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The determination of small amounts of zinc in ores Francis, Marion David 1949

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\i*<i fit THE DETERMINATION OF SMALL AMOUNTS OF ZINC IN ORES by MARION DAVID FRANCIS A T h e s i s Submitted 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 the Degree^ of MASTER OF ARTS i n the Department of CHEMISTRY THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1949 i ABSTRACT The d e t e r m i n a t i o n of z i n c by the back t i t r a t i o n o f excess potassium f e r r o c y a n i d e Is i n v e s t i g a t e d and found v e r y a c c u r a t e and simple t o perform. The use of the e x t e r n a l i n d i c a t o r method Is shown t o be i n a c c u r a t e and u n r e l i a b l e . A new I n d i c a t o r g i v i n g e x c e l l e n t r e s u l t s i s developed f o r the t i t r a t i o n o f f e r r o c y a n i d e s w i t h potassium permanganate• DEDICATION The work i n t h i s t h e s i s i s d e d i c a t e d to J . ALLEN HARRIS, PROFESSOR f o r h i s continuous encouragement and a s s i s t a n c e d u r i n g the course of t h i s r e s e a r c h . TABLE OF CONTENTS Page INTRODUCTION. 1 THE FERROCYANIDE METHOD. 2 The E x t e r n a l I n d i c a t o r and the E f f e c t of Ammonium C h l o r i d e . 2 The Back T i t r a t i o n Method w i t h Potassium Permanganate. 10 COMPARISON OF THE PERMANGANATE METHOD 22 CONCLUSIONS AND TREATMENT OF MATERIAL 26 REFERENCES 28 APPENDIX 1 I d e n t i f i c a t i o n o f the Manganese F e r r o c y a n i d e P r e c i p i t a t e . i Tables 1(a) & 1(b) i v Table 1(c) v Table I I v i l BIBLIOGRAPHY End o f T h e s i s TABLES I Volume V a r i a t i o n o f K „ F e ( C N ) c w i t h D i f f e r e n t C o n c e n t r a t i o n s of NH^Cl. 7 I I Volume V a r i a t i o n i n Blank D e t e r m i n a t i o n 7 I I I V a r i a t i o n i n Volume of KMnO^ with D i f f e r e n t C o n c e n t r a t i o n s of Hydrogen Ion. 12 IV V a r i a t i o n i n Volume of KMn0 4 w i t h NH^Cl C o n c e n t r a t i o n . 12 TABLE OF CONTENTS (Continued) TABLES (Continued) V V a r i a t i o n i n Volume of KMnO^ w i t h ( N H 4 ) g 3 0 4 13 VI V a r i a t i o n i n Volume of KMnO, w i t h Na^SO, 13 4 2 4 V I I D e t e r m i n a t i o n of Z i n c by Back T i t r a t i o n 18 V I I I Comparison of the Methods f o r the Determination o f Z i n c 25 FIGURES I T i t r a t i o n Curve Using K 4 F e ( C N ) g i n the -' Presence o f NB^Cl. . Between I I Curve Showing Per Cent E r r o r A g a i n s t ' 7 & 8 Co n c e n t r a t i o n o f NH"4C1. I l l & TV K 4 F e ( C N ) g - KMn0 4 P o t e n t i o m e t r i e T i t r a t i o n Face 15 V V a r i a t i o n o f P o t e n t i a l w i t h Time a t the End P o i n t Face 20 THE -DETERMINATION OF SMALL AMOUNTS OF  ZINC IN ORES INTRODUCTION Tho accurate quantitative determination of zinc has been a d i f f i c u l t problem for a long time. One of the most satisfactory and yet inconsistent methods in use, since the beginning of the twentieth century, has been the ferrocyanide method. In 1904 W. G. Waring^- published a comprehensive arti c l e on the ferrocyanide method, indicating the many ways in which errors could be made in the determination of zinc in ores. According to Waring the precipitation of zinc by ferrocyanide proceedes in two stages. 4ZnCl 2 +- 2K 4Fe(CN) 6 > 2ZBgFe(CN)fi +• 8KC1 (1) 6Zn gFe(CN) 6+ 2K 4Fe(CN) 6——> 4KgZa5 [FeCCNjJg ( 2 ) 2 In 1907 W. H. Seaman showed the great variety of results that could be obtained by varying the acidity, the temperature of the tit r a t i o n , and the concentration of the zinc and proposed optimum values at which to work. In the same year Wm. H. Keen outlined a method for purification of a standard zinc solution with which ho tested the accuracy of the ferrocyanide method. Since the time of the above work many other quantitative methods of analysis for zinc have been (2) proposed u s i n g a g r e a t v a r i e t y of reagents such as the pyrophosphate g r a v i m e t r i c method suggested by Stone^, the m e r c u r i c z i n c t h i o c y a n a t e method, suggested by S a r u d i , and many r e a c t i o n s i n v o l v i n g o r g a n i c reagents f o r both c o l o r i m e t r i c and g r a v i m e t r i c d e t e r m i n a t i o n s . I n t h i s t h e s i s , however, I s h a l l be c h i e f l y concerned with the Improvement of the f e r r o c y a n i d e method. THE FERROCYANIDE METHOD The E x t e r n a l . I n d i c a t o r and the E f f e c t . o f Ammonium C h l o r i d e . I t was e a r l y decided that the number of v a r i a b l e s a f f e c t i n g the a c c u r a t e f e r r o c y a n i d e t i t r a t i o n o f a sample c o n t a i n i n g z i n c were so numerous t h a t a systematic method f o r i n v e s t i g a t i o n must be used. The v a r i a b l e s are l i s t e d below with.the. treatment g i v e n them. A c i d i t y . The a c i d used was h y d r o c h l o r i c , the optimum c o n c e n t r a t i o n f o r the d e t e r m i n a t i o n b e i n g between 0.3 to 0.5 moles per l i t e r a c c o r d i n g to the.work of Waring 6 and 7 Seaman . The c o n c e n t r a t i o n used was 0.4 moles per l i t e r i n the f i n a l volume of the t i t r a t i o n s o l u t i o n . Temperature. The temperature was h e l d constant between (68 - 72)°C. by means of a steam bath, optimum temperature 8 c o n d i t i o n s b e i n g i n t h i s range . I n d i c a t o r . A s a t u r a t e d s o l u t i o n of the standard e x t e r n a l I n d i c a t o r , u r a n y l a c e t a t e , was used. I n l a t e r work (3) f e r r o u s ammonium s u l f a t e was used. Z i n c S o l u t i o n . A s o l u t i o n o f 0.005184 grams per l i t e r was used. S i n c e no high: grade p u r i t y z i n c was a v a i l a b l e 9 10 at the time, a combination o f Waring ' a and Keen's method f o r p u r i f y i n g the C. P. grade z i n c f o r use as a standard was c a r r i e d out. E s s e n t i a l l y the process was the f o l l o w i n g . The m e t a l l i c z i n c was washed with NH 4C1 and NH, (6N), r i n s e d t hree times with d i s t i l l e d water o and f i v e times with r e d i s t i l l e d a l c o h o l and c a r e f u l l y d r i e d . Approximately t en grams was then a c c u r a t e l y weighed, d i s s o l v e d i n d i l u t e h y d r o c h l o r i c a c i d , t r e a t e d with excess bromine which was then e x p e l l e d , and then made b a s i c with excess NH^OH and NH 4C1. The r e s u l t i n g s m a l l p r e c i p i t a t e o f F e ( 0 H ) 3 i m p u r i t y was washed c a r e f u l l y , r e d i s s b l v e d and r e p r e c i p i t a t e d three, times, and each time the f i l t r a t e was added t© the. main z i n c s o l u t i o n . The p r e c i p i t a t e o f F e ( 0 H ) 3 was then burned and determined as FegOg and s i l i c a and t h i s was deducted from the o r i g i n a l weight of z i n c . L a t e r a s o l u t i o n r e q u i r i n g the presence of no NH 4C1 was made up from s l a b z i n c s p e l t e r s from the U. S. Bureau o f Standards o f a s p e c i a l h i g h grade, with an a n a l y s i s o f 99.995 per cent p u r i t y . Both o f the above s o l u t i o n s were checked f o r content o f z i n c by f i r s t , the forma t i o n o f ZnNH 4P0 4 and second, the form a t i o n of ZngPgO^. Potassium f e r r o c y a n i d e . A s o l u t i o n 0.2096N (combining n o r m a l i t y ) was made