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ft}  THE SULFUR DIOXIDE, OXYGEN, SULFURIC ACID CELL  A t h e s i s submitted i n p a r t i a l f u l f i l l m e n t o f the requirements for  t h e Degree o f M a s t e r o f A p p l i e d S c i e n c e i n t h e Department o f C h e m i s t r y  JAMES A . L E I T H , B . A . S c .  The U n i v e r s i t y o f B r i t i s h C o l u m b i a  June,  1946.  ACKNOWLEDGEMENT  I w i s h t o acknowledge t h e  helpful  s u g g e s t i o n s and a s s i s t a n c e o f D r . W . F . S e y e r , under whose d i r e c t i o n the p r e s e n t work was c a r r i e d o u t .  I would a l s o l i k e  to  t h a n k D r . B . P . S u t h e r l a n d o f the C o n s o l i d a t e d M i n i n g and S m e l t i n g Company f o r s u p p l y i n g some much needed equipment w h i c h c o u l d n o t be o b t a i n e d from the r e g u l a r s o u r c e s .  TABLE OF CONTENTS, Page INTRODUCTION  1  OUTLINE OF THE PROBLEM  2  HISTORICAL  3  GENERAL THEORY  6  Gas Electrodes  •••  Hydrates i n Aqueous Solution  6 7  PROBABLE MECHANISM OF CELL. REACTION  8  Anode Reactions  8  Cathode Reactions  8  O v e r a l l C e l l Reaction  9  DESIGN OF THE CELL OPERATION OF THE CELL  12 '  14  P l a t i n i z i n g the Electrodes Degassing the Electrodes  14 •••  Changing the C e l l RESULTS  14 14 15  V a r i a t i o n of C e l l P o t e n t i a l with Cone. . . . .  15  E f f e c t o f the Rate of Sulfur Dioxide into the C e l l on the E . M . F  24  CONCLUSION  25  REFERENCES  26  BIBLIOGRAPHY  27  L I S T OF ILLUSTRATIONS  Page 1.  The V a r i a t i o n o f C e l l V o l t a g e w i t h Time U s i n g Different  Electrode M a t e r i a l  4  2.  Theoretical E.M.F. of C e l l  11  3.  S u l f u r D i o x i d e , Oxygen, S u l f u r i c A c i d C e l l . . . .  13  4.  C e l l Voltage v s . Concentration  of Electrolyte,.  17  5.  Freezing P o i n t Chart o f S u l f u r i c A c i d Hydrates.  20  6.  The V a r i a t i o n o f C e l l V o l t a g e w i t h Time  23  KEY TO SYMBOLS  f  -  f r e e energy change,  -  Faraday  96,500 coulombs  M -  mol f r a c t i o n  N -  normality  n  valence  -  T = temperature, t  -  time,  calories  degrees Centigrade  hours  X = per cent  acid  x  = n o r m a l i t y o f e l e c t r o l y t e a t time  J  = conversion factor:  "t"  1 c a l . - 4 . 1 8 3 joules 1 j o u l e = 9.87 cc atm.  THE SULFUR DIOXIDE« OXYGEN*. SULFURIC ACID CELL  P r o d u c t i o n o f s u l f u r i c a c i d "by p r e s e n t s t a n d a r d methods l e a v e s l i t t l e t o be d e s i r e d , elemental are u s e d .  sulfur,  e s p e c i a l l y when  s u l f i d e o r e s and some b y - p r o d u c t  gases  However, the u t i l i z a t i o n o f the f r e e energy o f  s u l f u r d i o x i d e i n the form o f e l e c t r i c a l production o f sulfuric acid offers  energy f o r  interesting  the  possibilities.  The c e l l r e a c t i o n o f such a p r o c e s s i s g i v e n b y : SO., +- $Q% + H 0 t  The f r e e energy change reaction  H SO^ 4  involved i n t h i s  i s made up o f the f r e e energy o f f o r m a t i o n o f  s u l f u r i c a c i d p l u s the f r e e energy o f d i l u t i o n o f acid. •49,100  sulfuric  The A F change i n f o r m i n g pure s u l f u r i c a c i d ' i s c a l o r i e s p e r m o l e , and f o r a more d i l u t e 1  acid  it  2 would be even g r e a t e r .  Any r e a c t i o n w h i c h i s accompanied  by a l a r g e n e g a t i v e f r e e energy change i s  spontaneous,  hence from t h e above f r e e energy change we r e a l i z e i t would "be q u i t e p o s s i b l e t o produce s u l f u r i c a c i d i n an e l e c t r o l y t i c c e l l and a t the same t i m e draw o f f e l e c t r i c a l e n e r g y . By t h e use o f s u l f u r d i o x i d e i n the method i n d i c a t e d , a number o f o b j e c t i v e s c o u l d be a c h i e v e d .  