by a magnetic s t i r r e r . P o t e n t i a l and pH readings were recorded a f t e r they had remained constant f o r 5 minutes. Frequent measurement of the s o l u t i o n temperature showed that i t remained between 24 i 1.0°C. 2.8(b) C e l l f o r Electrode P o l a r i z a t i o n Measurements The three e l e c t r o d e c e l l (Figure 9) used f o r e l e c t r o d e p o l a r i z a t i o n measurements was constructed a f t e r the design of Tolun and Kitchener (39). The platinum e l e c t r o d e (5 cm x 2 cm) could e a s i l y be removed from the c e l l f o r i n f r a r e d a n a l y s i s by the ATR method. The p o t e n t i a l measurements using a Model 610 B K e i t h l e y electrometer were made through a Luggin capillary connected to an Ag/AgCl/4 M KC1 reference electrode as described previously. Current measurements were made with a Simpson ammeter connected in series with the polarization c i r c u i t . Xanthate solutions were purged with purified argon for one hour in the auxiliary vessel and then introduced into the c e l l under argon atmosphere. Electrode potential readings were taken after 1 minute of polar-ization. Concentrations of 1 M, 0.1 M and 0.01 M KC1 electrolyte were used in the xanthate solutions along with a corresponding concen-tration of KC1 only in the salt bridge and reference c e l l . No appreciable differences in shape of the polarization curves was noticed between the various electrolyte concentrations. Algon purge K&dbthley Electrometer Reference Electrode Corning Model 10 pH meter 1 <2> \ Luggin f c a p i l l a r y platinum f o i l 1 Reference electrode 1 M KC1 so l u t i o n Figure 8. Apparatus for measuring pH and rest p o t e n t i a l . Argon purge Keithley Simpson E l e c t r o - Ammeter mete. •reference electrode QaTt>bridge Argon ~ r4 purga 11 -Platinum f o i l Luggin c a p i l l a r y Working C e l l Reference C e l l gure 9. Apparatus for p o l a r i z a t i o n studies. CHAPTER 3 RESULTS AND DISCUSSION 3.1 Decomposition of KEtX i n Aqueous A l k a l i n e S o l u t i o n s Experiments on the a l k a l i n e decomposition of aqueous xanthates have been discussed by a number of workers and most consider that the decomposition i s f i r s t order w i t h respect to xanthate i o n . However, the long r e a c t i o n times have l e d to s c a t t e r e d r e s u l t s as the r e a c t i o n approached completion and some doubt could be cast on the assumption that the f i r s t order r a t e was c o r r e c t . F i n k e l s t e i n (37) evaluated a number of f i r s t order r a t e constants f o r KEtX, but no i n f o r m a t i o n was given regarding the l e n g t h of time the r e a c t i o n was c a r r i e d out. Rao and P a t e l (28) a l s o s t u d i e d the decomposition of aqueous e t h y l xanthate s o l u t i o n s but were unable to f i t t h e i r data to any r a t e law. The r e s u l t s of Pomianowski and L e j a (18) suggested that oxygen a c t i v e l y p a r t i c i p a t e d i n the xanthate decomposition mechanism. However, F i n k e l s t e i n (37) was unable to demonstrate any s i g n i f i c a n t d i f f e r e n c e between the decomposition r a t e s f o r KEtX s o l u t i o n s under n i t r o g e n or oxygen atmospheres, and he concluded that oxygen was not i n v o l v e d i n the decomposition process. His study i n v o l v e d the use of NaH^PO^ and Na2B^0^ pH b u f f e r i n g agents, and i t could be argued that these a d d i t i v e s a f f e c t e d the r a t e and the mechanism of decomposition. This s e r i e s of experiments were undertaken to determine the true order of the r e a c t i o n and to evaluate the e f f e c t of oxygen on the decomposition r a t e without the presence of pH b u f f e r s . Two groups of experiments were c a r r i e d out under i d e n t i c a l c o n d i t i o n s i n a constant temperature bath (22 + 1°C) w i t h the samples s h i e l d e d from d a y l i g h t . The decomposition r a t e s of 5 samples each of aqueous KEtX were observed i n (1) p u r i f i e d argon atmosphere over deoxygenated double d i s t i l l e d water and, (2) p u r i f i e d oxygen atmosphere over oxygen saturated double d i s t i l l e d water. The f o l l o w i n g measurements were made during the course of a run: 1. the absorbance of the 301 my band of the u l t r a v i o l e t spectrum was used to determine the e t h y l xanthate con c e n t r a t i o n . 2. the pH was measured 3. the d i e t h y l dixanthogen concentration was measured by the hexane e x t r a c t i o n method. (Section 2.5, not obtained f o r every sample). Since volumes of aqueous sample were removed from the r e a c t i o n v e s s e l and the volume replaced w i t h e i t h e r p u r i f i e d argon or oxygen, the c o n c e n t r a t i o n of the gaseous products, notably CS2, could not be measured. A second group of experiments i n which the r e a c t i o n v e s s e l was completely f i l l e d are reported i n Section 3.1.5. 3.1.1 Determination of Rate Law The data recorded during a t y p i c a l run are given i n Table 1. Examples of co n c e n t r a t i o n - time p l o t s f o r two decomposition runs under each of argon and oxygen atmospheres are given i n Figure 10. The method of determining the true order of a r e a c t i o n by the measurement of i n i t i a l r a t e s i s described by L a i d l e r (56). The slopes Decomposition of E t h y l Xanthate under Argon Atmosphere at 22°C, Series 1 #2. Sample Time Absorbance E t h y l Xanthate pH D i e t h y l Number (hours) 301 my Concentration dixanthogen (moles l i t e r ~ l x 10^) co n c e n t r a t i o n 1 0 0.453 0.259 7.2 l O " 7 2 42 0.418 0.239 7.2 — 3 110.5 0.348 0.199 7.3 — 4 160 0.338 0.193 7.5 l O " 7 5 207 0.318 0.181 7.5 — 6 259 0.308 0.176 7.4 l O " 7 7 313 0.281 0.161 7.6 — 8 408 0.253 0.145 7.6 l O " 7 From the.least squares analyses f o r slope and i n t e r c e p t (see Appendix 1) S l o p e d ^ = -2.68 + .65 x 1 0 _ 8 moles l i t e r ' ^ r " 1 • - 4 _ i Intercept = .244 ± 0.004 x 10 moles l i t e r 9.0 8.0 7.0 CU •H 6. 0 iH CO cu O e 5. 0 o X o •H J-l 4. 0 cfl 4—' c a C o o 3. 0 X 4-1 w w 2.0 1.0 J Ca) Ca) • S o l u t i o n s saturated w i t h oxygen 0 0 S o l u t i o n s under argon r 1 1 1 r— 50 100 150 200 250 Time i n hours 300 — i — 350 400 Figure 10. Decomposition of aqueous' e t h y l xanthate i n Ca) s o l u t i o n s saturated w i t h oxygen, (b). deoxygenated s o l u t i o n s under argon^atmosphere. of concentration-time p l o t s are r e l a t e d to the conc e n t r a t i o n of a reactant by the equation -d[EtX~] n 7 - v - i n r a t e = L - j ^ — - = k[EtX J where k = r a t e constant n = order of the r e a c t i o n By taking-the common logarithms of each side l o g r a t e = l o g k + n l o g .[EtX -] the order of the r e a c t i o n was found from the slope of the double l o g a r -i t h m i c p l o t of r a t e against i n i t i a l KEtX con c e n t r a t i o n . The slope obtained f o r KEtX decomposition i n argon was found to be 0.965 (Figure 11). S i m i l a r i l y , the r e s u l t f o r KEtX decomposition i n oxygen was 0.975 (Figure 12). Both values are very n e a r l y 1.0 and confirm the decomposition r e a c t i o n i s f i r s t order i n KEtX. The observed r a t e law can be w r i t t e n _ d l g x j . = k l E t x - ] 3.1.2 E f f e c t of Oxygen on the Decomposition Rate The decomposition r a t e constants observed f o r KEtX s o l u t i o n s under argon or oxygen atmospheres were found to be equivalent (Tables 2 and 3 ) . The r e s u l t s are i n agreement w i t h the observations made by F i n k e l s t e i n (37) and summarized i n Table 4. Decomposition Rates f o r Aqueous v E t h y l Xanthate Under Argon Atmosphere at 22°C. Rate I n i t i a l (moles liter~~-'-hr -"'' x 10^) Concentration (moles l i t e r _ 1 x 1(T) l o g ( - r a t e ) l o g I n i t i a l Concentration (moles l i t e r - ! x i o 4 ) F i r s t Order r a t e constant k ^ h r - 1 x 10 4) -2.68 ± 0.65 -7.05 ± 0.87 -24.8 ± 3.3 -61.5 ± 9.3 -649 ± 44 .244 ± 0.009 .984 ± 0.012 3.44 ± 0.04 7.23 ± 0.09 94.6 ± 0.5 -3.57 ± 0.11 -3.15 ± 0.05 -2.61 ± 0.06 -2.21 ± 0.07 -1.15 ± 0.05 -4.612 ± 0.015 -4.007 ± 0.005 -3.463 ± 0.005 -3.141 ± 0.005 -2.024 ± 0.002 10.9 ± 0.7 7.1 ± 0.9 7.2 ± 0.4 8.5 ± 1.0 6.9 ± 0.5 l o g C o -Figure 11. Log-log p l o t of the decomposition r a t e of aqueous e t h y l xanthate vs. i n i t i a l e t h y l xanthate c o n c e n t r a t i o n f o r deoxygenated s o l u t i o n s under an argon atmosphere. Decomposition Rates f o r Aqueous E t h y l Xanthate under Oxygen Atmosphere at 22°C Rate I n i t i a l log (-rate) l o g I n i t i a l F i r s t Order (moles l i t e r _ l h r _ l x 10 ) Concentration Concentration r a t e constant (moles l i t e r " 1 x.10 4) k ( h r - 1 x 10 4) 1 2.15 ± 0.30 .207 ± .004 -3.67 ± 0.06 -4.684 ± 0.008 10.4 ± 0.3 6.98 ± 0.62 .944 ± 0.009 -3.16 ± 0.04- -4.025 ± 0.004 7.4 ± 0.7 26.6 ± 2.0 3.66 ± 0.02 -2.58 ± 0.03 -3.437 ± 0.003 7.3 ± 0.2 i 72.0 ± 5.0 9.71 ±,.0.06 -2.14 ± 0.03 -3.012 ± 0.003 7.2 ± 0.6 ^ 754 ± 40 95.1 ± 0.5 -1.12 + 0.02 -2.022 ± 0.003 7.9 ± 0.5 ' -5.0 -4.6 -4.2 -3.8 . S3..4 ' -3.0 -2.6 -2.2 -1.8 lo g C o Figure 12. Log-log p l o t of the decomposition r a t e of aqueous e t h y l xanthate vs. i n i t i a l e t h y l xanthate concentration f o r s o l u t i o n s saturated w i t h oxygen. Complete e x c l u s i o n of oxygen from the r e a c t i o n f l a s k was not expected i n the experiments using argon. However, the procedure employed i n deoxygenating the water was designed to reduce the oxygen content of the water (normally 43 mg/1 at 22°C) by at l e a s t two orders of magnitude. Since no v a r i a t i o n i n r a t e constant between argon and oxygen s o l u t i o n s was observed, i t was concluded that oxygen was not i n v o l v e d i n the r a t e c o n t r o l l i n g steps of the decomposition process. 3.1.3 E f f e c t of Hydroxyl Ion on the Decomposition Rate The pH of the samples taken during the course of the runs f o r xanthate decomposition under argon atmosphere are given i n Figure 13. The r e s u l t s f o r decomposition under argon and oxygen atmospheres are presented i n Table 4 where comparison w i t h the values reported by F i n k e l s t e i n can be made. The observed r a t e s obtained i n unbuffered s o l u t i o n s are i n general agreement w i t h the r e s u l t s obtained by F i n k e l s t e i n , and the constancy of the rat e s i n d i c a t e that the r e a c t i o n r a t e i s unaffected by [OH J i n the re g i o n pH 7 to 11. Since the samples were not b u f f e r e d , i t was necessary to take the pH measurement immediately a f t e r the sample was withdrawn from the r e a c t i o n f l a s k . A f t e r a short time ( l e s s than 5 minutes) the absorption of C0 2 from the atmosphere caused the pH to f a l l r a p i d l y and such readings would be u n r e l i a b l e . During the course of a run, the pH increased- i n d i c a t i n g t h a t OH was a product of the decomposition r e a c t i o n . In Table 5, the change i n IOH J has been r e l a t e d to the decrease i n l E t X ]. A comparison of [EtX ] = 94.6 x 10 M o o -[EtX ] = 7.23 x 10 4M [EtX ] = 3.44 x 10 4M o EtX ] = 2.58 x 10 M o [EtX ] = 0.98 x 10 i 200 — i — 250 —I— 300 — 1 — 350 Figure 13. 100 150 time in hours pH shift of ethyl xanthate decomposition runs in deoxygenated water under argon atmosphere. the f i g u r e s shows a general correspondence between the o v e r a l l pH increase and xanthate decrease, but because of the d i f f i c u l t i e s i n pH measurement i t was not p o s s i b l e to o b t a i n a q u a n t i t a t i v e r e l a t i o n s h i p between [EtX ] decomposed and [OH ] produced. 3.1.4 E v a l u a t i o n of Reaction Products The measurement of diethyl dixanthogen concentration during the KEtX decomposition experiments revealed that t h i s compound was not a major decomposition product (Table 1) f o r KEtX i n e i t h e r deoxygenated water or oxygen saturated water. has been recognized as a major decomp-o s i t i o n product along w i t h p o s s i b l y other gases such as COS or H^S, and quantitative data were obtained f o r these products. The gaseous products from the decomposition were bubbled through CCl^, and an i n f r a r e d spectrum of the r e s u l t a n t CCl^ s o l u t i o n s was taken; Only one major peak at 1515 cm \ a t t r i b u t a b l e to CS^ a b s o r p t i o n , was observed. 50 ml of the gas was i n j e c t e d i n t o a Pye Gas Chromatograph which was equipped w i t h a 10 foot column c o n t a i n i n g a n e u t r a l s u b s t r a t e . The s i n g l e peak was confirmed as CS^ from the r e t e n t i o n time i n the column. No other peaks were observed. 