up from c r y s t a l s from the U. S. Bureau (4) o f Standards with a p u r i t y o f 99.97 per cen t . Time. The time allowed f o r complete r e a c t i o n was h e l d constant at f i v e minutes, the minimum r e q u i r e d time f o r for m a t i o n of a s t a b l e p r e c i p i t a t e b e i n g t h r e e m i n u t e s . ^ The time of a d d i t i o n of K 4Fe(CN)g s o l u t i o n was h e l d constant a t s i x t y seconds.. F a s t e r a d d i t i o n formed an unbreakable c o l l o i d a l s o l u t i o n with consequent h i g h a b s o r p t i o n o f K^Fe(CN)g• S t i r r i n g . Mechanical s t i r r i n g was used as b e i n g the most e f f i c i e n t and s a t i s f a c t o r y . D i l u t i o n . The volume o f the s o l u t i o n was a d j u s t e d so tha t the f i n a l volume would always be 2 0 0 — 2 m i l l i l i t e r s . E l e c t r o l y t e . The e l e c t r o l y t e used was r e c r y s t a l l l z e d ammonium c h l o r i d e In v a r y i n g concentrations, from 0»00 to 1.500 moles per l i t e r . Other f a c t o r s . The p o s s i b i l i t y o f f a c t o r s as y e t unknown t h a t would a f f e c t the accuracy o f t i t r a t i o n with f e r r o - cyanide are to be s e r i o u s l y c o n s i d e r e d . The f i r s t i n v e s t i g a t i o n c o n s i s t e d i n h o l d i n g a l l o f the above l i s t e d c o n d i t i o n s constant except the c o n c e n t r a t i o n o f ammonium c h l o r i d e , and ob s e r v i n g the v a r i a t i o n s from accuracy. The volume o f K 4 F e ( C N ) g added was found to s t e a d i l y decrease as the c o n c e n t r a t i o n o f NH^Cl i n c r e a s e d , f i n a l l y r e a c h i n g a minimum optimum v a l u e . G r e a t e r (5) c o n c e n t r a t i o n s o f NH^Cl then g r a d u a l l y i n c r e a s e d the volume of f e r r o c y a n i d e r e q u i r e d to g i v e an end p o i n t . With i n s u f f i c i e n t e l e c t r o l y t e i n s o l u t i o n the f o r m a t i o n of the c o l l o i d a l ZngFe(CN) g Is f a v o r e d and consequently l a r g e amounts of absorbed potassium f e r r o c y a n i d e are occluded with the c o l l o i d . With too l a r g e c o n c e n t r a t i o n s o f NH 4C1 i t i s i m p o s s i b l e to prevent i t from c r y s t a l l i z i n g out around the edges o f the hot s o l u t i o n and so p o s s i b l y c r y s t a l l i z i n g a l s o , s m a l l amounts o f K„Fe(CN)_ a t the edges b e f o r e I t has a chance to be s t i r r e d c ompletely i n t o the s o l u t i o n . This. 13 an o b j e c t i o n to "the mechanical method o f s t i r r i n g . D u ring the. course of the t i t r a t i o n s i t was observed t h a t the time r e q u i r e d f o r development of the brown u r a n y l f e r r o c y a n i d e p r e c i p i t a t e , (u"0g)gFe(CN) 6, i n the spot p l a t e , v a r i e d tremendously f o r the a c t u a l t i t r a t i o n and f o r the blank; the b l a n k , b e i n g ten to f i f t e e n times slower i n developing, t h e . c o l o r . The blank was ©f course the free, b l a n k , the s o l u t i o n reagents a l o n e . I c o n s i d e r e d t h a t the mo3t obvious reason f o r the slower r e a c t i o n was, t h a t without the presence of the z i n c f e r r o c y a n i d e p r e c i p i t a t e , . t h e brown u r a n y l f e r r o  cyanide p r e c i p i t a t e d v e r y s l o w l y and f i n e l y , making e a r l y d e t e c t i o n d i f f i c u l t . That i s , the potassium z i n c f e r r o  cyanide p r e c i p i t a t e would a f f e c t the end p o i n t d e t e r m i n a t i o n i n two ways: f i r s t , the excess K.Fe(CN) A adsorbed on the (6) s u r f a c e o f the c o l l o i d a l zinc, p r e c i p i t a t e , and thus presented more s u r f a c e f o r the u r a n y l a c e t a t e to act;, and second, the brown u r a n y l ferr©cyanide.formed, adsorbed on the s u r f a c e of the z i n c p r e c i p i t a t e and.thus the brown c o l o r developed and s e t t l e d , much, more r a p i d l y . The d i f f e r e n c e i n the c o l o r development time v a r i e d the bl a n k "end p o i n t " by as much as 0.05 to 0.20 m i l l i l i t e r s , an e r r o r o f one per cent, s i n c e the end p o i n t i n the e x t e r n a l i n d i c a t o r i s determined on a set time b a s i s (one minute a f t e r a d d i t i o n o f the drop of t e s t s o l u t i o n ) . On the b a s i s o f t h i s r e a s o n i n g then, I thought i t a d v i s a b l e to r e - r u n a l l the determinations., each time f i l t e r i n g the z i n c p r e c i p i t a t e , washing i t thoroughly with hot water c o n t a i n i n g HH^CI, and then t r a n s f e r r i n g i t t o the b l a n k s o l u t i o n . The two t a b l e s o f r e s u l t s below r e p r e s e n t a t l e a s t two and i n some cases (where r e s u l t s seem to be i n c o n s i s t e n t ) as many as s i x t i t r a t i o n s . The r e s u l t s t a b u l a t e d are the averages. (V) TABLE I Volume V a r i a t i o n o f K ^ e t C N j g with D i f f e r e n t C o n c e n t r a t i o n s o f N H 4 C 1 C o n c e n t r a t i o n o f C o r r e c t e d Volume Per Cent E r r o r i n N H 4 C I ( m / 1 ) of T i t r a t i o n ( m / 1 ) T i t r a t i o n s 0.000 20.71 3.03 0.0467 20.42 1.60 0.0934 20.35 1.25 0.140 20.30 1.00 0.187 20.24 0.70 0.234 2 0 * 2 0 0 * 5 0 0 . 2 8 0 20.31 1.05 0.327 20.34 1.20 0.374 20.34 1.20 0.420 20.33 1.15 0.560 20.35 1.25 0.934 20.38 1.40 1.401 20.42 : 1.60 ( T h e o r e t i c a l Volume of K 4Fe(CN)gRequired a 20.10 ml.) TABLE I I Volume V a r i a t i o n i n Blank D e t e r m i n a t i o n Cone, ©f V o l . Of V o l . o f Per Cent E r r o r Per Cent E r r o r NH,C1 Blank Blank In Blank In Det»n TtyVf (No ppt) (In p r e s . Due to Blank o f ppt) 0.000 0.70 0.75 7.0 0.25 0.0467 0.57 0.70 19.0 0.70 0.0934 0.58 0.69 16.0 0.60 0.140 0.55 0.62 11.0 0.40 0.187 0.50 0.59 15.0 0.50 0.234 0.53 0.61 13.0 0.40 0.280 0.51 0.61 16.0 0.50 0.327 0.59 0.68 13.0 0.50 0.374 0.56 0.66 15.0 0.50 0.420 0.57 0.67 15.5 0.50 0.560 0.43 0.49 12.0 0.30 0.934 0.40 0.50 20.0 0.50 1.401 0.42 0.64 34.0 1.05 F i g u r e I , Curve (1.) and Curve (2) show n i c e l y the e f f e c t o f the NE AC1 on the b l a n k d e t e r m i n a t i o n . The f r m IT) ^ 3 * b N r e a e n t In the Blank. ooytuiide jipt. n the Blank. actual t i t r a t i o n of C o n c e n t r a t i o n ©f NH.Cl (M/L) (8) minimum p o i n t s ©f Curve (1.) and Curve (2) f o r the blanks correspond n i c e l y , a l s o , with the minimum p o i n t f o r Curve (3), the a c t u a l t i t r a t i o n of the z i n c . The d i s t a n c e o f Curve (2) from Curve (1) shows a l s o the constant amount o f a b s o r p t i o n o f f e r r o c y a n i d e on the z i n c p r e c i p i t a t e . T h i s data would then i n d i c a t e the p o s s i b i l i t y , a l s o , of i n t e r n a l a b s o r p t i o n o f K 4 F e ( C N ) g i n t h e p r e c i p i t a t e a3 i t formed. I n t e r n a l a b s o r p t i o n c o u l d not be removed by washing- and t h e r e f o r e would lower the a c t u a l r e a d i n g o f the b l a n k below i t s proper v a l u e , even i n the presence of the p r e c i p i t a t e . T h e r e f o r e In the l i g h t o f these readings i t would be n e c e s s a r y to run a t r u e b l a n k i n the f o l l o w i n g manner. With each t i t r a t i o n f o r an unknown q u a n t i t y of z i n c , a d u p l i c a t e , would have to be run on a s i m i l a r