These  i n c l u d e p r o d u c t i o n o f cheap d i l u t e a c i d f o r i n d u s t r i a l u s e s such as p i c k l i n g s t e e l o r l e a c h i n g copper o r o t h e r  ores;  p r o d u c t i o n o f c o n c e n t r a t e d a c i d where economic ( r o n d i t i o n s w a r r a n t , and abatement o f a t m o s p h e r i c p o l l u t i o n .  OUTLINE OF THE PROBLEM  I n the f o l l o w i n g e x p e r i m e n t s i t was p r o p o s e d t o determine the e l e c t r o m o t i v e f o r c e o f a c e l l s i m i l a r Pt,  to  SO,,: H^SO,,.: Oj, , P t  at various concentrations o f s u l f u r i c a c i d .  The above  cell  was s t u d i e d i n 1945 b y T . C . A s s a l y , and from h i s work i t was shown t h a t f o u r h y d r a t e s o f s u l f u r i c a c i d were s t a b l e i n solution.  The w r i t e r proposed t o s t u d y the same c e l l w i t h  d i f f e r e n t e l e c t r o d e m a t e r i a l t o t e s t the v a l i d i t y o f Assaly's results. The o v e r a l l e f f i c i e n c y o f the p l a t i n u m - e l e c t r o d e c e l l was b e l o w 50 p e r c e n t o v e r the g r e a t e r p a r t o f the  acid  r a n g e , so a p r e l i m i n a r y s t u d y was made t o f i n d an e l e c t r o d e  c o m b i n a t i o n w h i c h would i n c r e a s e the c e l l e f f i c i e n c y .  Lead  e l e c t r o d e s were u s e d f i r s t b u t were soon d i s c a r d e d s i n c e an o x i d e f i l m formed i m m e d i a t e l y on b o t h t h e anode and cathode producing a v e r y low v o l t a g e .  T e s t s were t h e n c a r r i e d o u t  u s i n g t a n t a l u m m e t a l a t b o t h e l e c t r o d e s and a c o n s t a n t v o l t a g e was o b t a i n e d a f t e r  about 24 h o u r s .  The  o f t h i s c e l l was b e l o w t h a t o f A s s a l y ' s b u t i t  efficiency appeared  t h a t a c o m b i n a t i o n o f p l a t i n u m and t a n t a l u m m i g h t g i v e the desired results.  A c e l l made up o f a p l a t i n u m anode and a  t a n t a l u m cathode was t h e n t e s t e d b u t t h i s s t i l l was n o t satisfactory.  The e l e c t r o d e c o m b i n a t i o n was t h e n r e v e r s e d ,  and w i t h t a n t a l u m as anode and p l a t i n u m as cathode a g r e a t i n c r e a s e i n e f f i c i e n c y was o b t a i n e d .  F i g . 1 shows t h e s e  results graphically. As a r e s u l t o f t h e s e e x p e r i m e n t s i t was d e c i d e d t o d e t e r m i n e the a c t u a l e l e c t r o m o t i v e f o r c e o f the  cell:  T a , S 0 : H^SO^: 0,,, P t 4  at various concentrations of s u l f u r i c  acid. \.  HISTORICAL  I n 1916 M e s s r s . M . De Kay Thompson and IT. J . Thompson* i n v e s t i g a t e d the c u r r e n t e f f i c i e n c i e s o f the o x i d a t i o n o f s u l f u r o u s a c i d and t h e y showed t h a t  this  o x i d a t i o n t a k e s p l a c e w i t h h i g h c u r r e n t e f f i c i e n c i e s even  5 i n strong s u l f u r i c acid solutions.  Also for a given  s u l f u r i c a c i d c o n c e n t r a t i o n the c u r r e n t e f f i c i e n c y d e c r e a s e s with increasing current  density.  M e s s r s . M . De Kay Thompson and A . P .  Sullivan  s t u d i e d the d e p o l a r i z a t i o n e f f e c t s o f s u l f u r d i o x i d e i n an e l e c t r o l y t i c c e l l u s i n g a p l a t i n u m anode.  They found t h a t  i t was p o s s i b l e to d e c r e a s e the p o l a r i z a t i o n a t the anode and t h u s d e c r e a s e the e q u i l i b r i u m v o l t a g e o f the c e l l b y the a d d i t i o n o f s u l f u r d i o x i d e .  