3.1.5 Q u a n t i t a t i v e Measurement of Carbon D i s u l f i d e In s l i g h t l y a l k a l i n e s o l u t i o n s (pH 7-9) the absorbance of OH i n the u l t r a v i o l e t r e g i o n (400 my to 195 my) i s n e g l i g i b l e . Under these c o n d i t i o n s , the aqueous CS^ concentrations were determined d i r e c t l y from the absorbance at 206.5 my (e = 72,000 l i t e r mole "''cm "*", S e c t i o n 2.4). E f f e c t of pH on the Decomposition of E t h y l Xanthate L i t e r a t u r e Atmosphere Values Temp. pH k (hr 1 ) Atmosphere Present Temp. Study pH k 1 ( h r ' -1) A i r 22±3°C 6 8.8 x .10 4 ( 1 ) Nitrogen 25° 6 185 X 1 0 _ 4 ( 2 ) 7 26.5 X l O " 4 Argon 22±2°C 7.0 7.1 ± 0.9 x 10" 4 8 -4 -4 13.3 X 10 8.0 ' 7.2 ± 0.4 x 10 -4 -4 9 12.2 X 10 9.0 8.5 ± 1.0 x 10 -4 -4 10 12.1 X 10 10.5 6.9 ± 0.5 x 10 -4 11 12.4 X 10 Oxygen 25° 6 185 X l O " 4 Oxygen 22+2°C 6.9 7.4 ± 0.7 x 10~ 4 -4 7 28 X 10 -4 8 13.3 X 10 -4 -4 9 12.3 X 10 9.5 7.2 ± 0.6 x 10 -4 -4 10 11.1 X 10 10.5 7.9 ± 0.5 x 10 -4 11 12.3 X 10 (1) Pomianowski and Le j a (18). (2) F i n k e l s t e i n (37). R e l a t i o n s h i p Between E t h y l Xanthate Decomposition and pH Increase Decomposition Under Argon Atmosphere Decomposition Under Oxygen Atmosphere KEtX Decomposed (moles l i t e r ^) OH Increase (moles l i t e r "*") Xanthate Decomposed (moles l i t e r OH Increase (moles l i t e r ^) 1.14 x 10 2.88 x 10 8.5 x 10 2.20 x 10 2.24 x 10 -5 -5 -5 -4 -3 5 4 x 10 x 10 5.0 x 10 1.2 x 10 1.3 x 10 -7 -5 -4 -3 8.5 x 10 2.8 x 10 9.0 x 10 2.5 x 10 2.5 x 10 -6 -5 -5 -4 -3 6 x 10 10" 5 4 _ x 10 1.3 x 10 5 • x 10 -8 -5 -4 -4 CS^ i s s p a r i n g l y s o l u b l e i n water (ca. 2 gms/1 at 22°C (1)) and a new s e r i e s of decomposition runs were undertaken i n quartz U.V. c e l l s f i t t e d w i t h t e f l o n plugs. The c e l l s were f i l l e d w i t h aqueous KEtX s o l u t i o n so that no gas volume was present, then stoppered and sealed w i t h wax. The r a t e of decomposition of KEtX and the e v o l u t i o n of CS^ were observed. In Table 6, the m a t e r i a l balance c a l c u l a t i o n s showed that CS^ e v o l u t i o n corresponded to the xanthate i o n decrease. The r a t e of CS^ formation was unaffected by the oxygen d i s s o l v e d i n the water. 3.1.6 Mechanism of Xanthate Decomposition i n A l k a l i n e S o l u t i o n Most workers agree that CS^ r e s u l t s from aqueous decomposition of xanthate, but a l a r g e number of other products have a l s o been proposed. Glembotskii (57) reported K^CS^ and K^CO^ as other major products, but he was unable to confirm t h e i r presence. P h i l i p p and F i c h t e (32) described a decomposition mechanism as due to a slow h y d r a t i o n of the carbon s u l f u r bond i n the xanthate which r e s u l t e d i n an intermediate of the s t r u c t u r e S~ I R-O-C-SH I OH The h y d r a t i o n product was unstable and decomposed i n t o other products, namely carbonate, h y d r o s u l f i d e , d i t h i o c a r b o n a t e and t r i t h i o c a r b o n a t e . Hovenkamp (33) concluded from experiments w i t h e t h y l xanthate i n d i l u t e a l k a l i n e s o l u t i o n t h a t : 0.7 4-1 in r-l o e, CM o C n) o t-i C o C O u 0.6 0.5 • 0.4 0.3 0.2 0.1 • d [ E ^ ] = -4.38 + 0.8 x 10 8 mole l iter 1 h r 1 d [ C S 2 ] 8 i i d t = 4 . I x 1 0 ° + 0.4 mole l iter V 1 i I 50 100 Time in hours —I— 150 —I— 200 Figure 14. Decomposition of e t h y l xanthate i n deoxygenated water i n a sealed c e l l , pH 7.4. (a) rat e of xanthate decomposition, (b) r a t e of [©S? formation. M a t e r i a l Balance Between E t h y l Xanthate Decomposed and CS^ Evolved from Sealed C e l l Measurements Condi t i o n I n i t i a l pH F i r s t Order Rate Constant ( h r _ 1 ) [EtX ] Decomposed (moles l i t e r ) [CS ] Evolved (moles l i t e r ) Ratio [CS 2]/[EtX ] Deoxygenated 7.2 7.4 7.1 ± 0.8 x 10 4 , 8.0 x 10 6 7.9 ± 1.6 x 10 -4 1.77x 10 -5 7.3 x 10 -6 1.78 x 10 -5 0.91 1.00 Oxygen Saturated 7.5 7.3 7.0 ± 0.5 x 10 -4 8.8 ± 0.8 x-10 -4 7.3 x 10 -6 1.19x 10 -5 6.4 x 10 -5 1.08 x 10 -5 0.87 0.91 "As to the decomposition of e t h y l xanthate, i n our o p i n i o n a r e a c t i o n i n which carbon d i s u l f i d e i s l i b e r a t e d has to be considered to be the main r e a c t i o n . " 'C 2H 5OCS 2 + H + * C 2H 5OH + CS 2" Recently Dautzenburg and P h i l i p p (34) p o s t u l a t e d that the a l k a l i n e decomposition of e t h y l xanthate can be w r i t t e n C 2H 5OCS 2 + H 20 » C 2H 5OH + • CS + OH i n the region pH 8 to 13. Above 1 M NaOH, the CS 2 was considered to recombine w i t h the excess OH to produce a di t h i o c a r b o n a t e i o n which decomposed r a p i d l y to carbonate and s u l f i d e according to the scheme CS 2 + OH • CS2OH — l CS 20 + H + CS 20 + 20H » C0 3 + HS The r e s u l t s presented i n t h i s s e c t i o n are i n complete agreement w i t h Dautzenburg and. P h i l i p p (34) f o r the r e g i o n pH 8 to 13. Experimental evidence i n support of t h i s r e a c t i o n scheme can be summarized i n t o the f o l l o w i n g p o i n t s : 1. the decomposition r a t e i s f i r s t order w i t h respect to e t h y l xanthate i o n , i n d i c a t i n g that only one molecule of e t h y l xanthate i s invol v e d i n the r a t e determining step. 2. the r e a c t i o n was found to be unaffected by oxygen or base. 3. the i n c r e a s e i n pH i n d i c a t e d that OH was product of the r e a c t i o n . 4. CS^ was the only s u l f u r c o n t a i n i n g product observed during the r e a c t i o n , and a q u a n t i t a t i v e measure showed that one mole of CS^ was produced f o r each mole of e t h y l xanthate decomposed. 3.1.7 Decomposition of KEtX i n the Presence of P o s s i b l e C a t a l y t i c Agents Xanthate decomposition experiments under argon and oxygen atmos-pheres at n e u t r a l to s l i g h t l y b a s i c c o n d i t i o n s (pH 7 to 9) were c a r r i e d out i n the presence of three p o s s i b l e c a t a l y t i c agents 1. methylene blue 2. p l a t i n i z e d platinum _3 3. f e r r o u s s u l f a t e (10 M). 3.1.7(a) Methylene blue F i n k e l s t e i n had reported that an e q u i l i b r i u m e l e c t r o c h e m i c a l p o t e n t i a l between e t h y l xanthate and d i e t h y l dixanthogen could be measured w i t h a redox i n d i c a t o r such as methylene b l u e . The p o s s i b i l i t y that (E t X ) ^ would be c a t a l y t i c a l l y formed by methylene blue as an a l t e r n a t e product to the h y d r o l y s i s r e a c t i o n was i n v e s t i g a t e d . Under argon atmosphere, the decomposition r a t e was slower than s i m i l a r observations made f o r e t h y l xanthate decomposition without methylene blue (Table 7). However, under oxygen atmosphere, EtX decomposed r a p i d l y and only about 10% remained a f t e r 30 minutes. A true l i n e a r f i r s t order r a t e was not observed, but because of the r a p i d Decomposition of Aqueous E t h y l Xanthate i n the Presence of P o s s i b l e C a t a l y t i c Agents D e s c r i p t i o n of Run Temp. I n i t i a l I n i t i a l Xanthate pH Concentration [E t X ~ l F i r s t Order Rate Constant ( h r " 1 ) Methylene Blue (2 x 10 1. Oxygen free 2. Oxygen saturated -5 M) 23 9C 23°C 8.1 8.5-6.3 8.7 x 10 6.8 x 10 -5 -5 2.3 ± 0.5 x 10 400 ± 100 -4 P l a t i n i z e d Platinum 1. Oxygen free 2. Oxygen saturated 23°C 23°C 8.0 7.6 5.6 x 10 7.3 x 10 -5 -5 9.7 ± 0.7 x 10 15.7 ± 1.0 x 10 -4 -4 -3 Ferrous S u l f a t e (10 M) 1. Oxygen free 2. Oxygen saturated 23°C 23°C 8.3 8.6 1.0 x 10 8.5 x 10 -4 -5 3.6 ± 0.5 x 10 42. ± 0.5 x 10 "4 -4 r e a c t i o n the concentrations of EtX were d i f f i c u l t to measure a c c u r a t e l y . For comparison, the approximate f i r s t order r a t e constant was 400 hr or s i x orders of magnitude f a s t e r than the decomposition r a t e of xanthate i n oxygen f r e e water. No (EtX)^ could be found by hexane e x t r a c t i o n e i t h e r during or at the end of these runs. Methylene b l u e , or 3,9-bis-dimethylaminophenazothonium c h l o r i d e contains a p o s i t i v e s u l f u r and'four amine groups. The e t h y l xanthate i o n was presumbly capable of combining w i t h the p o s i t i v e l y charged s u l f u r atom to form a weak d i s u l f i d e which undergoes decomposition r e a c t i o n s i n the presence of oxygen. Another p o s s i b i l i t y was the complexing of the xanthate i o n w i t h the amino groups, r e s u l t i n g i n the formation of adducts which may a l s o have undergone r a p i d decomposition. The r e s u l t pointed out that o x i d a t i o n - r e d u c t i o n i n d i c a t o r s cannot be used w i t h xanthates unless care i s taken to i n s u r e that the i n d i c a t o r does not decompose the xanthate. 3.7.1(b) P l a t i n i z e d Platinum The decomposition r a t e of KEtX i n water c o n t a i n i n g p l a t i n i z e d 2 platinum (20 cm geometrical surface area i n 900 ml R^ O w i t h strong a g i t a t i o n ) was observed under both argon andAoxygen atmospheres (Table 7). Each value i s a composite of two runs, where a d i f f e r e n t surface pretreatment f o r the platinum was used f o r each run. In the f i r s t run, the platinum was p l a t i n i z e d , cleaned by cathodic p o l a r i z a t i o n where H. was evolved from a d i l u t e H„S0. s o l u t i o n , then washed w i t h I 2 4 copious q u a n t i t i e s of d i l u t e KOH and d i s t i l l e d water and then placed i n the r e a c t i o n f l a s k . The second platinum pretreatment i n v o l v e d c l e a n i n g i n aqua r e g i a , then d i l u t e KOH. and d i s t i l l e d water. No d i f f e r e n c e s i n the decomposition r a t e s were noted and the two r e s u l t s f o r each run i n both argon and oxygen saturated s o l u t i o n s are p l o t t e d together. The r a t e of decomposition i s approximately doubled i n oxygen saturated KEtX s o l u t i o n s over s i m i l a r r a t e s found f o r s o l u t i o n s not c o n t a i n i n g platinum. This r a t e , however, i s s t i l l slow and i t appears that p l a t i n i z e d platinum i s not an e f f e c t i v e o x i d a t i o n c a t a l y s t . The d i e t h y l dixanthogen content of the s o l u t i o n , determined a f t e r 30,80 and 120 hours showed no s i g n i f i c a n t concentration present. A f t e r completion of the run, the platinum f o i l was removed from the r e a c t i o n v e s s e l and washed w i t h hexane. The U.V. spectrum of the e x t r a c t showed no evidence of d i e t h y l dixanthogen. 3.1.7(c) Ferrous S u l f a t e The p o s s i b i l i t y that f e r r o u s and f e r r i c i o n s , <which are common to a l l f l o t a t i o n systems, are a c t i v e c a t a l y s t s i n the o x i d a t i o n of xanthate to dixanthogen was a l s o i n v e s t i g a t e d . The decomposition -3 r a t e s i n oxygen and argon were found to be l e s s r a p i d i n 10 M f e r r o u s s u l f a t e than the rates observed f o r KEtX only under the same c o n d i t i o n s . Rao and P a t e l (29) a l s o observed that the decomposition r a t e of e t h y l xanthate was slower i n the presence of ferrous i o n . These authors considered that small amounts of d i e t h y l dixanthogen were produced during the decomposition but dixanthogen determinations c a r r i e d out at 2 and 30 hours i n the present experiments revealed no measurable q u a n t i t i e s (< 10 mole l i t e r ^) at these time i n t e r v a l s . The p o s s i b l e c a t a l y t i c agents f o r converting xanthate to dixanthogen i n the presence of oxygen d i d not y i e l d f r u i t f u l r e s u l t s . However, I j [ | | | other m e t a l l i c ions such as Pb , Cu , Zn could s i g n i f i c a n t l y a c c e l e r a t e the decomposition r a t e , but t h i s was not i n v e s t i g a t e d . The experiments on the behavior of aqueous xanthate and oxygen were c a r r i e d out i n a homogeneous environment, and thus cannot be a p p l i e d d i r e c t l y to f l o t a t i o n systems. The presence of s u l f i d e minerals creates a heterogeneous environment i n which the c a t a l y t i c o x i d a t i o n of xanthate to dixanthogen i s l i k e l y . The subject of c a t a l y t i c o x i d a t i o n w i l l be discussed f u r t h e r i n the Summary and Conclusions chapter of t h i s t h e s i s . 3.2 Decomposition of Dixanthogen In Aqueous A l k a l i n e S o l u t i o n In the previous t e s t s , the o x i d a t i o n of e t h y l xanthate was c a r r i e d out under argon and oxygen atmospheres and the products were shown not to be d i e t h y l dixanthogen, but CS^ a n ^ OH formed by a h y d r o l y t i c r e a c t i o n . To f u r t h e r d e f i n e the chemical nature of the aqueous xanthate-dixanthogen system, the r e a c t i o n of hydroxyl i o n and other reagents w i t h d i e t h y l dixanthogen was s t u d i e d . I t was t e n t a t i v e l y b e l i e v e d that the r e d u c t i o n of d i e t h y l dixanthogen w i t h base would r e s u l t i n n o n - s t o i c h i o m e t r i c production of xanthate, i . e . one mole of dixanthogen reacted w i t h OH would not produce two moles of xanthate. This formed the i n i t i a l p art of the study; the second p a r t was the measurement of the r a t e and order of the r e d u c t i o n r e a c t i o n w i t h respect to each of the reactants so that a mechanism could be p o s t u l a t e d . 