c o n c e n t r a t i o n of z i n c of known v a l u e , and the t h e o r e t i c a l volume of f e r r o c y a n i d e s u b t r a c t e d from the a c t u a l c a l c u l a t e d volume r e q u i r e d and. t h i s volume s u b t r a c t e d as the t r u e b l a n k . During the e a r l y course o f t i t r a t i o n s i t was observed that another source of e r r o r i n the b l a n k d e t e r  m i n a t i o n was In the s i z e o f the drop o f u r a n y l a c e t a t e used i n the spot p l a t e . T h i s e r r o r was e a s i l y e l i m i n a t e d by u s i n g the same eye dropper h e l d a t the same angle each time • Determinations u s i n g f e r r o u s ammonium s u l f a t e e x t e r n a l i n d i c a t o r gave a s e t o f r e s u l t s analogous to (9) those f o r u r a n y l a c e t a t e , the maximum-error i n the presence o f the p r e c i p i t a t e b e i n g 0.5 per cent a t optimum c o n c e n t r a t i o n o f ammonium c h l o r i d e . In the l i g h t o f d a t a set down above, i t i s c o n c l u s i v e l y e v i d e n t t h a t h i g h accuracy cannot be ob t a i n e d by the use o f the e x t e r n a l i n d i c a t o r . I t i s Impossible to o b t a i n r e s u l t s o f g r e a t e r c e r t a i n accuracy than 0.5 to 1.0 per cent. Unless optimum a c i d i t y , c o n c e n t r a t i o n of z i n c s o l u t i o n , amount o f u r a n y l . a c e t a t e used f o r i n d i c a t o r , time o f development of i n d i c a t o r c o l o r , c o n c e n t r a t i o n of e l e c t r o l y t e , presence of the p r e c i p i t a t e i n the blank, and other f a c t o r s a l r e a d y mentioned.are s t r i c t l y adhered t o , e r r o r s as h i g h as 5.0 percent w i l l be encountered s i n c e the e f f e c t s are a d d i t i v e . Of . s p e c i a l s i g n i f i c a n c e i s the c o n c e n t r a t i o n of t h e e l e c t r o l y t e . Table I and Table I I show the c r i t i c a l v a l u e of the c o n c e n t r a t i o n of ammonium, c h l o r i d e i n obta i n i n g , c o r r e c t r e s u l t s . F i g u r e I I shows the v e r y steep drop and the steep r i s e again i n the per cent e r r o r with i n c r e a s i n g c o n c e n t r a t i o n . T h i s phase of the work on e x t e r n a l i n d i c a t o r s was then c o n s i d e r e d c o n c l u d e d , i n so f a r . a s g a i n i n g g r e a t e r a c c u r a c y . Comparison w i l l be.made l a t e r with other methods developed. (10) THE BACK TITRATION METHOD The Back T i t r a t i o n of Excess F e r r o c y a n i d e w i t h Potassium  Permanganate * In 1941 an a r t i c l e appeared by Z. S. 32 Mukhina and M. L. Mironenko. s t a t i n g t h a t z i n c c o u l d be determined by p r e c i p i t a t i o n w i t h potassium f e r r o c y a n i d e and potassium f e r r l c y a n i d e and then back t i t r a t e d w i t h potassium permanganate u s i n g Indigo carmine as an i n d i c a  t o r . S i nce the o r i g i n a l r e f e r e n c e was not a v a i l a b l e , I f i r s t had t o r u n checks on the back t i t r a t i o n a c c u r a c y by running b l a n k d e t e r m i n a t i o n s on the potassium f e r r o  cyanide s o l u t i o n u s i n g potassium permanganate. C o n d i t i o n s of the t i t r a t i o n i n the presence of z i n c were s i m u l a t e d . The f e r r o c y a n i d e was p i p e t t e d i n t o a s o l u t i o n 0.5 molar In h y d r o c h l o r i c a c i d c o n t a i n i n g a l s o 0.23 moles per l i t e r of ammonium c h l o r i d e . The s o l u t i o n , h o t a t (60 - 65)°C» was then t i t r a t e d w i t h KMn0 4« I t was found f o r t h i s t i t r a t i o n t h a t no I n d i c a t o r was necessary, s i n c e the l i g h t y e l l o w green c o l o r of the f e r r l c y a n i d e at t h i s d i l u t i o n (200 ml) does not h i n d e r the d e t e c t i o n o f the end p o i n t * In the f i r s t t i t r a t i o n s t h a t were r u n on the above method a c o n s i d e r a b l e e r r o r ( 0.5 - 2.0 per cent ) was found t o occur and was a t t r i b u t e d to the forma t i o n of a f i n e g e l a t i n o u s p r e c i p i t a t e which i n t e r f e r e d (11) somewhat with the d e t e r m i n a t i o n of the tr u e end p o i n t . The p r e c i p i t a t e was determined t o be B/fc^FefCNjg and formed o n l y i n the presence o f an ammonium s a l t . # In view of the d i s c r e p a n c y mentioned above, I c o n s i d e r e d i t n e c e s s a r y t o make a thorough i n v e s t i g a t i o n o f the f e r r o c y a n i d e - permanganate t i t r a  t i o n . The f o l l o w i n g method was used. E x a c t l y 40 m i l l i  l i t e r s o f potassium f e r r o c y a n i d e were p i p e t t e d i n t o 400 m i l l i l i t e r beakers and the s o l u t i o n s made up to 190 m i l l i l i t e r s with s u l f u r i c a c i d and water, t o c o n t a i n a t the end p o i n t , c o n c e n t r a t i o n s o f a c i d from 0.00 to 2.00 moles per l i t e r . A s e r i e s o f t i t r a t i o n s were made v a r y i n g the c o n c e n t r a t i o n o f a c i d and then v a r y i n g the c o n c e n t r a  t i o n of f i r s t , ammonium c h l o r i d e j second, ammonium s u l f a t e ; and t h i r d , sodium s u l f a t e ; w i t h a g i v e n c o n c e n t r a t i o n of a c i d . Blanks were run on the water and s u l f u r i c a c i d and i n no case, even i n the presence of the s a l t s , was th e r e found a g r e a t e r t i t r a t i o n v a l u e f o r the b l a n k than 0.02 m i l l i l i t e r s . Tables I I I , IV, V, and V I , show the r e s u l t s of these experiments. # Notes The complete i d e n t i f i c a t i o n of t h i s p r e c i p i t a t e i s g i v e n i n d e t a i l In the appendix. (12) TABLE I I I Variation in Volume of KMn<>4 With Different Concentrations of Hydrogen Ion Volume of Conc» E Volume of Difference % Error  K4Fe(CN)6 (m/1) KMnO-4 39.96 ml 0.00 (End Point Obscured) 0.10 16.51 0.01 0.06 0.50 16.48 0.04 0.24 1.00 16.50 0.02 0.12 1.50 16.46 0.06 0.24 2.00 16.43 0.09 0.54 (Theoretical Volume of KMnO^ Required = 16.52 ml) TABLE IV Variation in Volume of KMn04 With HH4CI Concentration Volume of Cone. H Cone, of Volume of D i f f . #Error K 4Pe(CN) 6 (m/1) NH4CI KMn04 39.96 0.10 0.000 16.52 0.00 0.00 0.10 0.047 16.45 0.07 0.42 0.10 0.140 16.43 0.09 0.54 0.10 0.187 16.40 0.12 0.73 0.30 0.093 16.40 0.12 0.73 0.30 0.140 16.38 0.14 0.85 0.30 0.187 (End Point Obscured)— (Theoretical Volume of KMn04 Required s 16.52 ml) (13) TABLE V V a r i a t i o n i n Volume of KMnOA With (HH.USO Volume of Conc.H Gone, of Volume of D i f f . % Erro] K 4 P e ( C N ) 6 ' (a/D (NH 4)gS0 4 KMn0 4 19.98 0.10 0.010 8.28 0.02 0.24 0.10 0.019 8.27 0.01 0.12 0.10 0.028 8.20 0.06 0.73 0.10 0.038 8.37 0.11 1.30 0.30 0.010 8.25 0.01 0.10 0.30 0.019 8.28 0.02 0.24 0.30 0.028 8.30 0.04 0.48 0.30 0.038 8.13 0.13 1.60 ( T h e o r e t i c a l Volume of KMn0 4 Required r 8.26 ml) TABLE VI V a r i a t i o n i n Volume of KMn0 4 With N a 2 S 0 4 Volume of Conc.H Cone, of Volume of D i f f . % E r r o r K 4 F e ( C N ) 6 (m/1) N a 2 S 0 4 KMn0 4 39.96 0.45 0.017 17.25 0.05 0.29 0.035 17.27 0.03 0.17 0.052 17.33 0.03 0.17 0.070 17.34 0.04 0.23 0.087 17.37 0.07 0.40 0.105 17.38 0.08 0.46 ( T h e o r e t i c a l Volume of KMn0 4 Required Z 17.30) (14) Table I I I shows a d e f i n i t e change In accuracy w i t h a v a r y i n g c o n c e n t r a t i o n of a c i d . The s a f e range of a c i d f o r