I t appeared from t h e i r  e x p e r i m e n t s t h a t the p o l a r i z a t i o n was due t o the a c c u m u l a t i o n o f f r e e oxygen a t the anode.  On a d d i t i o n o f s u l f u r d i o x i d e  t o the a n o l y t e a r e a c t i o n o c c u r r e d between t h i s gas and the f r e e oxygen r e d u c i n g the p o l a r i z a t i o n e f f e c t s . M r . T . C . A s s a l y s t u d i e d the a c t u a l e l e c t r o m o t i v e f o r c e o f the  cell: P t , S 0 : H^SO : 0 , P t l  A  X  a t v a r i o u s c o n c e n t r a t i o n s o f s u l f u r i c a c i d and a t temperatures.  different  He found t h a t i n p l o t t i n g c e l l v o l t a g e  versus  c o n c e n t r a t i o n o f e l e c t r o l y t e t h a t a s t e p - w i s e c u r v e was formed; each o f the f o u r s t e p s c o r r e s p o n d i n g t o a h y d r a t e o f sulfuric acid.  Other e x p e r i m e n t e r s have shown e v i d e n c e  for  t h e e x i s t e n c e o f t h r e e o f t h e s e h y d r a t e s b y f r e e z i n g methods. The e l e c t r o l y t i c method i n d i c a t e s t h a t the s u l f u r i c a c i d o  h y d r a t e s are s t a b l e a t 25 C as w e l l as a t t h e i r  freezing  points. The e l e c t r o m o t i v e f o r c e o f the above c e l l was shown t o be dependent o n the t e m p e r a t u r e o f the  electrolyte.  - 6  The v o l t a g e i n c r e a s e d s l i g h t l y w i t h a d e c r e a s e i n t e m p e r a t u r e and approached a c o n s t a n t v a l u e a t l o w t e m p e r a t u r e s ,  GENERAL THEORY  GAS ELECTRODES Gas e l e c t r o d e s commonly c o n s i s t o f a s o l i d c l a s s c o n d u c t o r , i n w h i c h a gas has been a b s o r b e d .  firstRecent  i n v e s t i g a t o r s have shown t h a t the emf d e v e l o p e d b y s o - c a l l e d gas e l e c t r o d e s  i s a f u n c t i o n o f the f i r s t - c l a s s  conductor  i n w h i c h the gas i s c o n t a i n e d as w e l l as o f the gas  itself.  I n o t h e r w o r d s , gas e l e c t r o d e s are r e a l l y g a s - m e t a l  electrodes  s i n c e m e t a l s are commonly used t o absorb t h e s e g a s e s .  The  p o t e n t i a l s d e v e l o p e d by such g a s - m e t a l e l e c t r o d e s are for  the p a r t i c u l a r m e t a l and g a s , and are r e l a t e d t o  a b s o r b i n g power o f the m e t a l f o r the g a s .  the  The p o t e n t i a l s  are known to v a r y g r e a t l y w i t h t h e n a t u r e o f the i n w h i c h the e l e c t r o d e s are immersed.  specific  electrolyte  I t has a l s o been found  t h a t t h e r e i s a somewhat d i f f e r e n t p o t e n t i a l d e v e l o p e d when the e l e c t r o d e i s s t a t i o n a r y t h a n when i t i s i n m o t i o n . latter  e f f e c t may be shown b y moving the e l e c t r o d e o r b y  k e e p i n g i t s t a t i o n a r y and moving the e l e c t r o l y t e . e l e c t r o l y t e may be moved by s t i r r i n g , c a u s i n g t h e t o f l o w p a s t t h e e l e c t r o d e , o r b y b u b b l i n g t h e gas the  This  The electrolyte through  electrolyte. I t has been o b s e r v e d t h a t the amount o f gas  needed  to b r i n g about i t s e f f e c t on the p o t e n t i a l o f a m e t a l , i s v e r y  small indeed.  1  I n f a c t , the p r e s s u r e o r s o l u b i l i t y o f the  gas has l i t t l e o r no e f f e c t on the p o t e n t i a l .  When a gas  is  b e i n g pumped o u t o f t h e s o l u t i o n l i t t l e e f f e c t i s o b s e r v e d on t h e p o t e n t i a l u n t i l the b u b b l i n g o u t o f the gas causes a stirring  effect.  