3.2.1 Stoichlometry of Reaction of D i e t h y l Dixanthogen w i t h Hydroxyl Ion I n i t i a l l y , stock s o l u t i o n s of known q u a n t i t i e s of e t h y l xanthate and d i e t h y l dixanthogen were prepared by the a d d i t i o n of K I ^ to n e u t r a l KEtX s o l u t i o n s . A small volume of concentrated KOH was added to pHi>ll and the f l a s k was s t i r r e d v i g o r o u s l y u n t i l the r e a c t i o n was estimated to have gone to completion. At t h i s time, the co n c e n t r a t i o n of remaining CEtX)^ was measured by the hexane e x t r a c t i o n technique (Section 2.5) and the EtX co n c e n t r a t i o n was measured by u l t r a v i o l e t a bsorption ( S e c t i o n 2.4). The molar r a t i o of the amount of EtX generated from the amount of ( E t X ) 2 decomposed was then c a l c u l a t e d . The r e s u l t of 18 r e a c t i o n s between OH and (EtX)^ are given i n Table 8. The value f o r the molar r a t i o c l e a r l y shows that one mole of  xanthate was generated f o r each mole of dixanthogen that was reduced. 3.2.2 E l i m i n a t i o n of P o s s i b l e I n t e r f e r e n c e by Iodide Since the c o n t r o l l e d amounts of ( E t X ) 2 were prepared by t i t r a t i o n of the KEtX s o l u t i o n w i t h K I ^ , there was some question that the r e s i d u a l I could have i n t e r f e r e d w i t h the r e s u l t s . Three h y d r o l y s i s runs (Series 2) were c a r r i e d out w i t h stock s o l u t i o n s of ( E t X ^ pre-pared by a g i t a t i n g l i q u i d (EtX)^ i n deoxygenated double d i s t i l l e d water, which e l i m i n a t e d i o d i d e i o n . These r e s u l t s were equivalent to those obtained by i o d i n e t i t r a t i o n . Further evidence was provided i n t e s t s -4 -3 i n which the r e d u c t i o n of (EtX)^ was attempted i n 10 M and 10 M K l . Under these c o n d i t i o n s , the r a t e was shown to be s u f f i c i e n t l y slow to be n e g l i g i b l e compared w i t h the r a t e of base r e d u c t i o n (Table 9). Reaction of D i e t h y l Dixanthogen w i t h Hydroxyl Ion at 22° C Run # Atmosphere Run Time pH [(EtX> 2] [EtX~] Molar Ratio hydrolysed recovered [EtX~]/[(EtX) ] - 1 5 - 1 5 (moles l i t e r x 10 )(moles l i t e r x 10 ) Series 1 #1 A i r 15 12.15 2.60 2.59 1.00 #2 A i r 15 11.92 0.45 0.49 1.09 Series 2 #1 Argon 45 11.05 1.35 1.54 1.14 #2 Argon 15 11.85 0.45 0.44 0.98 #3 Argon 15 11.58 1.00 1.05 1.05 Series 3 #1 Oxygen 85 10.35 1.10 1.12 1.02 Series 4 #1 A i r 45 11.70 2.20 2.15 0.98 #2 A i r 45 11.25 2.20 2.31 1.05 #3 A i r 45 11.53 2.20 2.25 1.02 #4 A i r 30 11.97 1.50 1.50 1.00 #5 A i r 30 12.12 1.50 1.50 1.00 Series 6 #1 A i r 40 11.40 1.12 1.10 0.98 #2 A i r - 60 11.08 1.08 1.12 1.04 #3 A i r 45 12.05 0.95 1.02 1.07 Series 7 #1 A i r 60 10.84 1.10 1.15 1.05 #2 A i r 50 11.15 1.05 1.01 0.91 #3 A i r 50 11.17 1.00 0.95 0.95 #4 A i r 50 11.18 0.95 0.98 1.03 Mean value f o r molar r a t i o [EtX ]/[(EtX) ] = 1.02 ± 0.05. 3.2.3 E v a l u a t i o n of Reaction Products The r e a c t i o n of (EtX)^ w i t h OH r e s u l t e d i n the formation of CS^ which could be measured q u a l i t a t i v e l y by f o l l o w i n g the u l t r a v i o l e t absorbance at 206.5 my. U n f o r t u n a t e l y , at pH >10, the OH absorption became s i g n i f i c a n t and the CS^ band was masked. Consequently, q u a n t i -t a t i v e e v a l u a t i o n of CS^ concentrations by t h i s method was not p o s s i b l e . E x t r a c t i o n of the r e a c t i o n products i n t o organic s o l v e n t s was i n v e s t i g a t e d , but t h i s r e s u l t e d i n simultaneous e x t r a c t i o n of (EtX)^ and d i d not y i e l d f r u i t f u l r e s u l t s . Because of the low c o n c e n t r a t i o n of products, an i n f r a r e d spectrum of the decomposed aqueous s o l u t i o n was not p o s s i b l e using the s o l u t i o n s at which the U.V. experiments were made. A separate experiment was devised whereby 5 N KOH was a g i t a t e d w i t h l i q u i d (EtX)^ f o r 24 hours i n a sealed f l a s k . The reacted s o l u t i o n was c e n t r i f u g e d to remove any excess (EtX)^ and an i n f r a r e d spectrum was obtained from the remaining s o l u t i o n using the c e l l described i n S e c t i o n 2.7(b). The i n f r a r e d spectrum over the region 1400 cm to 700 cm y i e l d e d -1 -1 -1 three strong peaks at 1060 cm , 1110cm and 1040 cm which correspond w i t h the a b s o r p t i o n peaks at 1058 cm \ 1110 cm and 1040 cm ^ f o r EtX ions p r e v i o u s l y reported by Poling, (5) (see Figure 28). A U.V. absorption spectrum of the s o l u t i o n showed that a p p r o x i -mately 1.6 moles of EtX was produced f o r each mole of (EtX)^ hydrolysed. The discrepancy between.this r e s u l t andithe previous experiments reported i n Table 8 can be explained by c o n s i d e r i n g that the h y d r o l y s i s was c a r r i e d out i n a s t r o n g l y b a s i c medium and some of the CS„ produced during the h y d r o l y s i s was l i k e l y to recombine w i t h the a l c o h o l a t e i n s o l u t i o n and regenerate e t h y l xanthate i o n s . The only r e a c t i o n products that were detected were CS^ and EtX . 3.2.4 C o n t r i b u t i o n of the React i o n Products to the 301 my Xanthate Absorption In the previous experiments on e t h y l xanthate decomposition, the pH was l e s s than 10 and the p o s s i b i l i t y of r e a c t i o n between the product CS^ and OH was avoided. In the d i e t h y l dixanthogen decomposition runs, the pH was greater than 11, and the p o s s i b i l i t y of other products c o n t r i b u t i n g to the 301 my xanthate absorption band was considered. Rovenkamp (57) l i s t s the r e a c t i o n s between CS^ and OH as f o l l o w s : CS„ + 0R~ y CS o0H y CS o0 + H + 2 2 -< 2 CS 20 + 20H y C0 3 + 2SH H 2CS 20 y H 2S + COS COS + 30H~ r a p l d > SET + C 0 3 = + H 20 CS„ + SH y HCS_ y CS„ + H + 2 3 -« 3 CS 2 + CS 20 y CS 3 + COS The U.V. spectra of CS 20 , SH , CS 2, CS 3 , CS^ , CS 30 and e t h y l xanthate have been reported (33) and are given i n f i g u r e 15. The c o n t r i b u t i o n of any of these products to the 301 my absorbance of e t h y l xanthate does not appear to be s i g n i f i c a n t , and f o r the present experiments, 1 301 my band was considered to be due s o l e l y to absorption by the e t h y l xanthate i o n . toooo a ImoC'crri1 5J300 ® CD-/ '\cs2o"' \ SHV a (cs,) / \ I I I 40 w 20- .1 a ImorW 15JDO0-(D \ / r® 10000 X V, Xanthate 5*00 n. ImoC lyroo 10JQO0 5TJ00 400 380 360 340 320 300 260 260 240 m i f i g u r e 15. U l t r a v i o l e t s p e c t r a of s u l f u r c o n t a i n i n g compounds i n the (1) d i t h i o c a r b o n a t e i n 5 N NaOH; (3) Na„S i n 0.1 N NaOH; (4) e t h y l monothiocarbonate system CS2~NaOH-alcohol. (2) CS, (C^OCOSK) ; (5) t r i t h i o c a r b o n a t e ( C S ^ ) ; (6) potassium e t h y l xanthate (C^OCSSK) ; (7) tet r a t h i o p e r c a r b o n a t e (CS^ -) ; (8) t r i t h i o p e r c a r b o n a t e s ( C S 3 0 ~ ) ; ( a f t e r Hovenkamp (57)) 3.2.5 Mechanism of the Decomposition of Dixanthogen The h y d r o l y s i s of d i e t h y l dixanthogen w i t h OH can be considered to be a s p e c i f i c example of the r e a c t i o n s between d i s u l f i d e s and n e g a t i v e l y charged bases or n u c l e o p h i l i c reagents. In a review paper by Davis (58), the general form of these types of r e a c t i o n was w r i t t e n : XS-SX + Y~ y XSY + XS~ The r e a c t i o n was considered to proceed by an S^2 or n u c l e o p h i l i c displacement mechanism. Such "a mechanism was f i r s t proposed by Foss (59), who considered that a l l d i s u l f i d e s , i n c l u d i n g dixanthogen undergo p o l a r f i s s i o n according to the scheme XS-SX «—y (XS +) (SXT) Foss p o s t u l a t e d that a t t a c k by n u c l e o p h i l i c agents stronger than the xanthate i o n would r e s u l t i n displacement of the xanthate and the forma-t i o n of a new d i s u l f i d e adduct. The r e a c t i o n between d i e t h y l dixanthogen and OH can be i l l u s t r a t e d f o l l o w i n g the mechanism proposed by Foss (59). The i n i t i a l step of the displacement r e a c t i o n can be w r i t t e n S S S S M II _ II H _ ROCSSCOR + OH y ROCSOH + ROCS One of the products, a s u l f e n i c a c i d , i s known to be h i g h l y unstable. Danehy (60) has observed that p r o t e i n s are d i s p r o p o r t i o n a t e d by base h y d r o l y s i s and that a l k y l d i s u l f i d e s undergo rearrangement and decomp-o s i t i o n u n t i l a l k y l s u l f o n i c a c i d s are formed. The mechanism of xanthate s u l f e n i c a c i d decomposition has not been exp l a i n e d , although Danehy (60) has explained that only d i a l k y l d i s u l f i d e s undergo o x i d a t i o n to s u l f o n i c a c i d s , and a l l other d i s u l f i d e s undergo d i s p r o p o r t i o n a t i o n to a v a r i e t y of products. Carbon d i s u l f i d e was detected as a product of ( E t X ) ^ h y d r o l y s i s from the U.V. absorbance at 206.5 my. Attempts at q u a n t i t a t i v e measurement were unsuccessful because of i n t e r f e r e n c e by the OH absorbance which becomes s i g n i f i c a n t above pH 10. The d i s p r o p o r a t i o n -a t i o n of xanthate s u l f e n i c a c i d was considered to proceed by the f o l l o w i n g r e a c t i o n The r e a c t i v i t y of bases toward the s u l f u r - s u l f u r bond has been discussed by a l a r g e number of workers (61,62). Foss considered that the displacement of a s u l f i d e anion w i t h another t h i o l i s a measure of the base str e n g t h of the t h i o l . The displacement of thiocyanate, o - o - d i a l k y l dithiophosphate, xanthate and mercaptide ions from b i s (o-n i t r o p h e n y l ) d i s u l f i d e i l l u s t r a t e d the i n c r e a s i n g order of base s t r e n g t h of each n u c l e o p h i l e and was discussed i n terms of S - n u c l e o p h i l i c i t y . The order of r e a c t i v i t y of some n u c l e o p h i l e s toward the d i s u l f i d e bond has beencreported by Davis (58) and i s given below S ll ROCSOH ROH . + CS 2 + OH 3.2.6 Reaction of Dixanthogen w i t h Other Nucleophiles C ^ S > CgHgS > CN > 0H~ » 0 2 N - ^ ^ - S » N 3 >> SCN~ In ge n e r a l , the r e a c t i o n between t h i o l s and d i s u l f i d e s has been considered to be r e v e r s i b l e and has been w r i t t e n RSSR + R.S y RS-SR.. + RS 1 •* 1 Such a r e a c t i o n between d i e t h y l dixanthogen and t h i o s u l f a t e i o n was p o s t u l a t e d by Foss (59) and can be w r i t t e n S S S S II II = II _ II _ ROCSSCOR + S 2 0 3 ROCSS 20 3 + ROCS S S II = II _ ROCSS„0„ + S.0- y ROCS + S.O, 2 3 2 3 -< 4 6 The k i n e t i c s of t h i s r e a c t i o n have been stud i e d by..-Wells (24). The r e a c t i o n of d i s u l f i d e s w i t h non-sulfur n u c l e o p h i l e s such as CN and OH has been shown to r e s u l t i n unstable adducts which de-compose to other products and e q u i l i b r i u m r e a c t i o n s have not been observed. Cambron (2) observed that the r e a c t i o n of CN w i t h d i e t h y l dixanthogen r e s u l t e d i n the formation of thiocyanate (SCN ) and xanthogen monosulfide. The r e a c t i o n between d i s u l f i d e s and OH have been reported by Danehy (60) and are summarized i n Secti o n 3.2.5. Experiments on the r e a c t i o n between;- d i e t h y l dixanthogen and t h i o -s u l f a t e i o n , s u l f i t e i o n , and i o d i d e i o n are reported i n Table 9. The re d u c t i o n of (EtX) w i t h K 2 S 2 0 3 would have shown 2.0 moles of xanthate produced f o r each mole of ( E t X ) 2 decomposed had the r e a c t i o n gone to Stoichiometry of the Reaction Between Various Nucleophiles and D i e t h y l Dixanthogen Nucleophile Concentration pH [ (EtX) ] Reacted [EtX ] Recovered Molar Ratio [ E t X - ] / [ ( E t X ) 2 ] s 2 o 3 = 5 x 1 0 3 9 . 1 0 1 . 3 9 x 1 0 - 5 2 . 5 1 x 1 0 - 5 1 . 8 1 s o 3 = l O " 4 7 . 6 0 1 . 7 5 x 1 0 - 5 1 . 6 2 x 1 0 - 5 0 . 9 2 I - l O " 3 6 . 8 5 0 . 7 4 x 1 0 - 5 0 . 7 x 1 0 - 6 0 . 1 0 I - l O " 4 6 . 9 0 0 . 7 4 x 1 0 - 5 not detected 0 . 0 0 completion as p o s t u l a t e d by Foss (59). In the case of s u l f i t e reduc-t i o n , the i n a b i l i t y of the s u l f i t e anion to dimerize probably r e s u l t e d i n d i s p r o p o r t i o n a t i o n of the adduct, and the formation of CS^ and other r e a c t i o n products. Since no r e a c t i o n was observed between i o d i d e and d i e t h y l dixanthogen, the i o d i d e i o n has been considered to be a weaker base than the e t h y l xanthate i o n . From these r e s u l t s and an order of base strengths f o r n u c l e o p h i l e s given by Davis (58), an order f o r the base strengths of these n u c l e o p h i l e s can be p o s t u l a t e d . S CN~> 0H~ > S 0 3 = > S 2 0 3 = > ROCS" > SCN~ > i " 3.3 Rate Studies on the Reaction Between Dixanthogen and Hydroxyl Ion The r e a c t i o n between d i e t h y l dixanthogen and hydroxyl i o n was p o s t u l a t e d to proceed by a b i m o l e c u l a r displacement mechanism (or S^2 mechanism). Studies on S^2 displacement r e a c t i o n s i n carbon systems have been reviewed by Ingold, who demonstrated that the incoming group enters on the "backside" of the s u b s t r a t e and forms a weak bond w i t h the substrate i o n . The timing of the bond breaking and bond forming process r e q u i r e s that both the a t t a c k i n g group and substrate must be i n c l o s e p r o x i m i t y (or c o l l i s i o n ) before the r e a c t i o n can occur. The c o n c e n t r a t i o n of the intermediate i s d i r e c t l y p r o p o r t i o n a l to the number of bimolecular c o l l i s i o n s between them which i n t u r n i s p r o p o r t i o n a l to the r a t e constant. The general r a t e expression f o r a b i m o l e c u l a r r e a c t i o n can be w r i t t e n - d [ ( E t X ) 2 ] = k 2 [ ( E t X ) 2 ] m [ O H ] n dt In the case of an S^2 displacement mechanism, the r e a c t i o n becomes f i r s t order i n [ ( E t X ) 2 ] and [OH ], or m = n = 1. To provide evidence f o r the proposed mechanism, the order of the r e a c t i o n w i t h respect to each species was determined experimentally. The method of i n i t i a l r a t e s (56), which was used to determine the order of the e t h y l xanthate decomposition r e a c t i o n was not s u i t a b l e f o r e v a l u a t i n g the order of decomposition of d i e t h y l dixanthogen. Since the r e a c t i o n could be followed to 80% completion, and the [OH ] remained constant, the order of the r e a c t i o n could be determined from the i n t e g r a t e d form of the assumed f i r s t order r a t e expression. Consider m = 1 3.3.1 Reaction Order w i t h Respect to D i e t h y l Dixanthogen Concentration d.