acc u r a t e t i t r a t i o n appears to be from about (0.5 - 1*5) moles per l i t e r . Table IV and Table V i n d i c a t e the i n c r e a s i n g Inaccuracy Involved In the f e r r o  cyanide - permanganate t i t r a t i o n In the presence of I n c r e a s i n g c o n c e n t r a t i o n of ammonium i o n . In the case o f u s i n g ammonium c h l o r i d e as the e l e c t r o l y t e (Table I V ) , no r e a l l y a c c urate t i t r a t i o n was made, even i n the p r e s  ence of as l i t t l e as 0.05 moles per l i t e r . Furthermore, I n c r e a s i n g a c i d c o n c e n t r a t i o n seems to i n c r e a s e the e r r o r . In the case of the ammonium s u l f a t e , however, (Table V) I n c r e a s i n g a c i d c o n c e n t r a t i o n does m a t e r i a l l y a i d the accurac y of the t i t r a t i o n . T h i s i s more s a t i s  f a c t o r y , s i n c e an a c i d c o n c e n t r a t i o n of at l e a s t 0.3 moles per l i t e r i s d e s i r e d f o r the z i n c p r e c i p i t a t i o n . Table VI shows t h a t a t 0.45 moles per l i t e r of a c i d , any c o n c e n t r a t i o n o f sodium s u l f a t e from 0.00 t o 0.10 or l a r g e r i s s a t i s f a c t o r y . Thus we can see t h a t ammonium c h l o r i d e cannot be used as the e l e c t r o l y t e i n the p r e c i p  i t a t i o n of potassium z i n c f e r r o c y a n i d e when u s i n g potassium permanganate f o r the back t i t r a t i o n r eagent. The s a f e range f o r ammonium s u l f a t e appears t o be from |00.00 - 0.03) moles per l i t e r In an a c i d c o n c e n t r a t i o n 0.30 mole* per l i t e r or g r e a t e r . The use of such s m a l l c o n c e n t r a t i o n s of ammonium s u l f a t e should be s a t i s f a c t o r y , s i n c e the I o 8 TT i (15) theory and experiment suggest that ammonium sulfate i s a much stronger agent f o r coagulation than i s ammonium chloride, "being of the order 250 to 1 f o r some inorganic 13 sols and of the order 16 to 32 times better f o r other sols* The difference i n eff e c t of an ammonium s a l t and a non-ammonium s a l t on the ferrocyanide - per manganate t i t r a t i o n i s w e l l I l l u s t r a t e d by the two graphs. (Figures I I I and IV). Note that the region (A) bracketed i n Figure I I I i s a region of sharp increase i n p o t e n t i a l but i n region (B) instead of the p o t e n t i a l continuing to increase sharply i t l e v e l s off and then gives the f i n a l large jump. The region (A) i s accounted fo r by the fa c t that manganese ferrocyanide has precip i t a t e d and thus e f f e c t i v e l y removed some of the f e r r o  cyanide, giving a premature end point. At the point of sudden l e v e l i n g , however, the concentration of f e r r o  cyanide has been lowered (by i t s oxidation) below the concentration required to keep the pr e c i p i t a t e out of solution and so the pr e c i p i t a t e dissolves. The solution of the pr e c i p i t a t e then keeps the p o t e n t i a l almost constant u n t i l the p r e c i p i t a t e has completely dissolved. At complete solution of the p r e c i p i t a t e the po t e n t i a l again begins to r i s e r a p i d l y . Figure IV shows the smooth, normal oxidation curve and gradual.rise of the po t e n t i a l u n t i l the true end point, that i s , the t i t r a t i o n i s a (16) normal one and so should he, and i s , more a c c u r a t e than t h a t t i t r a t i o n i n the presence of ammonium i o n * S ince the use of an ammonium s a l t f o r an e l e c t r o l y t e does i n v o l v e a s l i g h t l y l a r g e r e r r o r than the use of sodium s u l f a t e , I thought i t would be I n t e r e s t i n g to attempt u s i n g o n l y sodium s u l f a t e as the e l e c t r o l y t e . In order to o b t a i n the b e s t r e s u l t s i n v o l v i n g the optimum c o n d i t i o n s f o r the p r e c i p i t a t i o n of a s t a b l e , constant composition p r e c i p i t a t e of z i n c , and t o o b t a i n maximum accuracy of the back t i t r a t i o n , the f o l l o w i n g method was f i n a l l y developed. E x a c t l y 19.98 m i l l i l i t e r s o f standard z i n c s o l u t i o n , 13.5 m i l l i l i t e r s of 6N s u l f u r i c a c i d , 137.5 m i l l i l i t e r s o f water and 2.00 grams o f sodium s u l f a t e were mixed t o g e t h e r , heated to 70°C, and e x a c t l y 19.98 m i l l i l i t e r s o f standard f e r r o c y a n i d e run i n a t the r a t e of 60 drops per minute* A f t e r the l a s t drop of f e r r o c y a n i d e had been added, the s o l u t i o n was s t i r r e d a t a constant speed f o r , e x a c t l y t h r e e minutes* The mixture was c o o l e d f o r f i v e minutes to (15 - 20)°C* In a water b a t h and then the second 19*98 m i l l i l i t e r s added r a p i d l y w i t h s t i r r i n g . Now 90 per cent of the r e q u i r e d standard potassium permanganate was run i n i n the c o l d . The mixture was then heated f o r three minutes to 65°C* over a Bunsen flame and the t i t r a t i o n completed a t t h i s temp e r a t u r e t o the end p o i n t . The end p o i n t of a l l these t i t r a t i o n s was determined by means of a r e s e a r c h model (17) Beckman pH Meter converted to p o t e n t i o m e t r i c r e a d i n g s , A h i g h temperature platinum e l e c t r o d e (#1281-X11), and a h i g h temperature calomel e l e c t r o d e (#8970-T) f o r r e f e r  ence, were used. The use o f the Beckman f o r the d e t e r  m i n a t i o n o f the hack t i t r a t i o n end p o i n t became a n e c e s s i t y , f o r the f o l l o w i n g r e a s o n . The normal c o l o r change u s i n g potassium permanganate as i t s own i n d i c a t o r , when t i t r a t i n g potassium f e r r o c y a n i d e , was from green ( c o l o r of the f e r r l c y a n i d e ) t o y e l l o w t o orange. The appearence of the f i r s t f a i n t orange c o l o r i s the end 1 5 p o i n t , not the p i n k • In the presence o f the z i n c potassium f e r r o c y a n i d e p r e c i p i t a t e , however, the c o l o r change was almost completely obscured and when t h i s method of i d e n t i f i c a t i o n of the end p o i n t was used, e r r o r s as h i g h as 15 per cent were i n c u r r e d . The use of the p o t e n t i o m e t r i c method, then, was d e v i s e d and i t was by t h i s method t h a t Table VII below, was compiled. The l a s t f o u r r e a d i n g s of Table V I I were determined by an i n t e r n a l i n d i c a t o r , o r t h o p h e n a n t h r o l i n e f e r r o u s complex. The l a s t two of the r e a d i n g s were determined i n the presence o f n i t r o b e n z e n e . TABLE V I I ^ Determination o f Z i n c by Back T i t r a t i o n . V a r i a t i o n With Na oS0.. V o l * o f V o l . of Cone, of Cone. . B a c k E q u i v . V o l . of C a l e . Theoret. %  ZnS0 4 K 4 F e ( C N ) 6 N a 2 S 0 4 of H T l t ' n V o l . of K 4Fe(CN)g Wt. of Wt. of E r r o r S t d . Added - With K 4 P e ( C N ) 6 Req'd Zinc Z i n c KMn0 4 by Z i n c 19.98 39.96 0.035 0.45 9.02 20.84 19.12 0.0978 0.0999 2.10 0.070 8.99 20.77 19.19 0.0982 1.70 0.105 8.95 20.68 19.28 0.0986 1.30 > 0.210 8.88 20.51 19.45 0.0995 > 0.40 15.00 0.035 11.06 25.55 14.41 0.0737 0.0750 1.73 0.070 11.02 25.46 14.50 0.0742 1.06 0.105 11.02 25.46 14.50 0.0742 1.06 > r 0.210 10.98 25.37 14.59 0.0746 0.53 10.00 0.035 13.03 30.10 9.86 0.0504 0.0500 0.80 0.070 13.18 30.45 