HYDRATES I N AQUEOUS SOLUTION The number o f m o l e c u l e s o f w a t e r i n c o m b i n a t i o n w i t h one m o l e c u l e o f t h e d i s s o l v e d s u b s t a n c e ,  frequently  i n c r e a s e s from the most c o n c e n t r a t e d t o t h e most d i l u t e s o l u t i o n s , as w i t h magnesium c h l o r i d e , manganese c h l o r i d e and copper c h l o r i d e .  W i t h some s u b s t a n c e s the number o f m o l e c u l e s  o f water h e l d i n c o m b i n a t i o n b y one m o l e c u l e o f the d i s s o l v e d substance may p a s s t h r o u g h a w e l l - d e f i n e d maximum as d i l u t i o n i s increased.  the  I n o t h e r c a s e s , the number o f  m o l e c u l e s o f w a t e r h e l d i n c o m b i n a t i o n b y one m o l e c u l e o f the d i s s o l v e d substance may r e a c h a maximum v a l u e as the  dilution  i s i n c r e a s e d ; t h i s maximum v a l u e may t h e n r e m a i n p r a c t i c a l l y c o n s t a n t w i t h f u r t h e r i n c r e a s e s i n the  dilution.  The q u e s t i o n a r i s e s whether t h e s e h y d r a t e s are c h e m i c a l compounds o r whether t h e y r e p r e s e n t form o f c o m b i n a t i o n .  some l e s s  true  stable  That t h e y are u n s t a b l e i s shown b y the  ease w i t h w h i c h t h e y are b r o k e n down b y h e a t .  Most o f the  w a t e r can be d r i v e n o f f from the above s o l u t i o n s a t a o  temperature  o n l y a l i t t l e above 100 C .  The more complex  h y d r a t e s a r e , t h e n , decomposed i n s o l u t i o n a t a c o m p a r a t i v e l y low temperature vapour.  and the water i s g i v e n o f f i n t h e form o f  I n t h e l i g h t o f t h e s e f a c t s the h y d r a t e s can s c a r c e l y  be r e g a r d e d as t r u e c h e m i c a l compounds.  I f however, we  i n s i s t on c a l l i n g them c h e m i c a l compounds, we must admit t h a t they represent a v e r y low order o f s t a b i l i t y . I t i s the g e n e r a l o p i n i o n t h a t b o t h m o l e c u l e s and i o n s combine w i t h w a t e r ,  forming hydrates.  I t seems t h a t  m o l e c u l e s are c e r t a i n l y capable o f f o r m i n g h y d r a t e s , i n very concentrated  the  because  s o l u t i o n s where the i o n i z a t i o n i s v e r y  s m a l l , we o f t e n have c o n s i d e r a b l e h y d r a t i o n .  That i o n s are  capable o f c o m b i n i n g w i t h w a t e r i n s o l u t i o n i s shown by the magnitude o f the h y d r a t i o n i n many o f the d i l u t e  solutions,  where c h i e f l y i o n s and o n l y a few m o l e c u l e s are p r e s e n t .  PROBABLE MECHANISM OF CELL REACTIONS  ANODE REACTIONS The s u l f u r d i o x i d e f i r s t d i s s o l v e s i n the  sulfuric  a c i d e l e c t r o l y t e and t h e n adsorbs on t h e s u r f a c e o f the electrode: SO (gas) = SO^(solution) t  (1)  The s u l f u r d i o x i d e t h e n combines w i t h w a t e r as i n e q u a t i o n (2)  ' S O + H 0 = S0~ + 2 H % ^ ^ z  CATHODE  JL  (2)  REACTIONS S i m i l a r l y , oxygen i s d i s s o l v e d i n the  electrolyte  and t h e n adsorbed on the s u r f a c e o f t h e e l e c t r o d e . • 0 (gas) 4  '=:"  O^Csolution)  (3)  The oxygen t h e n combines w i t h water as i n e q u a t i o n  £O  F C  *  H.0  +• 2 ( - )  -  20H  (4)  OVERALL CELL REACTION The  o v e r a l l c e l l reaction i s given by:  S0 +£ O i  mH^O -  2  The  Hj,S0  4  (5)  (N = X)  c e l l r e a c t i o n can be d i v i d e d i n t o two p a r t s  as s u c h : SO 2. +• J 0  a  z  +• io^  (6)  mH O - H^SO^. (N - X )  (7)  t  2 H % SO^ SO  +- H O = H^SO^. a  + mH O = H SO^ ( N =• X) t  A d d i t i o n o f equations overall c e l l reaction. r e a c t i o n s o f equations  (8)  z  (6) and (7) g i v e s t h e  Hence a d d i t i o n o f t h e f r e e  energy o f  (6) and (7) w i l l g i v e the o v e r a l l f r e e  energy change w i t h i n the c e l l .  