[ (EtX) 2 ] = k 2 I ( E t X ) 2 ] I0H ] n dt w i t h JOH l n constant - d i (EtX) 2 ] - k [ ( E t X ) 2 ] k x = k 2 JOH J n dt -d l n [ ( E t X ) 2 ] dt l n [ ( E t X ) 2 ] = C - k x t In Table 10, the r e s u l t s of a t y p i c a l d i e t h y l dixanthogen h y d r o l y s i s run are given. The [ ( E t X ) ^ ^ was c a l c u l a t e d i n d i r e c t l y from measurements of [EtX ] . Since the s t o i c h i o m e t r i c runs had shown that one mole of xanthate was formed f o r each mole of (EtX) reacted [ ( E t X ) 2 ^ was determined by hexane e x t r a c t i o n at the beginning of each run. The c o n t r i b u t i o n of ( E t X ) 2 to the 301 my absorbance was estimated from the height of the EtX b a s e l i n e and subtracted from the absorbance. Semilogarithmic p l o t s of [ ( E t X ) 2 ^ vs. time are given f o r a l l S e r i e s #4 runs i n Figure 16. A l i n e a r p l o t was obtained over 90% of the r e a c t i o n and e s t a b l i s h e d that r a t e was f i r s t order, c o n s i s t e n t w i t h the p o s t u l a t e d mechanism. 3.3.2 Reaction Order w i t h Respect to Hydroxyl Ion Concentration The order of the r e a c t i o n w i t h respect to hydroxyl i o n was evaluated by c o n s i d e r i n g the r a t e of decomposition of d i e t h y l dixanthogen at a number of pH values [(EtX) ] = [(EtX).] - : [EtX ] + [ E t X _ ] l t z o t o - d [ ( E t X ) 2 ] = k 2 [ ( E t X ) 2 ] I O H J n dt -d In I ( E t X ) 2 ] = k x = k 2 [OH ] n dt l o g k± = l o g k 2 + n l o g [OH ] = log k_ - n pOH H y d r o l y s i s of Dixanthogen w i t h Hydroxyl Ion (Series 4 #3) Sample No. Time pH Absorbance [EtX ] f c [ ( E t X ) 2 ] f c = [ ( E t X ) 2 ] q - [ E t X ] t+[EtX ] l o g [ ( E t X ) ] (minutes) 301 my -.4 . 1 _ -1 ..4 , ,.^ -1 x 10 mole l x t e r x 10 mole l i t e r 1 0 11.54 1.010 0.577 0.210 -4.678 2 2.00 - 1.119 0.639 0.168 -4.775 3 4.00 - 1.168 0.677 0.140 -4.854 4 6.84 11.53 1.244 0.711 0.097 -5.013 5 9.17 - 1.263 0.722 0.086 -5.066 6 12.00 - 1.304 0.745 0.062 -5.208 7 14.30 - 1.326 0.758 0.050 -5.301 8 17.50 11.53 1.353 0.773 0.034 -5.469 9 21.00 - 1.372 0.785 0.023 -5.638 10 26.50 11.53 1.391 0.795 0.013 -5.886 From l e a s t squares a n a l y s i s , k = 4.5 ± 2 -1 1.4 x 10 min ON 3x10 2xH?0 10 " 6 4 OpH 11.25 QpH 11.53 A p H 11.70 •pH 11.97 • pH 12.-12 i 10 i 20 Figure 16. 5 10 15 20 25 Time i n minutes F i r s t order r a t e p l o t f o r the r e a c t i o n of aqueous d i e t h y l dixanthogen w i t h hydroxyl i o n , S e r i e s 4. = l o g k. - n ,(pKr - pH) z w = ( l o g k 0 - n pK ) + n pH z w Tabulated values of k^ and c a l c u l a t e d values f o r k^, along w i t h the e r r o r at 95% confidence l i m i t s are given i n Table 10. A p l o t of l o g k^ vs. pH, given i n Figure 17, y i e l d e d a slope of 0.995 ± 0.08 which confirmed that n = 1 and the r a t e expression was w r i t t e n -d[(EtX) ] = k 2 [ ( E t X ) 2 ] [ O H ] which was c o r r e c t f o r a S„2 displacement r e a c t i o n . A model f o r the r e a c t i o n intermediate between d i e t h y l dixanthogen and hydroxyl i o n i s drawn below; f o l l o w i n g examples f o r such i n t e r -mediates given by Gould (65). S II S-C-OR s II R-O-C OH The model has been shown to be c o n s i s t e n t w i t h s i m i l a r models drawn f o r S^2 r e a c t i o n s f o r a l i p h a t i c systems (65). S i m i l a r intermediates would be expected f o r the r e a c t i o n between ( E t X ) 2 and other n u c l e o p h i l i c reagents such as CN , , and SO^ a l l of which are stronger bases than the e t h y l xanthate i o n , and would d i s p l a c e e t h y l xanthate i o n from d i e t h y l dixanthogen. Evaluation of Second Order Rate Constants f o r H y d r o l y s i s of D i e t h y l Dixanthogen Run No. pH pOH k, x 10 min l o g k.. l o g k 9 k„ l i t e r mole min Series 4 #1 11.70 2. 30 7. 4 + 1.0 -1. 13 + 0.06 1.17 + 0.06 14.0 + 2.0 #2 11.25 2. 75 2. 9 + 0.1 -1. 53 + 0.02 1.12 + 0.02 13.2 + 0.6 #3 11.53 2. 47 4. 5 + 1.4 -1. 34 + 0.01 1.13 + 0.01 13.5 + 0.3 #4 11.97 2. 03 11. 9 + 0.4 -0. 92 + 0.01 L l l + 0.01 12.9 + 0.3 m 12.12 1. 88 18. 2 + 1.9 -0. 74 + 0.05 1.14 + 0.05 13.8 + 1.7 Series 5 #1 11.30 2. 70 3. 1 + 0.6 -1. 51 + 0.08 1.19 + 0.08 15.5 + 3.0 Series 6 #1 11.40 2. 60 3. 5 + 0.2 -1. 46 + 0.03 1.14 + 0.03 13.8 + 1.0 #2 11.08 2. 92 1. 44 + 0.08 -1. 84 + 0.02 1.08 + 0.02 12.0 + 0.6 Se r i e s 7 #1 10.84 3. 16 0. 92 + 0.04 -2. 09 + 0.02 1.12 + 0.02 13.2 + 0.6 #2 11.15 2. 85 2. 1 + 0.2 -1. 69 + 0.04 1.14 + 0.04 13.8 + 1.4 #3 11.17 2. 83 2. 1 + 0.1 -1. 68 + 0.02 1.15 + 0.02 14.2 + 0.6 #4 11.18 2. 82 2. 2 + 0.2 -1. 66 + 0.03 1.16 + 0.03 14.5 + 1.0 3.4 S o l u b i l i t y of D i e t h y l Dixanthogen i n Water Following the development of an a n a l y t i c a l method f o r determining the c o n c e n t r a t i o n of d i e t h y l dixanthogen i n water, a s e r i e s of experiments were undertaken to determine the c o n c e n t r a t i o n of saturated d i e t h y l dianthogen s o l u t i o n s . Two methods were used: 1. S o l u t i o n s of deoxygenated-double d i s t i l l e d water were a g i t a t e d w i t h l i q u i d ( E t X ) ^ f o r s u i t a b l e periods of time. Samples were removed, c e n t r i f u g e d and analysed f o r ( E t X ^ content by the hexane e x t r a c t i o n method (Section 2.5). 2. S a t u r a t i o n was estimated by i o d i n e t i t r a t i o n and measurement of t u r b i d i t y (Section 2.6). 3.4.1 Measurement of S a t u r a t i o n by E x t r a c t i o n Technique The l e n g t h of time of a g i t a t i o n of the d i e t h y l dixanthogen^water s o l u t i o n was t e s t e d by determining the amount of d i e t h y l dixanthogen that had d i s s o l v e d a f t e r 4, 8, 16 and 30 hours at 22 + 1°C.After 4 hours the c o n c e n t r a t i o n of d i s s o l v e d d i e t h y l dixanthogen had reached 1.0x10 ^ moles l i t e r ^ , a f t e r ;8 hours 1.26x10 ~* moles l i t e r ^ and a f t e r . 16" hours 1.25 moles l i t e r 1 ;(see Figure 18, pH = 6.1).. A f t e r 16 h o u r s . a g i t a t i o n ' the s o l u t i o n s .were considered to be saturated w i t h ( E t X ^ -Some v a r i a t i o n i n s o l u b i l i t y was observed, as i s noted i n Figure 18. The d i f f i c u l t y w i t h t h i s procedure was that any minute traces of l i q u i d ( E t X ) ^ that may have remained suspended i n the s o l u t i o n would create l a r g e anomalous readings. Hence, a l l samples were analysed i n d u p l i c a t e and when a l a r g e v a r i a t i o n was found, the a n a l y s i s was repeated u n t i l the p r e c i s i o n was i 0.1 x 10 ;^M. 3.4.2 Measurements of S a t u r a t i o n by T u r b i d i m e t r i c Technique This method was found to be more s e n s i t i v e f o r e v a l u a t i n g s o l u b i l i t y than the d i s s o l u t i o n method. The e f f e c t of K I , and excess KEtX has been discussed i n Secti o n 2.5. Three measurements of (EtX)^ c o n c e n t r a t i o n , determined at pH 5.8,6.8 and 7.5 are given i n Figure 18. These values compare f a v o r a b l y w i t h the measurements by the a g i t a t i o n technique. The average value f o r the l i m i t of s o l u b i l i t y , using both methods, was observed to be 1.25 x 10 M over a pH range of 2.8 to 8.4. This value compares w i t h 1 . 3 x 10 ^ M .- as determined by Pomianowski and Le j a (18) at pH 6. In a l l cases during these t e s t s , v a r y i n g amounts of EtX were present. The e f f e c t of changing EtX concentrations d i d not appear to a f f e c t the s o l u b i l i t y of ( E t X ) 2 -The s o l u b i l i t y measurements were l i m i t e d to pH 9 because above t h i s v a l u e the r a t e of the h y d r o l y s i s r e a c t i o n became s i g n i f i c a n t and i n t e r f e r e d w i t h the s o l u b i l i t y determination. The low pH re g i o n was not completely explored, although a q u a l i t a t i v e t e s t showed that (EtX)£ was s t a b l e at pH 2 f o r at l e a s t 20 hours. These r e s u l t s expanded on the data of Pomianowski and L e j a (18) who obtained a value of 1 . 3 x 10 M f o r the s o l u b i l i t y of (EtX) 2 at pH 6. A value of 10 was reported by G o l d s t i c k (42) who -measured the T y n d a l l e f f e c t of aqueous-organic mixtures c o n t a i n i n g ( E t X ^ - The present measurements e s t a b l i s h e d the s a t u r a t i o n concentra-t i o n of (EtX)^ which was necessary f o r e v a l u a t i n g the a c t i v i t y of CEtX)„ s o l u t i o n s used i n the e l e c t r o c h e m i c a l experiments. • Determined by d i s s o l u t i o n method rj Determined by t u r b i d i m e t r i c method -ri-r**-*"0™* T -6 -1 I ~L 1 x 10 mole l i t e r 1 1 i i i t i 1 2 3 4 5 6 PH Figure 18. E f f e c t of pH on the s o l u b i l i t y of d i e t h y l dixanthogen. 3.5 E l e c t r o c h e m i c a l Studies on Aqueous E t h y l Xanthate 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 of xanthate to dixanthogen was w r i t t e n "2(EtX) + 2e~ 2EtX~ Values f o r the standard r e d u c t i o n p o t e n t i a l (E°) of t h i s r e a c t i o n have been determined by r e s t p o t e n t i a l methods using platinum e l e c t r o d e s (38,39,40), po t e n t i o m e t r i c t i t r a t i o n methods (41,42) and redox methods (37,63), and are summarized i n Table 12. Although the m a j o r i t y of E° values obtained using the v a r i o u s methods are i n general agreement w i t h each other, some questions regarding the r e v e r s i b i l i t y of the r e a c t i o n s at the e l e c t r o d e surfaces remained unanswered. S p e c i f i c a l l y , two reasons are apparent f o r t h i s problem. 1. The r e a c t i y i t y of the platinum e l e c t r o d e toward aqueous xanthate was not known. This included the p o s s i b i l i t y that platinum xanthate may be formed on the surfaces of platinum e l e c t r o d e s , causing a s i g n i f i c a n t o v e r p o t e n t i a l . 2. The standard s t a t e f o r dixanthogen has been set at the s a t u r a t i o n of dixanthogen i n water. The e f f e c t oh the r e s t p o t e n t i a l of concentrations of dixanthogen l e s s than s a t u r a t i o n has not been i n v e s t i g a t e d . The r e s u l t s reported i n t h i s s e c t i o n evaluated the e l e c t r o c h e m i c a l r e s t p o t e n t i a l s r e s u l t i n g from v a r y i n g concentrations of ethyl xanthate, diethyl dixanthogen, oxygen and hydroxyl i o n on a platinum e l e c t r o d e , along w i t h q u a l i t a t i v e a n a l y s i s of the surface products by i n f r a r e d s p e c t r o -scopy. A s e r i e s of p o l a r i z a t i o n experiments were a l s o c a r r i e d out, along w i t h a n a l y s i s of the surface products. 3.5.1 D e r i v a t i o n of the Nernst Equation of 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 of E t h y l Xanthate to D i e t h y l Dixanthogen For the e t h y l x a n t h a t e - d i e t h y l dixanthogen couple, the f o l l o w i n g r e a c t i o n s apply: CEtX) 2 + 2e - - j — * - 2EtX then yCEtX)„ + 2 y e " = 2 yCEtX~) 2 y e " 2 y ( E t X ) P ( E t X ) 2 -2FE .= 2y e_ = 2 y ° E t X _ ) + RT In a ^ - - RT In a ^ ^ 2FE° = - [ 2 W ° ( E t x _ ) - V ( E t X ) 2 ] E = E ° - | | In ^ 1 2 F a ( E t X ) 2 The measured p o t e n t i a l i s c o r r e c t i f the assumption yi ( i n Pt) = u g ( s o l u t i o n ) i s v a l i d . The standard state f o r the a c t i v i t y of ethyl xanthate ion a. = 1.0, has been established, when [EtX ] = 1.0 M. The standard (.EtX ; state f o r d i e t h y l dixanthogen (a = 1.0) has been chosen at i t s maximum s o l u b i l i t y i n water which has been determined experimentally to be 1.27 x 10 5 moles l i t e r " 1 . 