9.51 0.0487 2.60 0.105 13.13 30.33 9.63 0.0493 1.40 f > 0.210 13.07 30.19 9.77 0.0500 0.00 3 .00 22.98 0.105 8.70 20.10 2.88 0.01473 0.0150 1.80 0.210 8.67 20.03 2.95 0.01540 2.60 0.210 8.68 20.05 2.93 0.01498 0.13 < > 0.210 1 8.68 20.05 2.93 0.01498 '1 0.13 (19) C o n s i d e r a b l e technique was r e q u i r e d i n a p p l y i n g the Beckman to the r e c o g n i t i o n o f the t r u e end p o i n t . The potassium permanganate g i v e s a good p o t e n t i a l jump, as i n most other s t r a i g h t forward o x i d a  t i o n r e a c t i o n s , but i n f i v e t o e i g h t seconds a f t e r the l a r g e p o t e n t i a l r i s e a t the end p o i n t , the p o t e n t i a l b e gins to f a l l o f f again v e r y r a p i d l y . In approximately one to t h r e e minutes, depending on how f a r past the end p o i n t the t i t r a t i o n has been run, the p o t e n t i a l remains constant at about 0.75 to 1.5 m i l l i v o l t s above the poten t i a l where the f i r s t r i s e began. On a d d i t i o n o f each e x t r a drop o f potassium permanganate to the s o l u t i o n the n e g a t i v e p o t e n t i a l surges up from approximately (6.5 - 10) m i l l i v o l t s , remains constant at t h i s r e a d i n g f o r f i v e to e i g h t seconds and then s t e a d i l y drops back a g a i n . S i n c e the a c t u a l peak o f the p o t e n t i a l l a s t s f o r such a sh o r t time, i t was n e c e s s a r y to be sure t h a t the o x i d a t i o n was complete, t h a t i s , t h a t the f a l l was not due to a slow r e a c t i o n of the permanganate on the f e r r o c y a n i d e . T h i s c o n s i d e r a t i o n i s taken care of by c a r r y i n g out the t i t r a  t i o n a t 65°C. At t h i s temperature the r e a c t i o n of perman ganate and f e r r o c y a n i d e i s v e r y r a p i d . A graph showing the v a r i a t i o n of p o t e n t i a l with time f o r one drop o f potassium permanganate i s shown i n F i g u r e V. T h i s curve shows t h a t the end p o i n t has r e a l l y been reached, s i n c e even a f t e r three minutes the p o t e n t i a l i s s t i l l w e l l po te nt io me tr ic  Re ad in g (M il li vo lt s)  v (20) above the s t a r t i n g p o i n t of the p o t e n t i a l r i s e . The curve, F i g u r e V, was obtained by c a r r y i n g the t i t r a t i o n to j u s t one drop (0.06 ml.) from the true end p o i n t . The drop was then added and zero time taken as t h a t time when the drop j u s t touched the s u r f a c e of the s o l u t i o n . Readings o f time i n seconds and p o t e n t i a l were then taken over a p e r i o d of three minutes from the time the drop touched the s o l u t i o n . a f t e r the p o t e n t i a l p l a t e a u was reached was due to the f a c t t h a t secondary r e a c t i o n s c a t a l y s e d by the p r e c i p i t a t e o f z i n c , began to set i n . In these r e a c t i o n s the perman ganate i s g r a d u a l l y reduced w i t h the subsequent p r o d u c t i o n o f manganese d i o x i d e . Displacement of a i r by n i t r o g e n , over the s o l u t i o n t o be t i t r a t e d d i d not i n c r e a s e the p l a t e a u p e r i o d by any a p p r e c i a b l e time. S i n c e the p o t e n t i o m e t r i c method i s somewhat more d i f f i c u l t than i s to be d e s i r e d , I thought that the use of an i n t e r n a l i n d i c a t o r would perhaps i n c r e a s e the s e n s i t i v i t y of the d e t e r m i n a t i o n . The i n d i c a t o r f i n a l l y d e c i d e d on was o r t h o p h e n a n t h r o l i n e f e r r o u s complex". As The g r a d u a l f a l l i n g o f f of the p o t e n t i a l 2Mn04*M- 16H + - 4 - 1 0 € SMn*"*1-!- 10H„0 >J (-1.52) (-1.33) # Note. T h i s i n d i c a t o r i s manufactured by the G. F r e d e r i c k Smith Company and i s s o l d under the name of " F e r r o l n " . (21) was to be expected the c o l o r change a t the end p o i n t was not permanent. The change, however, from the reduced form to the o x i d i z e d and back a g a i n to the reduced form was so r a p i d t hat the d e t e r m i n a t i o n of the end p o i n t was extremely d i f f i c u l t . I t was n e c e s s a r y then to f i n d some t h i n g t o I n h i b i t the e f f e c t of the p r e c i p i t a t e of z i n c . Nitrobenzene was found to slow down the r a p i d i n d i c a t o r change. The f o l l o w i n g method u s i n g o r t h o p h e n a n t h r o l i n e f e r r o u s complex as I n d i c a t o r was developed and found s a t i s f a c t o r y . The r e q u i r e d volume of z i n c depending upon the c o n c e n t r a t i o n of the s o l u t i o n , was p i p e t t e d ^ i n t o 15 m i l l i l i t e r s o f 6N s u l f u r i c a c i d , and 6 grams of anhydrous sodium s u l f a t e . Enough d i s t i l l e d . w a t e r was then added so t h a t the f i n a l Volume of s o l u t i o n at the end of the t i t r a  t i o n would be approximately 200 m i l l i l i t e r s . The a c i d s o l u t i o n was then heated over a Bunsen f o r f o u r minutes to 70°C. and the r e q u i r e d volume o f f e r r o c y a n i d e added drop by drop to g i v e j u s t a s l i g h t excess. The s o l u t i o n was s t i r r e d f o r e x a c t l y three minutes and then p l a c e d i n a water bath and c o o l e d to (15 - 20)°C. f o r f i v e minutes. Three m i l l i l i t e r s o f pure n i t r o b e n z e n e were now added wit h v i g o r o u s s t i r r i n g and then the second p o r t i o n o f f e r r o c y a n i d e f o r the back t i t r a t i o n added. Approximately # Note. The p r e c i p i t a t e of z i n c should not c o n t a i n more than 0.10 grams of z i n c . (22) 90 per cent o f the r e q u i r e d potassium permanganate was now run i n and the s o l u t i o n heated t o (60 - 65)°C. To the hot s o l u t i o n were added two drops of 0.05M F e r r o i n and then the t i t r a t i o n was completed by the a d d i t i o n o f permanganate to the end p o i n t . The c o l o r change at the end p o i n t i s from orange to a c l e a r green c o l o r which l a s t s from f i v e to ten seconds b e f o r e g r a d u a l l y t a k i n g on the orange t i n t a g a i n . A white base and good back ground l i g h t i n g are e s s e n t i a l . The use of F e r r o i n as an i n d i c a t o r a p p l i e s e i t h e r i n the presence o f z i n c or out of the presence o f z i n c . The use o f t h i s i n d i c a t o r i n the s t r a i g h t forward t i t r a t i o n o f f e r r o c y a n i d e i s almost i d e a l and i s f a r more s a t i s f a c t o r y f o r the i d e n t i f i c a t i o n o f the permanganate - f e r r o c y a n i d e end p o i n t than the use o f the permanganate i t s e l f as p r e v i o u s l y suggested. A l l those d e t e r m i n a t i o n s i n Table I I I and Table VI which were made p r e v i o u s l y can be made even more a c c u r a t e l y i n the presence o f F e r r o i n . COMPARISON OF THE PERMANGANATE METHOD A comparison o f the f i n a l methods developed f o r the a n a l y s i s o f z i n c was c o n s i d e r e d n e c e s s a r y f o r a f i n a l examination of the permanganate method. S i n c e t h i s was to be a comparison the a n a l y s i s was done on f o u r d i f f e r e n t ore samples. These ores were t r e a t e d to e x t r a c t the z i n c and then t h i s e x t r a c t e d z i n c was analysed by the (23) e x t e r n a l i n d i c a t o