The v a l u e o f A F f o r  (6) i s o b t a i n e d from t h e d i f f e r e n c e o f t h e f r e e  equation  energy o f  f o r m a t i o n o f s u l f u r i c a c i d and t h e sum o f t h e f r e e  energies  o f f o r m a t i o n o f s u l f u r . d i o x i d e , w a t e r and o x y g e n . is* H s o + -  A  ~ 176,000 c a l o r i e s  z  ^is"  \s.o£,  ^ **° u » o F  Oj.  -  -  71,740 c a l o r i e s  -  -  56,690 c a l o r i e s  -  zero  T h e r e f o r e A F change = - 4 9 , 1 0 0 c a l o r i e s . The the free  number o f a c c u r a t e measurements from w h i c h  energy o f d i l u t i o n o f s u l f u r i c a c i d may be  calculated i s limited.  T a b l e s are a v a i l a b l e on the  free  energy o f d i l u t i o n o f t h e a c i d o n l y f o r d i l u t e s o l u t i o n s . 8  B r o n s t e d s t u d i e d the f r e e  energy o f d i l u t i o n o v e r a r a t h e r  wide range o f c o n c e n t r a t i o n s o f s u l f u r i c a c i d b u t a t • 9 t e m p e r a t u r e s r a n g i n g o n l y up t o 9 C . Harned and S t u r g i s I O  and L e w i s and R a n d a l l a l s o s t u d i e d the f r e e energy o f t h i s a c i d , b u t o n l y o v e r a s m a l l range o f  concentration.  A s a t i s f a c t o r y t a b l e o f the f r e e  energy o f d i l u t i o n  a t 25°C f o r c o n c e n t r a t i o n s up t o 0 . 2 m o l f r a c t i o n s u l f u r i c a c i d has been worked o u t b y R a n d a l l and Cushman.* r e s u l t s w h i c h are the f r e e e n e r g i e s * F , o f the 2H + +  S 0 ^ + mH 0 = R \ S 0 z  reaction:  (N-X)  4  Their  (7)  are g i v e n i n the t h i r d column o f T a b l e I , and the  correspond-  i n g v a l u e s o f mol f r a c t i o n M and p e r c e n t a c i d X i n t h e f i r s t and second columns r e s p e c t i v e l y . M  TABLE I A F . Ccal.")  X(%)  AF,  (k c a l . )  E(volts)  1.00  100.00  0  -49.1  1.065  .20  57.70  5270  -43.8  .950  .13  44.90  2915  -46.2  1.000  .10  37.70  1645  -47.5  1.030  .08  31.60  702  -48.4  1.048  .065  27.55  -85  -49.2  1.067  .05  22.28  -865  -50.0  1.084  .03  14.42  -2048  .02  10.00  .01 .002 .0009  -51.1  1.108  -2735  -51.8  1.122  5.21  -3702  -52.8  1.143  1.08  -5613  -54.7  1.185  -6547  -55.6  1.204  .487 A F  A  £x F  x  -  r e p r e s e n t s the o v e r a l l c e l l  reaction  r e p r e s e n t s the f r e e energy o f d i l u t i o n  By s u b s t i t u t i o n i n t h e f o r m u l a , E -  -&F)(J) n f  a calories J = 4.183 j o u l e s / c a l o r i e ©  assuming a temperature  o f 25 C , t h e t h e o r e t i c a l v a l u e s o f  t h e emf t h a t would be o b t a i n e d i n a c o m p l e t e l y r e v e r s i b l e c e l l are o b t a i n e d . Table I .  The v a l u e s are g i v e n i n column 5 o f  I n F i g , 2 t h e v a l u e s o f t h e emf g i v e n i n T a b l e I  are p l o t t e d as o r d i n a t e s a g a i n s t c o n c e n t r a t i o n o f s u l f u r i c a c i d as a b s c i s s a .  A h y p o t h e t i c a l c u r v e was drawn from  0 . 2 M t o 1 M s u l f u r i c a c i d , the range o f c o n c e n t r a t i o n where i n f o r m a t i o n on the f r e e energy o f d i l u t i o n was u n o b t a i n a b l e . DESIGN OF THE. CELL-  A diagrammatic sketch o f the c e l l i s g i v e n i n F i g . 3.  I t c o n s i s t e d o f two s t o p p e r e d f r i t t e d g l a s s v e s s e l s ,  anode and cathode compartments, each c o n t a i n i n g s u l f u r i c a c i d o f the same c o n c e n t r a t i o n .  Each o f t h e s e v e s s e l s had  a g l a s s tube e x t e n d i n g from i t , about two i n c h e s above fritted disc.  the  These v e s s e l s were connected b y means o f a  r u b b e r tube c o n t a i n i n g a c l a y d i a p h r a g m . E l e c t r i c a l c o n t a c t was made b y two m e r c u r y f i l l e d glass leads.  