3.5.2 Rest P o t e n t i a l Measurements with Platinum Electrode 3.5.2(a) Ethyl Xanthate Solutions Free of Diethyl Dixanthogen The s e r i e s of experiments on the electrode rest p o t e n t i a l s were ca r r i e d out i n the apparatus described i n Section 2.8(a). The stock solutions were prepared from p u r i f i e d KEtX, which was s u b s t a n t i a l l y free from ( E t X ) 2 , and were added to the c e l l where p o t e n t i a l measurements were recorded as argon purging proceeeded. After the p o t e n t i a l had become constant, oxygen was purged through the solutions to generate mixed p o t e n t i a l s . The change from argon to oxygen purge was accompanied by a rapid change to anodic p o t e n t i a l as can be seen i n Figure 19. The concentration of ( E t X ) 2 was measured a f t e r argon purging and af t e r oxygen purging of the neutral (pH 7-8) so l u t i o n . Determinations for (EtX)£ were c a r r i e d out during argon purging of the solutions, and af t e r oxygen purging. In each case, (EtX-)2'was not detected. Even a f t e r runs l a s t i n g 24 hours, no appreciable ( E t X ) 2 was found. This r e s u l t concurs with the r e s u l t s of KEtX decomposition tests reported i n the section on a l k a l i n e decomposition of KEtX i n the presence of p l a t i n i z e d platinum where ( E t X ) 2 was also.not detected, The e f f e c t of electrode pretreatment was important i n determining the value of the maximum cathodic p o t e n t i a l that could be reached under +0.190 • +0.120 +0.110 > +0.100 . w •u g +0.090 A w +0.080 +0.070 -+0.060 . +0.050 0 ^ Argon purge ( -5 9.8 x 10 M KEtX If -4 4.96 x 10 M KEtX 9.67 x 10 4 M KEtX Oxygen purge 20 —r~ 40 - r -60 i 80 100 120 Time i n minutes Figure 19. E f f e c t of oxygen on the r e s t p o t e n t i a l of e t h y l xanthate s o l u t i o n s f r e e from d i e t h y l dixanthogen, pH 7.0-8.0. 140 I 1®§ — i — 180 oo 200 argon purge. E l e c t r o d e s that were prepared by anodic treatment ( i . e . , anodic p o l a r i z a t i o n i n d i l u t e KOH followed by washing w i t h d i s t i l l e d water) were i n i t i a l l y at more p o s i t i v e p o t e n t i a l s than e l e c t r o d e s pre-pared under cathodic c o n d i t i o n s ( i . e . , cathodic p o l a r i z a t i o n i n d i l u t e H^SO^ followed by washing w i t h d i l u t e KOH and d i s t i l l e d water). As the s o l u t i o n was purged w i t h argon a l a r g e r p o t e n t i a l drop was observed f o r the a n o d i c a l l y t r e a t e d platinum than f o r the c a t h o d i c a l l y t r e a t e d m a t e r i a l . A f t e r oxygen a d d i t i o n , . t h e p o t e n t i a l changes were l a r g e r w i t h the a n o d i c a l l y t r e a t e d platinum. 3.5.2(b) E t h y l Xanthate S o l u t i o n s Saturated w i t h D i e t h y l Dixanthogen So l u t i o n s of KEtX containing (EtX)^ at pH 7-8 were prepared e i t h e r by a d d i t i o n of small q u a n t i t i e s of K I ^ to the stock KEtX s o l u t i o n or by a g i t a t i n g l i q u i d ( E t X ) ^ w i t h d i s t i l l e d water f o r 16-24 hours and subsequent a d d i t i o n of KEtX to the s o l u t i o n . The e f f e c t of the r e s i d u a l I produced by t i t r a t i o n method was teste d by measuring -4 -4 -3 the r e s t p o t e n t i a l of 10 M KEtX s o l u t i o n c o n t a i n i n g 10 and 10 M K l . No s i g n i f i c a n t change i n p o t e n t i a l was observed, and the r e s u l t s u s i n g the two techniques are reported together. For KEtX s o l u t i o n s saturated w i t h ( E t X ^ , the r e s t p o t e n t i a l s r a p i d l y reached a constant value w i t h argon purging. Upon i n t r o d u c t i o n of oxygen, the p o t e n t i a l was observed to become more anodic, which agreed w i t h the r e s u l t s of Tolun and Kitchener (39). The magnitude of the p o t e n t i a l d i f f e r e n c e between argon and oxygen purges was found to be a -3 maximum of 18 mV f o r 10 M KEtX and decreased as the KEtX con c e n t r a t i o n -3 was decreased below 10 M. This was a t t r i b u t e d to the establishment w CO CO > cn o > rC w +0.210 +0.200 4 +0.190 +0.180 J +0.170 j +0.160 +0.150 +0.140 +0.130 +0.120 +0.110 +0.100 Argon purge f M-A-To" Oxygen purge 30-A.A—A-5.4 x 10 5 M KEtX, 1.2 x 1 0 - 5 M (EtX). 4 x 10 4 M KEtX, 1.2 x 10~ 5 M (EtX), 9.32 x 10 M KEtX, 1.3 x 1 0 - 5 M (EtX). — i r r 50 70 90 Time i n minutes© 110 130 150 Figure 20. Effect of oxygen on the rest p o t e n t i a l of ethyl xanthate solutions saturated with d i e t h y l dixanthogen, pH 7.0-8.0. oo o of mixed p o t e n t i a l s r e s u l t i n g from the coupling of the cathodic r e d u c t i o n of oxygen w i t h the anodic o x i d a t i o n of e t h y l xanthate. The measurements w i t h a platinum e l e c t r o d e i n d i s t i l l e d water at known concentrations of oxygen have been reported by Natarajan and Iwasaki (66) (Figure 21). Using -nitrogen purging f o r removal of the d i s s o l v e d oxygen they reported a p o t e n t i a l decrease of 60 mV f o r a t e n - f o l d change i n the d i s s o l v e d oxygen content. The maximum change over the experimental region was reported as 160 mV which corresponded to a change i n d i s s o l v e d oxygen content of 2.8 orders of magnitude. These authors explained that the slope of 60 mV/loglO^] r e s u l t e d from adsorption of oxygen i n the e l e c t r o d e followed by a one e l e c t r o n t r a n s f e r step such as the one given by the equation (0 2)ads + e~ + (0 2~)ads This mechanism i s i n agreement w i t h Hoare (67) who considered that ( 0 2 )ads underwent f u r t h e r r e d u c t i o n to produce (H 20 2)ads. The small p o t e n t i a l d i f f e r e n c e observed f o r argon and oxygen purged e t h y l xanthate s o l u t i o n s which were saturated w i t h d i e t h y l -dixanthogen Indicated that the d i s s o l v e d oxygen content had a n o n - s t o i c h i o m e t r i c Influence, on the r e s t p o t e n t i a l . U n f o r t u n a t e l y , because of the complexity of the oxygen h a l f c e l l , no o v e r a l l r e a c t i o n could be assigned to t h i s mixed p o t e n t i a l value. S o l u t i o n s of KEtX saturated w i t h ( E t X ) 2 at pE 8 were l e f t i n the c e l l overnight under oxygen purge. The c o n c e n t r a t i o n of ( E t X ) 2 determined the f o l l o w i n g morning showed that the ( E t X ) 9 content had decreased from -0.56 H -0.54 J -0.52 1 -0.50 -0.48 J -0.46 -0.44 J -0.42 -0.40 +1.0 +2.0 , +3.0 +4.0 +5.0 l o § H 0 2 ] moles? l i t e r +6.0 -1 +7.0 Figure 21. Eh as a f u n c t i o n of d i s s o l v e d oxygen conentration (moles l i t e r d i s t i l l e d water, pH 6.8 ( a f t e r Natarajan and Iwasaki (66)). +8.0 xn 1.3 x 10 ^ if to 2.0 to 2.8 x 10 ^ M and was accompanied by a p o t e n t i a l r i s e of 5 mV. S i m i l a r t e s t s c a r r i e d out under argon purge a l s o showed a decrease i n ( E t X ^ content to the same v a l u e , and i n d i c a t e d that the r e d u c t i o n of dixanthogen proceeded independently from the amount of oxygen present i n the s o l u t i o n . The r e d u c t i o n i n d i e t h y l dixanthogen concentration was a t t r i b u t e d to the base decomposition r e a c t i o n which was proposed i n the previous s e c t i o n of t h i s t h e s i s ( S e c t i o n 3.2.5). The values obtained f o r the r e s t p o t e n t i a l s of platinum e l e c t r o d e s i n KEtX s o l u t i o n s saturated w i t h (EtX)^ (Figure 22) and purged w i t h argon (deoxygenated) are compared w i t h the r e s u l t s obtained by previous workers (Table 12). S o l u t i o n s saturated w i t h (EtX)^ were at the standard s t a t e a ( E t X ) = 1' t' i e e c l u a t i o n became E = E° - J J In [EtX _] Upon e x t r a p o l a t i n g l o g [Et X - ] to '[EtX -] = 1.0 M, E = E° = -0.060 V. The l i n e a r behavior of the slope (60 mV/logI :EtX _]) i s c o n s i s t e n t w i t h the one e l e c t r o n r e a c t i o n f o r e t h y l xanthate o x i d a t i o n on the assumption that the d i e t h y l dixanthogen c o n c e n t r a t i o n i s always constant and at u n i t a c t i v i t y . This assumption was shown to be v a l i d by the e x p e r i -ments where the s o l u b i l i t y of (EtX) ^ i n water was determined. The (EtX)2 concentrations were shown to be constant at n e u t r a l pH f o r at l e a s t the d u r a t i o n of the present experiments, and over the pH ranges used i n t h i s i n v e s t i g a t i o n . +8.3 +0.2 +0.1 • -0.0 -0.1 9' 9  A s''A } • results of present experiments O Majima and Takeda (40) A Tolun and Kitchener (39) 0 Stepanov, et a l . (38) tM Du Rietz (41) F i n k e l s t e i n (37) i -3 i -4 — i — -6 -1 -5 i -7 log [KEtXJl Figure 22. Rest potentials of a platinum electrode i n solutions saturated with d i e t h y l dixanthogen and under an argon atmosphere. Standard Reduction Potentials of the Xanthate-Dixanthogen Couples (RX) + 2e 2RX E p o t e n t i a l (S.H.E.) Reference Method Temp. Methyl Ethyl n-Propyl Iso- n-Butyl Iso- n-Amyl Hexyl propyl Butyl Stepanov et a l (38) Tolun & Kitchener (39) Rest Potential Ambient -0.013 -0.037 -0.068 -0.100 -0.132 -0.157 Du Rietz (41) Potentiometric Ambient T i t r a t i o n Maj ima & Takeda (40) Rest Potential 25° Goldstock (42) Potentiometric 17.5° T i t r a t i o n 33° -0.069 -0.095 -0.120 -0.145 -0.140 -0.155 Rest Pote n t i a l Ambient F i n k e l s t e i n (37) Redox Indicator 25° Foss (63) D i s u l f i d e Exchange Ambient -0.003 -0,049 -0.092 -0.096 -0.127 -0.127 -0.160 -0.053 -0.053 -0.081 C-0.08--0.131 ca.-0.30 3.5.2(c) E f f e c t of pH on Rest P o t e n t i a l Unbuffered s o l u t i o n s of KEtX saturated w i t h ( E t X ) 2 at pH 6.0-7.0 were introduced i n t o the c e l l (Figure 8) and were purged w i t h argon u n t i l a constant p o t e n t i a l was a t t a i n e d (approximately 60 minutes). The pH was changed by dropwise a d d i t i o n of 1 M KOH s o l u t i o n to tbe c e l l w i t h a 1 ml syri n g e . The pH and r e s t p o t e n t i a l were recorded a f t e r reaching a value that remained unchanged f o r 10 minutes. The r e s u l t s of four such runs given i n Figure 23 show a constant p o t e n t i a l i n the pH region between pH 6 to 8. A pH > 8, a p o t e n t i a l decrease of approxi-mately 30 mV/pH was observed which was a t t r i b u t e d to two f a c t o r s : 1. The r e a c t i o n between hydroxyl i o n and ( E t X ) 2 had reached a s u f f i c i e n t r a t e to decrease the c o n c e n t r a t i o n of ( E t X ) 2 from s a t u r a t i o n c o n d i t i o n s . According to the Nernst equation t h i s change i n (EtX)^ c o n c e n t r a t i o n would d r i v e the p o t e n t i a l to more negative v a l u e s . 2. At pH 9, the I0H~] = 10~5 M which i s equal to the I ( E t X ) 2 ] . At higher pH the {OH ] i s l a r g e r than (EtX) and the negative change i n p o t e n t i a l could be a t t r i b u t e d to the formation of mixed p o t e n t i a l s between the xanthate, dixanthogen, H^ O and OH . Natarajan and Iwasaki (66) have shown that the r e s t p o t e n t i a l of a platinum e l e c t r o d e i n d i s t i l l e d water purged w i t h n i t r o g e n obeyed the r e l a t i o n s h i p Eh = 0.80 - 0.059 pH The p o t e n t i a l change of 30 mV/pH observed f o r the present experiments could not be a t t r i b u t e d s o l e l y to the e f f e c t of the oxygen p o t e n t i a l , but to mixed p o t e n t i a l s r e s u l t i n g from the combined e f f e c t s of oxygen +300 +250 H Deoxygenated d i s t i l l e d water +200 w S3 CO to > to •u rH O +150 +100 +50 ^ 5 x 10 M KEtX A ;0 - 4 M ,KEtX • 1 -3 ^ 10 M KEtX 5 x ,10-3 M KEtX 0.0 6.0 T 7.0 8.0 —i— 9.0 pH 10.0 11.0 12.0 Figure 23. 13.0 Effect of .pH on the rest potential of aqueous ethyl xanthate saturated with diethyl dixanthogen. and d i e t h y l dixanthogen on the r e s t p o t e n t i a l s . 3.5.2(d) E f f e c t of D i e t h y l Dixanthogen on the Rest P o t e n t i a l The r e s t p o t e n t i a l s of KEtX s o l u t i o n s c o n t a i n i n g concentrations of (EtX), l e s s than s a t u r a t i o n were al s o s t u d i e d . The r e s u l t s p l o t t e d i n Figure 24 were obtained by a d d i t i o n of small q u a n t i t i e s of K I ^ s o l u t i o n to a KEtX stock s o l u t i o n at pH 7.0. The (EtX)^ concentrations were measured by comparing the r e s u l t s of the hexane e x t r a c t i o n method (Sect i o n 2.5) w i t h the q u a n t i t i e s of normal i o d i n e s o l u t i o n added. The r e s u l t s obtained from these experiments were combined w i t h those obtained from a l l the previous t e s t s on the r e s t p o t e n t i a l s of argon purged e t h y l x a n t h a t e - d i e t h y l dixanthogen s o l u t i o n s are p l o t t e d i n Figure 25. The c a l c u l a t i o n s f o r the a c t i v i t y of (EtX)^ were made on the ba s i s that u n i t a c t i v i t y (Standard State = 1) was reached at maximum s o l u b i l i t y of 1.27 x 10 ^ M. The Nernst equation obtained f o r 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 of .xanthate was w r i t t e n p r e v i o u s l y as E « E° -M m a R t x F (a ) 1 / 2 U ( E t X ) 2 ; 1/2 The values of l o g a E t X ~ / ^ a ( E t . x ) ^ were p l o t t e d against the observed r e s t p o t e n t i a l and t h i s r e s u l t e d i n two separate p l o t s . Those e x p e r i -mental values obtained at pE 6 to 8 ( p l o t (a)) have a slope of 58 mV which i s i n agreement w i t h the r e s u l t s of previous workers reported 1/2 i n F i g u r e 22. The i n t e r c e p t of t h i s p l o t (log a„ -/(a, E i. v> ) ' = 0 ) t,ZX ^JitA.^2 was found to y i e l d E°= -0.063 + 0.01 V which i s a l s o i n agreement w i t h +0.140 +0.130 • +0.120 w w +0.110 CO > CO 4J O > +0.100 . +0.090 5 x 10 M KEtX [(EtX) ]= 0.5 x — A 10 M Saturated Solution (1.27 x 10 5 M) -O-V • / 9.8 x 10 6 M (EtX) • 7.9 x 10 M (EtX), Q 6.8 x 10 - 6 M (EtX), • 4.4 x 10 - 6 M (EtX), -o-i 0 —r-10 20 30 - i — 40 O — — r -50 60 Time in minutes Figure 24. Effect of diethyl dixanthogen on the rest potential of a platinum electrode unde argon atmosphere and constant ethyl xanthate concentration, pH 6.9-7.7. w CO CO rH O > + 0 . 