r method, and by the back t i t r a t i o n method u s i n g b o t h the i n t e r n a l i n d i c a t o r and p o t e n t i o m e t r i c d e t e r m i n a t i o n . The methods and t h e i r v a r i o u s r e s u l t s are t a b u l a t e d i n Table V I I I . The f o u r ores were t r e a t e d by the f o l l o w i n g e x t e n s i v e method. The ore samples were d r i e d a t 100°C. f o r one hour. Approximately f o u r gram samples were taken and weighed a c c u r a t e l y , p l a c e d i n an erlenmeyer, and 40 m i l l i  l i t e r s o f concentrated h y d r o c h l o r i c a c i d added w i t h warming u n t i l e f f e r v e s c e n c e ceased. The s o l u t i o n was then evaporated to 20 m i l l i l i t e r s and 20 m i l l i l i t e r s o f concen t r a t e d n i t r i c a c i d added and the whole mixture then b o i l e d to near dryness. The s o l u t i o n was then c o o l e d and 40 m i l l i t e r s o f c o n c e n t r a t e d s u l f u r i c a c i d added. T h i s mixture was heated over an open flame to fumes of s u l f u r t r i o x i d e . S ince i n the permanganate back t i t r a t i o n method i t i s i m p e r a t i v e t h a t no n i t r a t e be p r e s e n t , the above s u l f u r i c a c i d s o l u t i o n was heated almost to dryness^, e x a c t l y 100 m i l l i l i t e r s of water was added t o the c o o l e d mixture and then 20 m i l l i l i t e r s , more of c o n c e n t r a t e d s u l f u r i c a c i d . P e l l e t s o f pure aluminum were added and the mixture b o i l e d f o r 10 minutes and f i l t e r e d i n t o a beaker through paper c o n t a i n i n g p e l l e t s of aluminum. # Note, I t has been the experience o f t h i s worker t h a t HNOg i s not completely d r i v e n o f f with the f i r s t few minutes of s u l f u r t r i o x i d e fuming. I t was found n e c e s s a r y to evaporate the s u l f u r i c a c i d mixtures almost to dryness b e f o r e o b t a i n i n g complete e x p u l s i o n of n i t r i c a c i d . (24) The f i l t e r e d s o l u t i o n was evaporated to 125 m i l l i l i t e r s and 6N ammonium hydroxide added u n t i l the p r e c i p i t a t e o f z i n c hydroxide j u s t f a i l e d to r e d i s s o l v e . Then 100 m i l l i l i t e r s o f IM c i t r i c a c i d 1 6 were added and made n e u t r a l to methyl orange by adding 6M ammonium hy d r o x i d e . An excess o f c i t r i c a c i d , 100 m i l l i l i t e r s of IM s o l u t i o n , was now added and a l s o 100 m i l l i l i t e r s o f a 20 per cent s o l u t i o n o f ammonium s u l f a t e . S ince no c o b a l t was present the use o f ammonium t h i o c y a n a t e f o r t h i s ore was not ne c e s s a r y . The volume was then a d j u s t e d t o 800 m i l l i l i t e r s , the s o l u t i o n heated to 6 0 ° C , and a i r r e p l a c e d with hydrogen s u l f i d e . The mixture was then heated to 95°G. and the e x i t tube c l o s e d . I t was allowed to c o o l and become s a t u r a t e d w i t h hydrogen s u l f i d e under the p r e s s u r e of the generator, and was shaked f r e q u e n t l y f o r 40 minutes, allowed to s e t t l e , separated by f i l t r a t i o n , and washed with a O i l M s o l u t i o n o f c i t r i c a c i d s a t u r a t e d with hydrogen s u l f i d e . When the washings were complete the f i l t e r paper and r e s i d u e (ZnS) were dropped i n t o a beaker and b o i l e d with 40 m i l l i l i t e r s o f 6N s u l f u r i c u n t i l no more hydrogen s u l f i d e remained i n the s o l u t i o n . The f i l t e r paper was f i l t e r e d out while hot and washed r e p e a t e d l y with s m a l l p o r t i o n s o f 200 m i l l i l i t e r s o f b o i l i n g h ot water. The f i l t r a t e c o n t a i n i n g the z i n c was then made up to e x a c t l y 250 m i l l i l i t e r s i n a v o l u m e t r i c f l a s k . (25) A l i q u o t s of t h i s s o l u t i o n were analysed f o r z i n e by the methods t a b u l a t e d i n Table V I I I . TABLE V I I I Comparison o f Methods f o r the De t e r m i n a t i o n o f Z i n c . Ore Sample Sample Back T i t ' n Method E x t . I n d i e Number 1 Number A B Weight (gms.) 4.7058 4.1090 P o t e n t i o  m e t r i c (gms.) 0.6369 0.5525 I n t e r n a l lAdi.c,. (gms.) 0.6349 0.5520 Method (gms.) 0.6482 0.5673 4 A B 4.1006 4.2711 0.6369 0.6627 0.6337 0.6632 0.6439 0.6650 6 A B 4.1853 4.0532 0.6925 0.6680 0.6905 0.6695 0.7052 0.6811 8 A . B 4.2769 4.1222 0.7789 0.7840 0.7764 0.7471 0.7890 0.7520 1 A B Thorn Smith A n a l y s i s it) 13.54 (JO 13.54 13.45 (jO 13.49 13.44 (JO 13.77 13.81 4 A B 15.49 15.53 15.52 15.45 15.53 15.70 15.57 6 A B 16.91 16.55 16.48 16.50 16.38 16.85 17.05 8 A B 18.17 18.21 18.15 18.15 18.12 18.45 18.24 An examination o f Table V I I I shows t h a t o f the two methods of d e t e r m i n a t i o n , p o t e n t i o m e t r i c or i n t e r n a l i n d i c a t o r , the most r e l y a b l e appears to be.the p o t e n t i o m e t r i c . The t a b l e i l l u s t r a t e s a l s o the extremely (26) c o n s i s t e n t r e s u l t s o b tained by e i t h e r method. I g n o r i n g Ore #6, the l a r g e s t s i n g l e d e v i a t i o n from the v a l u e s r e p o r t e d by Thorn Smith i s 0.10 per cent, and the l a r g e s t mean d e v i a t i o n f o r any one ore i s 0.06 per cent. The d i s c r e p a n c y in Ore #6 i s accounted for by the presence of a s l i g h t amount o f r e d u c i n g substance in the e x t r a c t e d solution due t o an e r r o r i n technique, thus i n c r e a s i n g s l i g h t l y the r e q u i r e d volume i n potassium permanganate and consequently r e d u c i n g the amount o f r e p o r t e d z i n c . As shown p r e v i o u s l y i n Table I I the e x t e r n a l i n d i c a t o r method g i v e s c o n s i s t e n t l y h i g h r e s u l t s . CONCLUSIONS AND TREATMENT OF MATERIAL 1. The e x t e r n a l I n d i c a t o r method while i t g i v e s c o n s i s t e n t r e s u l t s , w i l l not g i v e accuracy of g r e a t e r c e r t a i n value than 98.5 per cen t . 2. In the p r e c i p i t a t i o n of z i n c as potassium z i n c f e r r o c y a n i d e , an e l e c t r o l y t e i s not o n l y n e c e s s a r y but i t s c o n c e n t r a t i o n f o r maximum accuracy i s c r i t i c a l . T h i s a p p l i e s t o a l l d e t e r m i n a t i o n s I n v o l v i n g t h i s z i n c p r e c i p i t a t e . 3. The t i t r a t i o n of f e r r o c y a n i d e w i t h permanganate can be made extremely a c c u r a t e l y by any one of the f o l l o w i n g three methods,.provided t h a t ammonium Ion i s not prese n t i n the s o l u t i o n i n g r e a t e r c o n c e n t r a t i o n (27) than 0.05M. (a) By o b s e r v a t i o n of the end p o i n t by means of a po t e n t i o m e t e r . (b) By u s i n g potassium permanganate as I t s own i n d i c a t o r . (c) By the use of o r t h o p h e n a n t h r o l i n e f e r r o u s complex. Of these three methods the l a s t named i s by f a r the f a s t e s t , the e a s i e s t to handle, and the most s a t i s f a c t o r y . 4. The d e t e r m i n a t i o n o f z i n c i n a s o l u t i o n by the back t i t r a t i o n with potassium permanganate w i l l g i v e e x c e l l a n t accuracy I f care i s taken with the procedure developed and c a u t i o n i s used when deter m i n i n g the end p o i n t by e i t h e r the i n t e r n a l i n d i c a t o r or the potentiometer. (28) REFERENCES 1* W. G. Waring, J . Am. Chem. S o c , 26, 4 - 29, (1904). 