One end o f each l e a d was s e a l e d t o t h e  stem  o f i t s p a r t i c u l a r e l e c t r o d e ; the anode b e i n g a s t r i p o f t a n t a l u m and t h e cathode a p l a t i n i z e d p l a t i n u m g a u z e .  The  p o t e n t i a l was measured b y a p o t e n t i o m e t e r . The whole c e l l was s e t i n a c o n s t a n t  temperature o  b a t h w h i c h m a i n t a i n e d the temperature desired value.  w i t h i n 0 . 0 5 C o f the  13  14 OPERATION OF THE CELL  PLATINIZING THE ELECTRODES The p l a t i n u m e l e c t r o d e s were c o a t e d w i t h a l a y e r o f p l a t i n u m "black d e p o s i t e d e l e c t r o l y t i c a l l y from a t h r e e per cent s o l u t i o n o f c h l o r o p l a t i n i c a c i d .  The e l e c t r o d e s  were f i r s t c l e a n e d i n warm chromic a c i d and t h e n l o w e r e d i n t o the s o l u t i o n .  A 1 2 - v o l t c i r c u i t was u s e d and a  commutator a l l o w e d the c u r r e n t t o be r e v e r s e d a t d e s i r e d intervals. By means o f a s l i d i n g r e s i s t a n c e , the c u r r e n t was r e g u l a t e d so as t o produce a moderate e v o l u t i o n o f g a s .  The  d i r e c t i o n o f the c u r r e n t was r e v e r s e d e v e r y minute u n t i l a b l a c k and v e l v e t y c o a t i n g appeared on the s u r f a c e o f the electrodes. DEGASSING THE ELECTRODES A f t e r each r u n t h e e l e c t r o d e s were removed from the c e l l , washed i n d i s t i l l e d w a t e r , and degassed b y e l e c t r o l i z i n g them i n 6 N s u l f u r i c a c i d f o r a p p r o x i m a t e l y one h o u r . The d i r e c t i o n o f the c u r r e n t was r e v e r s e d e v e r y t e n m i n u t e s for  the f i r s t f i f t y m i n u t e s , t h e n e v e r y minute f o r the  ten minutes.  last  The p l a t i n u m e l e c t r o d e s were t h e n b o i l e d i n  d i l u t e n i t r i c a c i d f o r t e n m i n u t e s forremove any p o i s o n i n g agents.  F i n a l l y a l l t h e e l e c t r o d e s were washed i n b o i l i n g  d i s t i l l e d water f o r f i f t e e n minutes. CHANGING THE CELL The oxygen and s u l f u r d i o x i d e i n l e t and o u t l e t  c o n n e c t i o n s t o t h e c e l l were opened and the two h a l f - c e l l s were removed from the c o n s t a n t t e m p e r a t u r e b a t h .  15  After  f l u s h i n g w i t h warm d i s t i l l e d w a t e r , t h e h a l f - c e l l s were d r i e d i n a warm oven t h e n washed w i t h s u l f u r i c a c i d e l e c t r o l y t e . The c e l l was t h e n s e t up a g a i n and c o n n e c t i o n s made t o s u l f u r d i o x i d e and oxygen t a n k s .  the  Seventy-five cubic  c e n t i m e t e r s o f t e s t s u l f u r i c a c i d was t h e n added t o each h a l f c e l l and the e l e c t r o d e s p l a c e d i n t h e i r r e s p e c t i v e  half-cells.  The s u l f u r d i o x i d e and oxygen were o b t a i n e d from pressure tanks. for  The d e s i r e d c o n c e n t r a t i o n o f s u l f u r i c a c i d  each r u n was o b t a i n e d b y d i l u t i n g C P . a c i d w i t h  water.  distilled  The e x a c t s t r e n g t h o f the a c i d was d e t e r m i n e d b y  t i t r a t i n g a g a i n s t a s t a n d a r d NaOH s o l u t i o n u s i n g p h e n o l p h t h a l e i n as i n d i c a t o r .  RESULTS  VARIATION ffl? mgrT.- POTENTIAL WITH CONCENTRATION f  Measurements were made o f c e l l p o t e n t i a l a t v a r i o u s o a c i d c o n c e n t r a t i o n s , ,-ajad a t 25 C , and t h e s e are shown i n Table I I .  