1 ^ 0 +o.iao 4-0.1230 +0.1Q0 +0.1^0 +0.1(50 +0.1^0 +o.i(3;o +0.120 +0.110 +0.100 +0.090 -+0.080 +0.070 +0.060 +0.050 -4.4 3 -4.0 5 x 10 5 M KEtX, pH 8.2-11.5 5 x 10 4 M KEtX, pH 6.0-7.7 10 4 M - 5 x 10 5 M KEtX, pH 9.0-9.3 10 M KEtX, pH 7:0-8.0 V -3.6 -3.2 -2.8 -2.4 -2.0 l o g EtX a ( E t X ) ^ / 2 o Figure 25. C o r r e l a t i o n of the r e s t p o t e n t i a l s of e t h y l xanthate and d i e t h y l dixanthogen s o l u t i o n s w,ith the Nernst equation. the r e s u l t s obtained by previous workers (see Table 12). "The values which deviated from the s t r a i g h t , l i n e - p l o t (Figure 25) were obtained at. pH 9 or at low EtX and (EtX) concentrations. Under these c o n d i t i o n s , "dissolved oxygen, OH or minor contaminants could have, i n t e r f e r e d w i t h the attainment of e q u i l i b r i u m . Thus, no i n t e r p r e t a t i o n can be given f o r those r e s u l t s . 3.6 P o l a r i z a t i o n of Platinum Electrodes i n E t h y l Xanthate-D i e t h y l Dixanthogen S o l u t i o n s The- three e l e c t r o d e apparatus used i n the p o l a r i z a t i o n s t u d i e s was constructed according to the design given by Tolun and Kitchener (39) and i s described i n S e c t i o n 2.8(b). The s o l u t i o n s were prepared i n the s i d e v e s s e l and were purged w i t h argon before being added to the c e l l , which was a l s o under i n e r t atmosphere. The oxygen content could not be t e s t e d s i n c e any a n a l y t i c a l methods which depended on the q u a n t i t a t i v e r e d u c t i o n of oxygen f o r i t s determination would be poisoned by the presence of e t h y l xanthate. However from data s u p p l i e d by Natarajan and Iwasaki (66),the^concentration of oxygen i n —6 —1 d i s t i l l e d water was reduced to l e s s than 2 x 10 moles l i t e r (from -4 -1 10 mole l i t e r ) a f t e r purging w i t h n i t r o g e n f o r one hour. Since the c o n d i t i o n s used i n the present experiments were similar to those used by Natarajan and Iwasaki (66), the supply of oxygen to the e l e c t r o d e was considered to be reduced to n e g l i g i b l e q u a n t i t i e s i n comparison w i t h the concentrations of the other reagents. The three p o l a r i z a t i o n curves given i n Figure 26 were t y p i c a l of s i m i l a r curves obtained under a wide range of experimental c o n d i t i o n s . -3 D e s c r i p t i o n s of these curves, obtained f o r 10 M KEtX i n 1 M KC1 e l e c t r o l y t e were de t a i l e d as follows: 1. The base l i n e (Curve (1)) was established with e l e c t r o l y t e only and with 0.1 M ammonium acetate and 0.1 M sodium borate buffers and e l e c t r o l y t e . The current was noted to undergo through a hysteresis at the s t a r t of each experiment, i . e . , i n proceeding from anodic to cathodic sweeps, the current d i f f e r e d by 10 micro amperes at equivalent p o t e n t i a l s . This e f f e c t was i n s i g n i f i c a n t compared to the magnitude of the current when KEtX was present and had no e f f e c t on the subsequent p o l a r i z a t i o n curves. 2. The a p p l i c a t i o n of anodic overpotential onto the platinum -3 electrode i n argon purged 10 M KEtX resulted i n the electrochemical oxidation of KEtX to (EtX)„. At an applied p o t e n t i a l E„ „ _ = +0.42 V, the electrode was removed and an i n f r a r e d spectrum of the region 1400 cm ^ to 700 cm ^ was obtained using the ATR technique. The spectrum confirmed the presence of ( E t X ) 2 as can be seen i n Figure 29. The c i r c u i t was opened at +0.55 V and the p o t e n t i a l value decayed to the rest p o t e n t i a l value +0.123 V (S.H.E.) i n twenty minutes. The current observed during cathodic p o l a r i z a t i o n resulted from reduction of the adsorbed "(EtX) to EtX and proceeded u n t i l . . . r hydrogen was evolved. The anodic and cathodic curves were i d e n t i c a l for solutions buffered with ammonium acetate, sodium borate and without buffer. In the l a t t e r case, the pH of the so l u t i o n , i n i t i a l l y at 8.5 dropped to 5.5 during anodic p o l a r i z a t i o n when oxygen began to evolve ' and then rose to 9.5 during the cathodic p o l a r i z a t i o n sweep when hydrogen was evolved. 3. Mixed p o l a r i z a t i o n curves were obtained from KEtX. solutions -3 purged with oxygen. The rest p o t e n t i a l i n 10 M KEtX was +0.136 V Figure 26. P o l a r i z a t i o n curves f o r a platinum electrode i n 10 M e t h y l xanthate i n 1 M KC1 e l e c t r o l y t e . (S.H.E.) or 13 mV more anodic than the same s o l u t i o n under argon purge. S i m i l a r r e s u l t s are given i n F i gure 20. The anodic p o l a r i z a t i o n curve followed the o x i d a t i o n of a i r f r e e KEtX c l o s e l y u n t i l the p o t e n t i a l exceeded +0.3 V. The cathodic p o l a r i z a t i o n sweep i l l u s t r a t e d the r e d u c t i o n of oxygen along w i t h ( E t X ^ . The r a t e of oxygen r e d u c t i o n was s u f f i c i e n t l y r a p i d that the r e d u c t i o n of ( E t X ) 2 could not be observed. In a d d i t i o n to the observations presented i n Figure 26, s e v e r a l other experimental parameters were i n v e s t i g a t e d which d i d not r e s u l t i n any marked e f f e c t on the o v e r a l l shape of the curves. These r e s u l t s can be summarized as f o l l o w s : -4 -3 1. P o l a r i z a t i o n curves f o r 10 M and 5 x 10 M KEtX were obtained w i t h no r e s u l t i n g change i n t h e i r shape. The r e s t p o t e n t i a l f o r 1 0 - 4 M KEtX was +0.187 V (S.H.E.) and f o r 5 x 10~ 3 M KEtX was +0.103 V (S.H.E.). 2. E l e c t r o l y t e s o l u t i o n s of 0.1 M and 1.0 M KC1 had no e f f e c t on the shape of the p o l a r i z a t i o n curves, but s l i g h t l y lower currents were observed f o r 0.1 M KC1 s o l u t i o n s . 3. The r e s u l t s obtained from shiny and p l a t i n i z e d platinum elec t r o d e s were l i k e w i s e s i m i l a r . Current d e n s i t i e s obtained f o r p l a t i n i z e d platinum were l a r g e r than those observed f o r shiny platinum, due to the increased surface area of the p l a t i n i z e d e l e c t r o d e . The r e s u l t s presented i n Figure 26 were r e p l o t t e d i n Figure 27 to show the T a f e l behavior of the anodic and cathodic p o l a r i z a t i o n sweeps. In the place of the ammeter -used to o b t a i n the r e s u l t s presented i n Figure 26, a s p e c i a l potentiostat was employed which improved the current measuring accuracy of the apparatus. Anodic and cathodic p o l a r i z a t i o n p o t e n t i a l s were a p p l i e d i n steps of 10 mV and the steady s t a t e current -3 was measured. The p l o t f o r 10 M KEtX s o l u t i o n c o n t a i n i n g 1 M KC1 e l e c t r o l y t e purged w i t h n i t r o g e n i s given i n Figure 27. From the i n i t i a l anodic p o l a r i z a t i o n , a l i n e a r T a f e l r e g i o n was observed, xvith a slope of approximately 60 mV/log i . From the T a f e l equation r\ = a + b l o g i b = 2.303 ^ | r =0,059 „ 0.060 L ' U Suchaa value f o r n was expected f o r a one e l e c t r o n r e a c t i o n . During cathodic p o l a r i z a t i o n , a l i n e a r T a f e l r e g i o n was not observed. The slope of 80 mV/log i was obtained from a l i n e drawn from the r e s t p o t e n t i a l to a tangent of the cathodic curve (see Figure 27(b)). The n o n - l i n e a r l y may be explained by c o n s i d e r i n g that the supply of uncharged dixanthogen molecules was l i m i t e d to t h e i r r a t e of d i f f u s i o n to the e l e c t r o d e , and thus a re g i o n c o n t r o l l e d by a c t i v a t i o n over-p o t e n t i a l would be d i f f i c u l t to ,obtain. The exchange current d e n s i t y ( n = 0) was 4 x 10 1 amp/geometric cm . P o t t e r (68), i n a d i s c u s s i o n of the s i g n i f i c a n c e of exchange current values pointed out th a t i f o r v a r i o u s e l e c t r o d e processes cover o r —18 —2 2 a range from about 10 to 10 amps/geometrical cm of the e l e c t r o d e . -7 2 I f the value of i Q i s low (e.g., 10 amps/cm ) i t would be u n l i k e l y except i n systems of e x c e p t i o n a l p u r i t y that the r e v e r s i b l e p o t e n t i a l -4 gure 27. -3 +240 1 +220 +200 -+180 " +160 +140 +120 +100 ' +80 . +60 • +40 ~^ 2 -4 / J 1, 'O^ (b) o I — I — -3 -2 log 1 amperes/geometrical cm -3 P o l a r i z a t i o n diagram f o r a platinum electrode i n 10 M e t h y l xanthate, 1 M KC1 e l e c t r o l y t e (a) p o l a r i z a t i o n over a one v o l t range, (b) m a g n i f i c a t i o n of (a) to show T a f e l behavior. of the e l e c t r o d e process could be a t t a i n e d p r a c t i a l l y before some a l t e r n a t i v e process capable of s u s t a i n i n g a current d e n s i t y g r e a t e r than i assumed the c o n t r o l of the p o t e n t i a l . For the r e v e r s i b l e o _3 hydrogen e l e c t r o d e at platinum cathodes, i i s about 10 amp/geometrical 2 cm and i m p u r i t i e s that would interfere w i t h the r e v e r s i b l e hydrogen r e a c t i o n would be present i n e a s i l y detected q u a n t i t i e s . The exchange current betweeen e t h y l xanthate and diethyl dixanthogen was found to be 4 x 10 amps/geometrical cm . From the previous examples, t h i s current should be s u f f i c i e n t l y l a r g e to exclude the e f f e c t of any minor i m p u r i t i e s i n the s o l u t i o n , w i t h the r e s u l t that the r e s t p o t e n t i a l measurements would a c c u r a t e l y r e f l e c t the e l e c t r o -chemical e q u i l i b r i u m between e t h y l xanthate and d i e t h y l dixanthogen. 3.7 I n f r a r e d I n v e s t i g a t i o n s of Platinum E l e c t r o d e Surface The reference i n f r a r e d s p e c t r a f o r KEtX (Figure 28(a)) was obtained using the KBr p e l l e t technique, the spectrum of d i e t h y l dixanthogen was obtained from a c a p i l l a r y f i l m (Figure 28(b)) and the spectrum of aqueous e t h y l xanthate i o n (Figure 28(c)) was obtained using a s p e c i a l c e l l constructed from polyethylene f i l m . From observations of the s p e c t r a , the d i f f e r e n c e between xanthate dixanthogen ' and platinum xanthate can e a s i l y be d i s t i n g u i s h e d on the b a s i s of t h e i r c h a r a c t e r i s t i c peaks i n the region 1300 cm 1 to 700 cm \ The assignment of molecular v i b r a t i o n s i n the xanthate molecule to the i n f r a r e d absorption bands has been c a r r i e d out by L i t t l e , P o l i n g and L e j a (69). The strong band f o r KEtX at 1050cm" 1 and_at 1040cm" 1 for ( E t X ) 2 and (EtX ) has been a t t r i b u t e d to the C=S s t r e t c h i n g mode. The multipeaked band i n the region 1080 to 1180 cm 1 f o r KEtX was designated as r e s u l t i n g from s t r e t c h i n g v i b r a t i o n s of the C-O-C l i n k a g e . S i m i l a r C-O-C bands are observed at 1260 and 1240 cm 1 f o r (EtX)2 and at 1160 cm 1 f o r aqueous e t h y l xanthate i o n s . Spectra of the m a t e r i a l deposited on the surface of platinum e l e c t r o d e s were obtained using the m u l t i p l e ATR technique described i n Sectio n 2.7(c). A summary of th e d n f r a r e d r e s u l t s presented i n Figure 29 f o l l o w s : 1. The platinum f o i l , whether shiny or p l a t i n i z e d platinum y i e l d e d a f l a t base l i n e as can be seen i n Figure 29(a). The three s m a l l peaks observed at 1040, 925 and 770 cm 1 r e s u l t e d from the l i g h t t r a n s m i s s i o n c h a r a c t e r i s t i c s of the KRS-5 c r y s t a l and were not due to absorption bands from the platinum. - 3 2. A f t e r the platinum f o i l had been a n o d i c a l l y p o l a r i z e d i n 10 M KEtX at +0.43 Y (S.H.E.) f o r one minute.(see Figure 26) the el e c t r o d e was withdrawn from the s o l u t i o n and the spectrum, c h a r a c t e r i s t i c of (EtX)2 was obtained (Spectrum ( b ) ) . Ether washing r e s u l t e d i n removal of the adsorbed l a y e r from the e l e c t r o d e as was observed i n Spectrum ( c ) . 3. D i e t h y l dixanthogen, which had been deposited on the el e c t r o d e by anodic p o l a r i z a t i o n was then subjected to cathodic p o l a r i z a t i o n f o r one minute at -0.6 V (S.H.E.). The r e s u l t a n t spectrum (d) i l l u s t r a t e s the r e d u c t i o n of a l l of the (EtX)^ deposited by anodic p o l a r i z a t i o n . _3 4. P l a t i n i z e d platinum f o i l was al s o immersed i n 10 M xanthate s o l u t i o n without p o l a r i z a t i o n . Under b a s i c c o n d i t i o n s , pH 8 to 11, no (EtX)„ was observed, but pH 6.0, w i t h 30 minutes exposure time i n the o • H CO CO • H e CO c H C CU O S-i CU CM 50 100 50 100 50 100 (EtX) (liquid) (c) KEtX (aqueous) Pt(EfX) (solid) 1400 1300 1200 1100 1000 900 800 700 wavenumber cm 1 Figure 28. Infrared spectra of ethyl xanthate compounds, .