2. W. H. Seaman, I b i d , 29, I , 205 - 211, (1907). 3. ^W. H. Keen, I b i d , 30, 904, (1908). j 4. Stone, i b i d , 30, 904, (1908). 5. I . S a r u d l , O s t e r r . Chem. Z t g . , 42, 297-8, (1939). 6. W. G. Waring, l o c . c i t . 7. W. H. Seaman, l o c . c i t . 8. W. H. Seaman, l o c . c i t . 9. W. G. Waring, l o c . c i t . 10. W. H. Keen, l o c . c i t . 11. J . A . H a r r i s , Unpublished T h e s i s (1922) "The V o l u m e t r i c Determination of Z i n c " Pg. 5 (Graph). 12. Z. S. Mukhina and M. L. Mironenko-, Zavadskaya Lab., 10, 145 - 147, (1941). (C. A. 35, 5060, (1941). 13. A. W. Thomas, 1 s t Ed., London, McGraw-Hill, 1934, p. 178ff, 14. J . A. H a r r i s , Loc. c i t . v - / 15. c f . S n e l l and B i f f e n , Commercial Methods o f A n a l y s i s , 1 s t Ed., McGraw-Hill, p. 155. 16. S. A. Coleman, G. B. L. Smith, Ind. Eng. Chem., A n a l . Ed., 377 - 380, (1941). APPENDIX I d e n t i f i c a t i o n of the Mn 2Fe(CN) 6 P r e c i p i t a t e . Since the p r e c i p i t a t e appeared each time upon the addition of KMn04 to the K 4Fe(CN) 6 s o l u t i o n , i t was considered to be manganese dioxide, manganese f e r r o  cyanide, or manganese fe r r l c y a n i d e . Since the t u r b i d i t y produced was not dark and was soluble i n hot d i l u t e H2SO4, then the formation of both LtaCvj and Mn^ jPaCCNjjT^ a r e eliminated. Furthermore MnOg should not form i n an acid s o l u t i o n . Since the t u r b i d i t y seemed to appear more r a p i d l y on allowing the KMn04 to layer , then i t was thought perhaps that i t was due to a high concentration of Mn . Later work however, shewed that the p r e c i p i t a t e which formed was due to an uncommon ion eff e c t i n the presence of the ammonium i o n . A considerable number of s a l t s then were used, f o r example KgS0 4, Na 2 S0 4 , NH4C1, (NH 4 ) 2 S0 4 , and others, and i n each case the pr e c i p i t a t e was found to form only when an ammonium s a l t was used. The pr e c i p i t a t e on f i l t r a t i o n appeared a fa i n t grey blue to mauve color when wet and t h i s i s the cha r a c t e r i s t i c color of Mn 2Fe(CN) g. The pr e c i p i t a t e was then washed thoroughly with 50 per cent ethyl alcohol and dissolved i n hot very d i l u t e s u l f u r i c acid. On evaporating the solution i t takes on the b e a u t i f u l l i g h t blue color ( i i ) that i s c h a r a c t e r i s t i c of the decomposition of f e r r o  cyanide, that i s , the formation of Fe 4[pe(CN)^j 3» The evaporated solution plus n i t r i c acid and sodium b i s - muthate gave the d i s t i n c t rose color f o r Mn04 Ion, thus proving q u a l i t a t i v e l y the po s i t i v e presence of manganese i n the p r e c i p i t a t e . Further evidence that i t must be the ferrocyanide of manganese i s the f a c t that each time the solution cleared up just before the end point. Thus the oxidation of the ferrocyanide to ferrlcyanide reduced the concentration of Fe(CN)g" 4 ion below the s o l u b i l i t y product of the pr e c i p i t a t e at the concentration of the manganese ion present i n the f i n a l s o l u t i o n . On the basis of the q u a l i t a t i v e tests carried out above, I considered i t expedient to test the p r e c i p i t a t e q u a n t i t a t i v e l y f o r I t s composition. The tests applied to the weighed p r e c i p i t a t e were f i r s t , the exact amount of ferrocyanide present; second, the determination of the exact amount of manganese i n the p r e c i p i t a t e ; and t h i r d , the exact amount of ammonium present. The pr e c i p i t a t e f o r analysis was prepared i n the following rigorous manner. Pure standard potassium ferrocyanide solution (50 ml.) was di l u t e d to 180 ml., 20 ml. of 6N HgS04 and two grams of (NH 4)gS0 4 added, and 18 ml* ( i i i ) o f standard KMnO, t i t r a t e d i n . T h i s s o l u t i o n was then s t i r r e d g e n t l y and heated over a burner t o (50 - 55)°C. to h e l p coagulate the p r e c i p i t a t e which formed. The s o l u t i o n was then allowed to stand covered w i t h a watch g l a s s f o r one h a l f hour to a l l o w the p r e c i p i t a t e to s e t t l e . The mixture was f i l t e r e d through a c a r e f u l l y prepared . Gooch c r u c i b l e under a p r e s s u r e o f e x a c t l y e i g h t pounds. The p r e c i p i t a t e was then washed three times with c o l d d i s t i l l e d water and th r e e times w i t h c o l d v e r y d i l u t e s u l f u r i c a c i d (10 ml. 6N HgS0 4 to 190 m l . HgO). Each time the washing was sucked completely d r y . The Gooch c r u c i b l e was then d r i e d i n a constant temperature oven a t e x a c t l y 1 0 5 ° C , d e s i c c a t e d f o r e x a c t l y 30 minutes, and weighed Immediately. The p r e c i p i t a t e was then d i s s o l v e d with b o i l i n g d i l u t e s u l f u r i c a c i d (20 ml. 6N H g S 0 4 to 180 ml. HgO) by means o f po u r i n g s m a l l p o r t i o n s o f i t r e p e a t e d l y through the c r u c i b l e . The f i l t r a t e was then t i t r a t e d w i t h standard KMn0 4 f o r the a v a i l a b l e f e r r o c y a n i d e . The Gooch c r u c i b l e s were agai n d r i e d , d e s i c c a t e d and weighed under e x a c t l y the same c o n d i t i o n s as b e f o r e . The d i f f e r e n c e i n weight was the weight o f the p r e c i p i t a t e d i s s o l v e d . T h i s method was used i n p r e f e r e n c e t o the method of f i l t e r i n g the p r e c i p i t a t e through a weighed Gooch c r u c i b l e , d r y i n g , d e s i c c a t i n g and weighing the p r e c i p i t a t e and then d i s  s o l v i n g and t i t r a t i n g f o r a v a i l a b l e f e r r o c y a n i d e ; s i n c e i t was found t h a t v e r y s l i g h t i m p u r i t i e s c a t a l y s e d the ( i v ) f o rmation o f f e r r i c f e r r o c y a n i d e w i t h a consequent e r r o r i n the weight of the p r e c i p i t a t e . With the method used, however, f e r r i c f e r r o c y a n i d e I s not s o l u b l e i n the b o i l i n g d i l u t e s u l f u r i c a c i d , and t h e r e f o r e does not e n t e r i n t o the weight' of the p r e c i p i t a t e o b t a i n e d . The t i t r a t e d a v a i l a b l e f e r r o c y a n i d e was c a l c u l a t e d i n grams of Fe(CN)g~^ and t h i s weight was used to c a l c u l a t e the t o t a l weight of v a r i o u s , most p r o b a b l e , forms o f the p r e c i p i t a t e . These weights on comparison show q u i t e p l a i n l y the most c l o s e l y r e l a t e d forms by weight. A t a b l e of r e s u l t s below (Table I) i n d i c a t e the val u e of these comparisons. TABLE I (a) Sample number.... 1 2 3 A Wt. of A c t u a l p p t s . 0.0203 0.0570 0.0664 5 Wt. of Fe(CN) ~* by t i t r a t i o n 0.01298 0.0369 0.0434 TABLE I (b) C a l c u l a t e d Weights F o l l o w i n g Sample.,Number .... Mn 2Fe(CN) 6 Mn(NH 4) 2Fe(CN) 6 from B Forms. f o r the MnKgFe(CN) 6 Mn 3K 2 |Fe(CN)( Mn K 6 l e ( C N ) g . 