The f i r s t and second columns g i v e t h e n o r m a l i t y  N and t h e p e r c e n t a c i d X and t h e t h i r d column the measured emf o f the c e l l .  The v a l u e s o f t h e emf g i v e n i n T a b l e I I  are t h e v a l u e s o b t a i n e d a f t e r t h e s u l f u r d i o x i d e and oxygen had b u b b l e d t h r o u g h t h e c e l l f o r n e a r l y 24 h o u r s and the p o t e n t i a l approached a c o n s t a n t v a l u e .  cell  16 TABLE  III X 1  E  I  (voits)  0.98  4.69  0.8270  3.04  13.94  0.8120  4.80  21.23  0.8000-  6.22  26.75  0.7990-  8.10  33.60  0.7960--  10.24  40.82  0.7927  12.20  46.90  0.7896—  14.05  52.35  0.7889:  17.30  61.11  0.7882-  18.20  63.37  0.7690  21.35  70.79  0.7674-  24.31  77.17  0.7620^  25.50  79.50  0.7121^  26.61  81.70  0.678Q,  29.00  86.18  0.6752-  32.00  90.80  0.5800„  33.20  93.90  0.4770 .  35.60  97.10  0.4657  The v a l u e s o f the emf g i v e n i n T a b l e I I are i n F i g . 4 as o r d i n a t e u s i n g the n o r m a l i t y o f t h e a c i d abscissa.  plotted as  The e x p e r i m e n t a l r e s u l t s f a l l on a s t e p - w i s e c u r v e  w i t h each o f the f o u r s t e p s c o r r e s p o n d i n g t o a h y d r a t e o f sulfuric acid.  The f i r s t s t e p a t 3 0 . 9 N c o r r e s p o n d s t o  H S0^- H 0 ; t h e second a t 2 4 . 4 N t o H S 0 ^ 2 H 0 ; t h e t h i r d 2  a  a  t  at  • 18 1 7 . 1 N t o H S 0 - 4 H 0 ; t h e f o u r t h a t 5 . 0 1 If t o H S 0 - 20 R \ 0 . 4  4  Z  2  4  I t s h o u l d n o t be a t a l l s u r p r i s i n g t h a t  hydrates  o f s u l f u r i c a c i d are i n d i c a t e d i n an e l e c t r o l y t i c c e l l . H y d r a t e s o f s u l f u r i c a c i d are known t o e x i s t i n s o l u t i o n . S i n c e the f o r m a t i o n o f a h y d r a t e i n s o l u t i o n i s , a s s o c i a t e d w i t h a c o n s i d e r a b l e f r e e energy change, i t s h o u l d be a s s o c i a t e d w i t h a marked p o t e n t i a l change i n an e l e c t r o l y t i c cell.  The p o t e n t i a l s h o u l d v a r y w i t h t h e n a t u r e o f the  h y d r a t e formed, hence i t s h o u l d be p o s s i b l e t o d e t e c t  the  p r e s e n c e o f h y d r a t e s o f s u l f u r i c a c i d i n an e l e c t r o l y t i c c e l l . L e t u s c o n s i d e r the c u r v e shown i n F i g . 4 .  We w i l l  f i r s t c o n s i d e r the c e l l v o l t a g e a t a v e r y l o w a c i d c o n c e n t r a tion.  A t t h i s c o n c e n t r a t i o n we have a d e f i n i t e c e l l p o t e n t i a l ,  and as we i n c r e a s e the a c i d c o n c e n t r a t i o n we have a  decrease  i n the o v e r a l l f r e e energy change c a u s i n g t h e p o t e n t i a l t o drop.  The drop i s f a i r l y c o n s t a n t up t o a c o n c e n t r a t i o n o f -  5 N and o v e r t h i s range we have the h y d r a t e H S O ^ 2 0 H 0 . z  a  Beyond t h i s c o n c e n t r a t i o n we do n o t have enough w a t e r  present  t o form t h i s h y d r a t e so we have a n o t h e r h y d r a t e f o r m i n g , namely ^ 3 0 ^ - 4 ^ 0 .  From 5 N t o 1 7 . 1 N t h e s l o p e o f our c u r v e  i s c o n s t a n t and o v e r t h i s range we have t h e h y d r a t e H^S0^4H^0. Beyond 1 7 . 1 N we do n o t have enough w a t e r p r e s e n t t o form t h i s h y d r a t e so t h a t we have a change o v e r t o H S O ^ 2 H 0 accompanied t  b y a marked change i n p o t e n t i a l . explained  i  The o t h e r s t e p s c a n be  similarly. The p o r t i o n o f t h e t a n t a l u m - p l a t i n u m c u r v e A - B shown  i n F i g . 