(a) KEtX, solid, KBr pellet; (b) diethyl dixanthogen, capillary, film; (c) ethyl xanthate ions, aqueous solution; (d) platinum xamttehate, KBr pellet. (a) Pt f o i l a f t e r cleaning i n aqua regis and cathodic p o l a r i z a t i o n for 15 mins. IX 50 100 (b) 50 100 Pt f o i l a f t e r anodic p o l a r i z a t i o n at +0.2 V for 1 min. i n 10" 3 M KEtX OS) (b) a f t e r ether wash (d) (b) a f t e r cathodic p o l a r i z a -t i o n 50 100 Pt f o i l a f t e r 30 mins. i n 1 0 - 3 M KEtX (not^eolarized) pH 6.0 1400 Figure 29. 1300 1200 1000 -1 900 i 800 1100 Wavenumber cm Infrared (ATR) spectra of platinum f o i l a f t e r varying treatments. s o l u t i o n , weak bands a t t r i b u t e d t o t h e f o r m a t i o n o f ( E t X ) ^ were o b s e r v e d (Spectrum ( e ) ) . The f o r m a t i o n o f p l a t i n u m o x i d e s and h y d r o x i d e s a t t h e p l a t i n u m s u r f a c e has been s u g g e s t e d as c o n t r i b u t i n g t o t h e n o n - r e v e r s i b i l i t y of t h e Pt - 0 2-H20 system ( 6 7 ) . By a n a l o g y , p l a t i n u m x a n t h a t e s i n t h e P t -KEtX-IL^O system c o u l d a l s o be found on t h e e l e c t r o d e , and t h e p o t e n t i a l measurements would r e f l e c t a n o n - e q u i l i b r i u m p r o c e s s . P l a t i n u m x a n t h a t e was p r e p a r e d by a method d e s c r i b e d by Watt and McCormick (70) and i t s i n f r a r e d s p e c t r u m was o b t a i n e d u s i n g t h e KBr p e l l e t t e c h n i q u e ( F i g u r e 2 9 ( d ) ) . The a b s o r p t i o n bands c o r r e s p o n d e d w i t h t h o s e o b t a i n e d by Watt and McCormick (70) ( T a b l e 1 2 ) . The r e s u l t s p r e s e n t e d i n F i g u r e 29 show t h a t p l a t i n u m x a n t h a t e i s n o t o b s e r v e d under t h e c o n d i t i o n s i n v e s t i g a t e d . The amount o f p l a t i n u m x a n t h a t e r e q u i r e d t o b l o c k t h e e l e c t r o d e p r o c e s s e s between KEtX and ( E t X ) 2 can be assumed t o s u f f i c i e n t t o c o v e r t h e a c t i v e s i t e s , w h i c h i s l e s s t h a n a m o n o l a y e r . The c o v e r a g e o f p l a t i n u m x a n t h a t e on t h e e l e c t r o d e f o r t h e g e o m e t r i c s u r f a c e a r e a can be c a l c u l a t e d assuming °2 t h a t t h e a r e a o f a x a n t h a t e m o l e c u l e i s 28 A . The e l e c t r o d e w o u l d c o n t a i n x 321 gm" mole = 9 x 10T6 gm P t ( E t X l 2 2 16 ^2 e l e c t r o d e a r e a (4 x 10 cm x 10 A 2 cm 28A° 2 6 .02 x 1 0 2 3 m o l e c u l e s - • mole Because of s u r f a c e r o u g h n e s s , t h e a c t u a l a r e a o f t h e e l e c t r o d e s u r f a c e i s a t l e a s t t w i c e t h e g e o m e t r i c a r e a f o r a s h i n y p l a t i n u m e l e c t r o d e ( 6 8 ) . TABLE 13 V i b r a t i o n a l Band Assignments f o r the I n f r a r e d Spectrum of Platinum E t h y l Xanthate ( a f t e r Watt and McCormick (70)) P t ( E t X ) 2 Assignment D e s c r i p t i o n 1389 w 6 C C H 2 ) deformation 1371 m 6 C C I I 3 I deformation 1320 sh,w 1285 -vs . vCC-Oi s t r e t c h i n g 1144 w OJCCF^) wagging 1114 s v ( C = S ) s t r e t c h i n g 1058 vw v(CH 2) t w i s t i n g 1017 vs P (CH 3) roc k i n g 1004 m 853 m v (C-C) s t r e t c h i n g vw - very weak w - weak m - medium sh - sharp vs - very sharp P l a t i n i z i n g increased t h i s area by a f a c t o r of 10 to 100 times. Under these c o n d i t i o n s , monolayer coverages of platinum xanthate could be detected by i n f r a r e d spectroscopy. The absence of any i n f r a r e d bands that may be r e l a t e d to platinum xanthate i n d i c a t e s that i f i t were present, the coverage would be l e s s than a monolayer. 3.8 I n t e r p r e t a t i o n of the E l e c t r o c h e m i c a l Results The e l e c t r o c h e m i s t r y of xanthate s o l u t i o n s saturated w i t h dixanthogen has been wi d e l y s t u d i e d . Some of the r e s u l t s of these v a r i o u s s t u d i e s on e t h y l xanthate are presented i n Figure 2 2 - where the standard p o t e n t i a l was determined by e x t r a p o l a t i n g the measured E values at v a r i o u s xanthate concentrations to [EtX ] = 1.0 M. S i m i l a r i n v e s t i g a t i o n s w i t h other xanthate homologues have a l s o been reported and are given i n Table 12. The present r e s u l t s obtained f o r e t h y l xanthate and d i e t h y l dixanthogen were undertaken to add i n f o r m a t i o n about the e f f e c t of oxygen on the e l e c t r o d e p o t e n t i a l measured i n e t h y l xanthate s o l u t i o n s , and of the e f f e c t of d i e t h y l dixanthogen concentrations l e s s than s a t u r a t i o n on the e l e c t r o d e p o t e n t i a l . The r e s u l t s of t h i s work can be summarized as f o l l o w s : 1. The e l e c t r o d e p o t e n t i a l s observed f o r xanthate s o l u t i o n s saturated w i t h oxygen are considered to a r i s e from two sources: Ca) e i t h e r they represent a n o n - e q u i l i b r i u m r e a c t i o n at the e l e c t r o d e ' s u r f a c e , which r e s u l t s from immeasurably slow r e a c t i o n s between e t h y l xanthate and oxygen or; (b) the oxygen and e t h y l xanthate do not react at a l l , In which case the observed p o t e n t i a l i s a mixed p o t e n t i a l dependent upon the r e l a t i v e exchange currents of v a r i o u s species c o n t r i b u t -i n g to the e l e c t r o d e process. The evidence obtained i n support of t h i s c o n c l u s i o n can be b r i e f l y s t a t e d as f o l l o w s : ( i ) the r e l a t i v e concentrations of e t h y l xanthate and d i e t h y l dixanthogen were not i n f l u e n c e d by e i t h e r the pH or oxygen content of the s o l u t i o n i n the pH region 6 to 9. ( i i ) the d i f f e r e n c e i n p o t e n t i a l between argon and oxygen purged s o l u t i o n s of e t h y l xanthate saturated w i t h d i e t h y l dixanthogen was a maximum of 18 mV., although argon purging over a p e r i o d of 60 minutes would have reduced the oxygen content to 2 x 10 ^ moles l i t e r \ or 2% of s a t u r a t i o n (66). This i n d i c a t e s that oxygen has a n o n - s t o i c h i o m e t r i c e f f e c t on the e l e c t r o d e r e a c t i o n . ( i i i ) The n o n - l i n e a r r e l a t i o n s h i p between the p o t e n t i a l and the oxygen content i n d i c a t e s mixed p o t e n t i a l s were observed which could not be r e l a t e d to any o v e r a l l e l e c t r o d e r e a c t i o n . ( i v ) The value obtained f o r the exchange current of the e t h y l x a n t h a t e - d i e t h y l dixanthogen couple on a platinum e l e c t r o d e (4 x -4 2 10 amps/geometrical cm ) was s u f f i c i e n t l y l a r g e r than the value -9 obtained f o r the oxygen-hydroxyl couple ( v i z 1.3 x 10 amps/ 2 geometrical cm (67)) which i n d i c a t e s that the e t h y l xanthate-d i e t h y l dixanthogen couple would c o n t r o l the measured e l e c t r o d e p o t e n t i a l s . 2. W i t h i n the l i m i t a t i o n of these experiments, the o x i d a t i o n of e t h y l xanthate to d i e t h y l dixanthogen has been shown to be a r e v e r s i b l e r e a c t i o n i n an e l e c t r o c h e m i c a l sense. The evidence obtained i n support of t h i s c o n c l u s i o n can be b r i e f l y s t a t e d as f o l l o w s : ' Ci) The r e s t p o t e n t i a l s of v a r y i n g concentrations of e t h y l xanthate i n s o l u t i o n s saturated w i t h d i e t h y l dixanthogen c o r r e l a t e w i t h the Nernst equation i n that a l i n e a r p l o t w i t h a slope of 60 mV/log [EtX ] was observed. ( i i ) The r e s t p o t e n t i a l s of v a r y i n g concentrations of d i e t h y l dixanthogen at constant e t h y l xanthate concentration was teste d w i t h the Nernst equation and a l i n e a r p l o t w i t h a slope 1/2 of 59 mV/log a ^ - / ( a / r , t V . ) was observed. The e x t r a p o l a t e d E t A ( b t X ; 2 value f o r E° = -0.063 V c o r r e l a t e d w e l l w i t h the r e s u l t s of other workers. ( i i i ) The l a c k of evidence of platinum xanthate added support f o r the absence of chemical r e a c t i o n between the platinum e l e c t r o d e and the xanthate s o l u t i o n . The formation of platinum xanthate on the e l e c t r o d e surface as an intermediate step during the o x i d a t i o n of xanthate or the r e d u c t i o n of dixanthogen i s expected to be q u i t e p o s s i b l e . The present r e s u l t s have shown that t h i s r e a c t i o n does not i n t e r f e r e w i t h the o v e r a l l t r a n s f e r of e l e c t r o n s between xanthate and dixanthogen. S i m i l a r observations have been made by G o l d s t i c k (42) on the r e a c t i o n s between xanthate and the s i l v e r e l e c t r o d e . Although s i l v e r xanthate was formed on the e l e c t r o d e , the surface f i l m was found to be s u f f i c i e n t l y conducting so that the o x i d a t i o n p o t e n t i a l of xanthate to dixanthogen could be measured. From these measurements, G o l d s t i c k (42) concluded that the r e v e r s i b l e e l e c t r o d e p o t e n t i a l of the xanthate-dixanthogen couple could be measured w i t h s i l v e r e l e c t r o d e s . The adsorption of xanthate on copper surfaces r e s u l t e d i n the r a p i d growth of a cuprous xanthate f i l m which i n s u l a t e d the metal surface from the xanthate s o l u t i o n . The cuprous xanthate f i l m could not be removed by a p p l i c a t i o n of cathodic p o t e n t i a l as:was found f o r platinum and s i l y e r . Copper el e c t r o d e s were concluded to behave i r r e v e r s i b l y and were u n s u i t a b l e for the measurement of xanthate-dixanthogen e l e c t r o d e p o t e n t i a l s . CHAPTER 4 SUMMARY AND CONCLUSIONS The e v a l u a t i o n of e t h y l xanthate decomposition, d i e t h y l dixanthogen decomposition and the e l e c t r o c h e m i c a l behavior of the e t h y l xanthate-d i e t h y l dixanthogen couple has demonstrated that each of these systems undergo a d i f f e r e n t set of r e a c t i o n s i n n e u t r a l to m i l d l y a l k a l i n e c o n d i t i o n s . The main r e s u l t s r e l a t i n g to each system can be b r i e f l y described as f o l l o w s : (1) The decomposition of KEtX i n n e u t r a l to m i l d l y a l k a l i n e aqueous s o l u t i o n was shown to proceed by a h y d r o l y t i c mechanism given by the equation S II _ C2H5-0-C-S + H 20 y C 2 H 5 ° H + C S 2 + ° H The k i n e t i c s t u d i e s supported the po s t u l a t e d mechanism by p r o v i d i n g the f o l l o w i n g i n f o r m a t i o n ( i ) The qu a n t i t y of CS 2 evolved was i n a 1:1 molar r a t i o f o r the q u a n t i t y of KEtX decomposed (Table 6). ( i i ) The r a t e of CS 2 e v o l u t i o n was equal to the r a t e of KEtX decomposition,(Figure 14). ( i i i ) The r e a c t i o n was f i r s t order i n KEtX, i n d i c a t i n g that only one molecule of KEtX was i n v o l v e d i n the r a t e c o n t r o l l i n g steps of the decomposition process. The f i r s t order r a t e constant v was al s o measured (k^ = 7.6 + 1.0 x 10 4 hr "*") (Section 3.1.1). ( i v ) The decomposition r a t e was independent of pH over the range tes t e d (pH 7-12). The increase i n pH observed during the course of a decomposition run was a l s o r e l a t e d to the pos t u l a t e d r e a c t i o n (Figure 13). (v) (EtX)^ was not detected as one of the r e a c t i o n products. A u x i l l i a r y experiments were a l s o c a r r i e d out which explored the p o s s i b i l i t y that ( E t X ^ could be produced by o x i d a t i o n of aqueous KEtX w i t h oxygen i n the presence of c a t a l y t i c agents. Of three p o s s i b l e agents t e s t e d , ferrous s u l f a t e had a s l i g h t r e t a r d i n g e f f e c t 6n the decomposition r a t e of KEtX, w h i l e p l a t i n i z e d platinum and methylene blue a c c e l e r a t e d the r a t e by 2 and 10 times r e s p e c t i v e l y . ( E t X ) ^ was not detected during the course of the runs (Table 7). (2) The r e a c t i o n between d i e t h y l dixanthogen and hydroxyl i o n was found to proceed i n two steps i n which the f i r s t step was a n u c l e o p h i l i c a t t a c k of the s u l f u r - s u l f u r bond of (EtX)2 by OH which r e s u l t e d i n the formation of e t h y l xanthate and an unstable s u l f e n i c a c i d intermediate. In the second step of the r e a c t i o n ; the d i s p r o p o r t i o n -a t i o n of e t h y l xanthate s u l f e n i c a c i d r e s u l t e d i n the formation of CS2 and other products. The r e a c t i o n s are given by the f o l l o w i n g equations OH S S II II C 2H 50-C-S-0H + C H 0-C-S S W CoHr0-C-S-0H C 2H 50H + CS 2 + OH The observations which supported the proposed mechanism can be b r i e f l y s t a t e d as f o l l o w s : ( i ) M a t e r i a l balance s t u d i e d showed that one mole of EtX was generated f o r each mole of (EtX)2 decomposed (Table 8 ) . ( i i ) K i n e t i c s t u d i e s confirmed the r a t e to be f i r s t order w i t h respect to each of the concentrations of (EtX)2 and. OH (Sec t i o n 3.3). _ d [ ( E t X ) ] d t = k 2 [ ( E t X ) 2 ] [ 0 H ] This r a t e expression i s c o n s i s t e n t w i t h that r e q u i r e d f o r the po s t u l a t e d b i m