2 1 2 3 0.0197 0.0561 0.0659 0.0185 0.0528 0.0621 0.0191 0.0544 0.0640 0.0179 0.0512 0.0601 0.0211 0.0601 0.0707 0.0204 0.0581 0.0683 0.0218 0.0621 0.0731 (v) 1 2 3 -0.0003 -0.0009 -050005 -0.0015 -0,0052 -0.0043 -0.0009 -0.0026 -0.0024 -0.0021 -0.0058 -0.0063 0.0011 0.0031 0.0043 0.0004 0.0011 0.0019 0.0018 0.0051 0.0067 TABLE I (c) D e v i a t i o n from the Observed Weights A f o r the F o l l o w i n g Forms. Sample Number .... Mn 2Fe(CN) 6 Mn(NH 4) 2Fe(CN) g M n 3 ( N H 4 ) 2 | e ( G N l 6 ] 2 Mn(NH 4 ) 6 g e(CN) J2 MnKgFefCNjg Mn 3K g pe(CN), MnKg p ( G N ) g j 2 An examination of Table I (c) shows that the c l o s e s t formula by weight Is the form Mn 2Fe(CN)g. Two other formulae, namely Mn 3Kg J F e ( C N ) g J 2 and M n 3 ( N H 4 ) 2 JFe(CN)^j g, are too c l o s e by weight comparisons, to make a d e f i n i t e d e c i s i o n r e g a r d i n g a c t u a l formula o f the p r e c i p i t a t e . F o r t h i s reason two f u r t h e r t e s t s were made on the p r e c i p i t a t e ; one a q u a l i t a t i v e t e s t f o r potassium and the other a q u a n t i t a t i v e t e s t f o r ammonium. The f e r r o c y a n i d e p r e c i p i t a t e was prepared i n e x a c t l y the same manner as d e s c r i b e d p r e v i o u s l y ^ . The f i l t r a t e was evaporated almost to dry n e s s , 10 ml. of concentrated^'HCl added and then the s o l u t i o n a g a i n evaporated to a pas t e . The r e s i d u e was taken up i n 10 ml. of hot water and the f e r r i c f e r r o c y a n i d e p r e c i p i t a t e f i l t e r e d out and kept a s i d e f o r f u r t h e r t e s t s . To 10 ml. of the f i l t r a t e was added 5 ml. o f sodium c o b a l t i n i t r i t e # See APPENDIX Pp. I i and i i i ( v i ) s o l u t i o n (Na 3Co(N0g) 6 The s o l u t i o n was then b u f f e r e d w i t h 3 ml. of sodium a c e t a t e . The mixture was shaken and allowed to stand f o r one hour. S i n c e no p r e c i p i t a t e : formed, i t was assumed t h a t no a p p r e c i a b l e amount of potassium was p r e s e n t . T h i s . i s . l o g i c a l , s i n c e the p r e  c i p i t a t e used, weighed 0.0532 grams, and I f the form of potassium p r e s e n t . The c o b a l t . l n i t r i . t e . t e s t i s s e n s i t  i v e f o r 0.2 - 0.3 m i l l i g r a m s $. S i n c e there was a chance t h a t the potassium had been f i l t e r e d out i n the f e r r i c f e r r o c y a n i d e p r e c i p i t a t e i t was examined s p e c t r o s c o p i c a l l y f o r the presence of potassium. The s o l u t i o n was a l s o examined s p e c t r o s c o p i c a l l y . In n e i t h e r substance was t h e r e found even a t r a c e of potassium. The formula to be one of the most d i f f i c u l t .tests, undertaken. Of ten d e t e r m i n a t i o n s t h a t were made f i r s t , some appeared to correspond to a d e f i n i t e form of the o r i g i n a l p r e c i p  i t a t e , some.however, d i d not correspond to any d e f i n i t e compound and others were h i g h beyond a l l reason* A f t e r _a c o n s i d e r a b l e amount o f time was l o s t , the d i f f i c u l t y was found to be a p h y s i c a l c a r r y over of the sodium hydroxide i n the d i s t i l l a t i o n o f the ammonia. The d i f f i c u l t y was remedied by simply p l a c i n g a l ong d r y condenser v e r t i c a l l y i n the neck o f the f l a s k and from the top of t h i s , the steam was l e d through a t r a p and then there would be 6.3 m i l l i g r a m s Fe(CN) can thus be e l i m i n a t e d as a p o s s i b i l i t y . The q u a n t i t a t i v e t e s t f o r ammonium proved # Noyes and S w i f t , Qual. Chem. A n a l . , Macmillan, Pg. 298 ( v i i ) condensed i n a sh o r t c o l d condenser i n t o 50 ml. 0.2N HgSO^ standard s o l u t i o n . Of fou r runs u s i n g t h i s s e t up, the maximum weight o f NH^ found was 0.0005 grams on a p r e c i p i t a t e c o n t a i n i n g 0.0309 grams of F e ( C N ) 6 ~ 4 . T h i s maximum f i g u r e , however, does not correspond to any p o s s i b l e formula f o r the p r e c i p i t a t e under i n v e s t i g a t i o n . The r e s u l t s o f t h i s work show.that the formula cannot he M n 3 ( N H 4 ) 2 | e ( C N ) 6 ] 2 . The composition of the f e r r o c y a n i d e o f manganese must he then the one o r i g i n a l l y assumed # namely Mn 2Fe(CN) g. T h i s c o n c l u s i o n Is f u r t h e r e s t a b l i s h e d by the t a b l e below (Table I I ) assuming the above compos i t i o n o f the p r e c i p i t a t e . TABLE I I Weight  of ppt 0.0450 0.0570 0.0664 T o t a l wt  of Mn found 0.0154 0.0210 0.0240 Wt Mn  added ( T i t ' n ) 0.0015 0.0019 0.0023 D i f f e r e n c e C a l c u l - D e v i a t i o n (Wt Mn  i n ppt) 0.0139 0.0191 0.0217 ated wt  Mn from £Rt 0.0154 0.0195 0.0227 -0.0015 -0.0004 -0.0010 The manganese was determined by the bismuthate method on the t i t r a t e d p r e c i p i t a t e . The f e r r o  cyanide p r e c i p i t a t e which had been, t i t r a t e d w i t h a known volume o f standard KMnO^ was p l a c e d i n . a c o n i c a l f l a s k and 50 ml. of d i l u t e . n i t r i c a c i d (1 : 3) added, then 0.5 grams o f sodium bismuthate added. The s o l u t i o n was b o i l e d f o r f i v e minutes and the p i n k c o l o r o f the pe r  manganate formed was removed.by the dropwise a d d i t i o n of # M. D. F r a n c i s , Unpublished T h e s i s , "The Det'n of Small Amounts of Zinc i n Ores" ( v i i i ) f r e s h l y prepared sodium s u l f i t e . The oxides of nitrogen were then removed by vigorous b o i l i n g . The solution was cooled with running water to (10 - 15)°C*, 0.5 grams of sodium blsmuthate added, and a further 0.5 gram sample of sodium blsmuthate added to give an excess. The mixture was s t i r r e d f o r three minutes. To t h i s solution was added 50 ml*, of oxide f r e e , three per cent n i t r i c a c i d, and the whole solution f i l t e r e d through a f r e s h l y ignited Gooch c r u c i b l e . The f i l t r a t e to which were added the three per cent n i t r i c acid washings was over t i t r a t e d with ferrous ammonium sulfate solution and the excess back t i t r a t e d with standard potassium permanganate. The manganese added i n the ferrocyanide t i t r a t i o n was then subtracted from the t o t a l manganese determined above* The difference was the manganese present i n the p r e c i p i t a t e . Upon reviewing the evidence from both q u a l i t a t i v e and quantitative tests presented i n the argument, i t i s evident that the p r e c i p i t a t e must be of the composition MngPetCNjg* This i s the conclusion of the evidence f o r composition* BIBLIOGRAPHY B i f f e n , P. M., "Commercial Methods of A n a l y s i s " , 1st Ed., McGraw-Hill, p. 155. Coleman, S. A., I n d u s t r i a l E n g i n e e r i n g Chemistry, A n a l y t i c a l E d i t i o n , 377 - 380, (1941). H a r r i s , J . A., Unpublished T h e s i s (1922), "The V o l u m e t r i c Determination of Z i n c " p. 5 (Graph). Keen, W. H., J o u r n a l of the American Chemical S o c i e t y " , 26, 4 - 29, (1904). Mironenko, M. L., Zavadskaya L a b o r a t o r y , 10, 145 - 147, (1941). Mukhina, Z. S., Zavadskaya L a b o r a t o r y , 10, 145 - 147, (1941). S a r u d i , I . , O s t e r r Chemischen Z e l t u n g , 42, 297 - 298, (1939). Seaman, W. H., J o u r n a l of the American Chemical S o c i e t y , 29-1, 205 - 211, (1907). Stone, E. L., J o u r n a l of the American Chemical S o c i e t y , 30, 904, (1908). S n e l l , P. D., "Commercial Methods of A n a l y s i s " , 1 s t Ed., McGraw-Hill, p. 155. Thomas, A* W., " C o l l o i d Chemistry", l3t Ed., London, McGraw-Hill, 1934, p. 1 7 8 f f . Waring, W. G., J o u r n a l o f the American Chemical S o c i e t y , 26, 4 - 29, (1904). 

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