o l e c u l a r displacement mechanism. The experimental second order r a t e constant was measured (^2 = 14.4 + 0.3 l i t e r mole "'"min ^) . (3) The e l e c t r o c h e m i c a l experiments i n d i c a t e d that aqueous s o l u t i o n s of e t h y l xanthate and d i e t h y l dixanthogen came to a 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 e q u i l i b r i u m at a platinum surface. I t was a l s o shown that e t h y l xanthate was r e a d i l y o x i d i z e d to dixanthogen by the a p p l i c a -t i o n of a small anodic o v e r p o t e n t i a l , but not by d i s s o l v e d oxygen. The experimental evidence supporting the co n c l u s i o n that e t h y l xanthate and d i e t h y l dixanthogen behave r e v e r s i b l y can be b r i e f l y given as f o l l o w s : ( i ) Rest p o t e n t i a l measurements were obtained f o r v a r y i n g KEtX con c e n t r a t i o n at constant saturated aqueous (EtX)2 concentra-t i o n s (Figure 23). The r e s u l t s f i t t e d the Nernst equation f o r a one e l e c t r o n o x i d a t i o n r e a c t i o n . ( i i ) Rest p o t e n t i a l measurements were a l s o obtained f o r va r y i n g (EtX)„ conc e n t r a t i o n at constant KEtX c o n c e n t r a t i o n , and these a l s o i n d i c a t e d a one e l e c t r o n t r a n s f e r r e a c t i o n (Figure 25)• ( i i i ) The T a f e l slope r e s u l t i n g from the i n i t i a l stages of anodic p o l a r i z a t i o n of the platinum e l e c t r o d e i n d i c a t e d a one e l e c t r o n o x i d a t i o n r e a c t i o n . I n f r a r e d s p e c t r o s c o p i c examination of the platinum surface confirmed ( E t X ^ as the only d e t e c t a b l e r e a c t i o n product • (Figures 27 and 29). (i v ) No changes i n the ( E t X ^ content of KEtX s o l u t i o n s were noted a f t e r purging w i t h oxygen. This evidence supported the c o n c l u s i o n that d i s s o l v e d oxygen was not e f f e c t i v e i n the o x i d a t i o n of KEtX to (EtX) 2'.(Section 3.5.2(b)). Numerous studies on the decomposition of KEtX under a c i d i c c o n d i t i o n s have shown that the e t h y l xanthate anion i s protonated, forming x a n t h i c a c i d which decomposes i n t o e t h y l a l c o h o l and carbon d i s u l f i d e according to the f o l l o w i n g equation S S H - K I' + k 2 + C oH r0-C-S + H o0 -^-»- C„Hc0-C-SH + H C„Hc0H + CS0 + H 2 5 2 •< 2 5 2 5 2 A review of the published work on t h i s r e a c t i o n i s presented i n Appendix 2. Determinations of the d i s s o c i a t i o n constant f o r x a n t h i c a c i d -2 -1 formation g e n e r a l l y agree on a value of 2.8 ± 0.3 x 10 moles l i t e r The second order r a t e constant f o r x a n t h i c a c i d decomposition has been reported as k^ = 230 Z 20 l i t e r mole "'"min "*". The decomposition of aqueous e t h y l xanthate and d i e t h y l dixanthogen has been summarized i n Figure 30 where the h a l f - l i f e time of the decomposi-t i o n i s p l o t t e d against pH. The f i g u r e i l l u s t r a t e s the i n s t a b i l i t y of xanthate under a c i d i c c o n d i t i o n s and the i n s t a b i l i t y of dixanthogen under b a s i c c o n d i t i o n s . s o l u t i o n at 22°C. The present r e s u l t s a l s o provide some new i n f o r m a t i o n to the mech-anisms of depression of some m i n e r a l s , notably p y r i t e . KCN, ^ 2 8 2 0 ^ and CaO are depressants f o r p y r i t e and a p a r t i a l e x p l a nation f o r t h i s r e s u l t i s the r e a c t i v i t y of these reagents toward the adsorbed dixanthogen. The e f f e c t of t h i s r e a c t i o n i s only a p a r t i a l answer to the problem of e x p l a i n i n g depression s i n c e depressants a l s o a f f e c t the zeta p o t e n t i a l and adsorption of oxygen on the mi n e r a l surface. APPENDIX I Curve F i t t i n g and E r r o r A n a l y s i s Curve f i t t i n g was obtained by. f i r s t transposing the data to f i t a set of l i n e a r coordinates and then applying l i n e a r r e g r e s s i o n a n a l y s i s to those r e s u l t s . The c a l c u l a t i o n s were c a r r i e d out on a Hewlett Packard Model 9100 A c a l c u l a t o r which contained a program that y i e l d e d values f o r the i n t e r c e p t a, the slope b, and the c o r r e l a t i o n c o e f f i c i e n t r . The c a l c u l a t e d values of | r | are given i n Table 1. These are compared w i t h tabulated v a l u e s obtained from N e v i l l e and Kennedy (74) f o r a s i g n i f i c a n c e l e v e l of 95%. A genuine c o r r e l a t i o n at t h i s l e v e l i s shown to e x i s t when the c a l c u l a t e d v a l u e of | r | exceeds the tabulated value. The equation f o r a s t r a i g h t l i n e i s given as y =. a + bx where b i s the slope of the l i n e The e r r o r i n slope i s given by b +1 t R where t R i s the confidence l i m i t . The t v a l u e s f o r a 95% confidence l i m i t were obtained from a t a b l e i n N e v i l l e and Kennedy (74) which r e l a t e d the number of experimental p o i n t s to the p r o b a b i l i t y v a l u e . R i s defined by £ (y^y) 2-mE (y^-y) Cx,.-x) where x and y are the mean v a l u e s , x_^  and y_^  are the experimental values and n i s the number of t r i a l s . Values f o r t and R are a l s o \ given i n Table;1. The r e s u l t s obtained from s t u d i e s on the k i n e t i c s of xanthate and dixanthogen decomposition r e a c t i o n s and those obtained from the e l e c t r o c h e m i c a l experiments were t r e a t e d w i t h the same methods, and the l e v e l s of s i g n i f i c a n c e f o r the r e s u l t s are given w i t h those r e s u l t s . C o r r e l a t i o n C o e f f i c i e n t and E r r o r i n Rate Constants f o r the Oxidation of KEtX S o l u t i o n s D e s c r i p t i o n of Run | r | | r | t h e o r e t i c a l t R x 10 F i r s t Order Rate Constant ± t R Methylene Blue 1. oxygen free 0.95 2. oxygen saturated 0.92 0.81 0.93 2.132 2.015 .23 .50 2.3 ± 0.05 x 10 4 h r 1 400 ± 100 hr -1 P l a t i n i z e d Platinum 1. oxygen free 0.99 2. oxygen saturated 0.997 0.95 0.75 2.92 .25 9:7 ± 0.7 x 10 4 h r 1 2.015 .48 15.7 ± 1.0 x 1 0 - 4 h r - 1 Ferrous S u l f a t e 1. oxygen free 0.98 2. oxygen saturated 0.975 0.86 0.93 2.132 2.015 0.23 0.19 3.6 ± 0.5 x 10 4 h r 1 4.2 ± 0.5 x 1 0 ~ 4 h r - 1 APPENDIX I I Decomposition of E t h y l Xanthate In Ac i d S o l u t i o n A l k y l xanthates of Na + or K + i o n i z e r e a d i l y and react w i t h water under a c i d i c c o n d i t i o n s to form x a n t h i c a c i d . S ' S II II R-O-C-S + HO y- R-O-C-SH + OH Aqueous x a n t h i c a c i d i s h i g h l y unstable and decomposes r a p i d l y i n t o carbon d i s u l f i d e and a l c o h o l S II R-O-C-SH y ROH + CS The r e a c t i o n r a t e constant has been shown by Iwasaki and Cooke (75,76) to be second order,and the k i n e t i c s of the decomposition of x a n t h i c a c i d have been described by the f o l l o w i n g equation ^=g- = k (X")(H+) Guadin (72), i n h i s st u d i e s on the f l o t a t i o n of p y r i t e w i t h e t h y l xanthate reported that the decomposition of the xanthate at low pH had to be taken i n t o account to e x p l a i n the high xanthate consumption during h i s t e s t s . A number of st u d i e s have been c a r r i e d out to determine the i o n i z a t i o n constant and the decomposition r a t e of e t h y l xanthate. This s e c t i o n i s a summary of the works published on the decomposition of e t h y l xanthate i n a c i d s o l u t i o n . The f i r s t i n v e s t i g a t o r s of the a c i d xanthate r e a c t i o n concluded that the decomposition r e s u l t e d from combination of the xanthate i o n w i t h hydrogen i o n to form x a n t h i c a c i d , which decomposed i n t o carbon d i s u l f i d e and a l c o h o l , von Halban and Hecht (77) mixed e t h y l xanthate s o l u t i o n s w i t h h y d r o c h l o r i c a c i d , stopped the r e a c t i o n at s u i t a b l e times by the a d d i t i o n of sodium carbonate and then determined the remaining e t h y l xanthate by i o d i n e t i t r a t i o n . The i o n i z a t i o n constant was c a l c u l a t e d f o r the d i s s o c i a t i o n of e t h y l x a n t h i c a c i d (0.030) and the second order r a t e constant (16.0 l i t e r mole "*"min ^) f o r the decomposition r e a c t i o n was determined (see Table 14). King, e t . a l . (78,79,80) measured the decomposition r a t e by f o l l o w i n g the i n c r e a s e i n vapor pressure of the r e a c t i o n due to the formation of carbon d i s u l f i d e and other products. The r a t e s f o r a number of xanthate homologues were st u d i e d by Komylev (81) who c o r r e l a t e d the h a l f - l i f e of the r e a c t i o n s with. pE, temperature and chain length of the p a r t i c u l a r xanthate. Cook and Nixon (82), using a pE technique reported a value f o r the e q u i l i b r i u m constant considerably lower than those reported p r e v i o u s l y . In a l a t e r paper they admitted that t h e i r method was subject to a considerable e r r o r because of the non-stoichiometric: consumption of hydrogen ions i n the r e a c t i o n . A l l of the more recent s t u d i e s (since 1958) have been made employing the u l t r a v i o l e t spectrophotometric method f o r measuring the xanthate i o n c o n c e n t r a t i o n . Iwasaki and Cooke (75,76) using t h i s method evaluated the r a t e constant f o r e t h y l xanthate to be 4.3 min 1 and the i o n i z a t i o n constant as 0.020. K l e i n , e t . a l . (83), a l s o using spectro-scopic methods p o s t u l a t e d a decomposition mechanism based on an a c t i v a t e d complex where the hydrogen i o n i s in.the v i c i n i t y of the oxygen atom on the R-O-C chain. Majima (84), i n a d e t a i l e d i n v e s t i g a t i o n i n t o t h i s system, recognized that the c o n t r i b u t i o n of e t h y l x a n t h i c a c i d to the s p e c t r a l absorbance of the ethyl xanthate i o n was very s m a l l , a c o n s i d e r a t i o n overlooked by Iwasaki and Cooke. Majima a l s o evaluated the i o n i z a t i o n constants and the decomposition r a t e constants f o r the homologous s e r i e s and found that both constants decreased as the chain l e n g t h of the hydrocarbon was increased. His i n v e s t i g a t i o n a l s o showed that i o n i c s t r e n g t h had a small e f f e c t on the decomposition r a t e . A s i m i l a r study by Zahradnick (85) at 0.5°C showed no d e f i n i t e trend i n both constants i n passing from methyl to amyl xanthate, but then showed a sharp decrease i n decomposition r a t e w i t h the longer chain d e r i v a t i v e s . Pomianowski and L e j a (86) a l s o evaluated the decomposition r a t e of e t h y l xanthate as a pa r t of a study on the formation of complexes between e t h y l xanthate and a l k y l t r i m e t h y l ammonium bromide (C^TAB). They pointed out that the decomposition r a t e a n e u t r a l pH values was a f f e c t e d by small q u a n t i t i e s of m e t a l l i c ions which could act as oxygen c a r r i e r s or c a t a l y s t s . In a recent study, Hopstock (87) proposes that a r a p i d e q u i l i b r i u m i s reached between the xanthate i o n and x a n t h i c a c i d . A small concentra-t i o n of a z w i t t e r i o n i c form of x a n t h i c a c i d i s present as a r e s u l t of bonding of the hydrogen atom to one of the e l e c t r o n s on the oxygen atom. The z w i t t e r i o n i c form i s unstable and decomposes r a p i d l y i n t o a l c o h o l and carbon d i s u l f i d e . The r e s u l t s of these workers have been given i n Table 14 and show a reasonable consistency between the e t h y l xanthate decomposition r a t e constants obtained by a l l workers except those of the very e a r l y i n v e s t i g a t i o n s . The average value of the decomposition r a t e used i n the c a l c u l a t i o n i n Figure 30 was 230 + 20 l i t e r mole 1 min 1 obtained from an average of the r e s u l t s of the more recent workers, notably Majima and Eopstock. TABLE 15 D i s s o c i a t i o n Constants and Decomposition Rate Constants f o r E t h y l Xanthic Acid Method Reference Temp.°C D i s s o c i a t i o n Constant Decomp. Rate _v Constant iodometric t i t r a t i o n vapor pressure Spectrophoto-m e t r y von Halban & Hecht (77) King e t . a l . (78,79,80) Pomianowski & L e j a (86) 25 23 0.030 0.031 16.0 l i t e r mole m i n - 1 270 l i t e r mole mi n ~ l -1 -1 165 l i t e r mole m i n - 1 -1 pH determina-t i o n Spectrophoto-m e t r y Cook & Nixon (82) Iwasaki & Cooke (75) 23.5 0.007 0.020 4.3 min -1 Spectrophoto-m e t r y Cooke & Iwasaki (76) Spectrophoto- Majima (84) m e t r i c 21 25 0.0239 0.029 226 l i t e r mole m i n ~ l -1 Spectrophoto- T o e r n e l l (88) m e t r i c Spectrophoto- K l e i n (83) metr i c 25 Spectrophoto- Homylev (81) 25 metr i c 10 Spectrophoto- Hopstock (87) 23.5 metr i c 0.046 0.0255 0.0230 0.0263 185 l i t e r mole mi n ~ l -1 29.0 l i t e r mole min"! -1 10.5 l i t e r mole -.cm"-'-240 l i t e r mole mi n ~ l -1 -1 REFERENCES 1. Handbook of Chemistry and P h y s i c s , 50th E d i t i o n , The Chemical Rubber Company, 1970. 2. A. Cambron, Can. J . 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