4 was p l o t t e d b y m e a s u r i n g the c e l l p o t e n t i a l s t a r t i n g  a t a l o w c o n c e n t r a t i o n o f a c i d and w o r k i n g up t o c o n c e n t r a t i o n o f 17 IT.  19  a  The o t h e r end o f t h e c u r v e was  p l o t t e d b y m e a s u r i n g the c e l l p o t e n t i a l u s i n g C P . a c i d t h e n w o r k i n g back to 1 8 . 3 N .  T h i s gave the c u r v e C - D - E .  An  a p p a r e n t d i s c r e p a n c y i n the c u r v e i s shown d o t t e d between D and E .  To check t h i s p a r t o f the c u r v e t e s t s were c a r r i e d  o u t w i t h 1 8 . 3 N , 2 2 . 1 N and 2 4 . 2 N a c i d .  A f t e r 20 h o u r s ,  v a l u e s were o b t a i n e d f a l l i n g on the d o t t e d p o r t i o n o f the curve but a f t e r  a n o t h e r 12 h o u r s t h e s e approached t h e  solid  p o r t i o n o f the c u r v e D - E and remained c o n s t a n t f o r o v e r 5 hours. The c u r v e o b t a i n e d by A s s a l y i s a l s o shown i n F i g . 4 f o r comparison w i t h t h a t o b t a i n e d b y t h e w r i t e r .  The  e f f i c i e n c y o f the t a n t a l u m r p l a t i n u m c e l l i s c l e a r l y shown t o be much g r e a t e r t h a n t h a t o f A s s a l y s o v e r most o f t h e  acid  1  range.  The g r e a t drop i n e f f i c i e n c y a t the h i g h a c i d  c o n c e n t r a t i o n i s p r o b a b l y due t o the t a n t a l u m d i s s o l v i n g i n the s u l f u r i c a c i d . Other e x p e r i m e n t e r s have shown e v i d e n c e o f t h r e e h y d r a t e s o f s u l f u r i c a c i d b y f r e e z i n g methods. o b t a i n e d the h y d r a t e H S O ^ ^ O ; a  Giron^found the  Pickering hydrate  HgSO^SHaO; and Donk o b t a i n e d c r y s t a l l i n e s u l f u r i c a c i d monohydrate H S O R^O. fe  v  The f r e e z i n g p o i n t c h a r t o f s u l f u r i c a c i d i s shown i n F i g . 5 .  hydrates  B , D and G are the e r y o h y d r a t e p o i n t s and  C , E and H are t h e m e l t i n g p o i n t s o f the h y d r a t e s R \ S 0 . 4 5 ^ 0 , 4  Hj,S0 . 2 H 0 and R\S0,_- H 0 r e s p e c t i v e l y . +  t  x  From B t o C , D to E  10  o  10  20  3o  J  Fig.5  O  to  70  8°  9o  21 and  G to H these r e s p e c t i v e hydrates s o l i d i f y out.  The  e x i s t e n c e o f H^SO^- 2 0 H 0 i s n o t i n d i c a t e d i n t h e g r a p h b u t t  would p r o b a b l y be d e t e c t e d between A and B on c l o s e r examination. Measurements were made o f emf w i t h time f o r concentrations  of acid.,  The measurements f o r t h r e e  t r a t i o n s are g i v e n i n T a b l e I I I .  several  concen-  The f i r s t column g i v e s  the  t i m e , the s e c o n d , t h i r d and f o u r t h g i v e the emf measured w i t h 0.98 N , 27.2 N a i d 35.5 N a c i d r e s p e c t i v e l y . Time-voltage curves f o r these a c i d are shown i n F i g . 6 .  concentrations  The i n i t i a l v o l t a g e i n a l l c a s e s was  v e r y h i g h , d r o p p i n g o f f t o a c o n s t a n t v a l u e a f t e r about 24 hours.  A t the b e g i n n i n g an e q u i l i b r i u m - e x i s t e d between  the  t a n t a l u m m e t a l and the s u l f u r d i o x i d e i n s o l u t i o n b u t as t i m e goes on an o x i d e f i l m forms on the t a n t a l u m and an e q u i l i b r i u m i s s e t up between the t a n t a l u m o x i d e and the sulfur dioxide i i i solution.  The i n i t i a l d r o p i n v o l t a g e  o c c u r s w h i l e the o x i d e f i l m i s f o r m i n g t h e n the g r a d u a l l y approaches a c o n s t a n t v a l u e .  voltage  22  TABLE III  E(0.98N)  t  EC27.2N)  E(35.6N) volts  mins.  volts  0  1.1000  1.0553  1.0240  15  1.0235  0.9352  0.8500  40  0.9500  0.8755  0.7150  1  00  0.9250  0.8503  • 0.6680  1  30  0.9100  0.8255  0.6320  2  00  0.8995  0.8110  0.6100  2  30  0.8900  0.7950  0.5920  3  00  0.8810  0.7825  0.5710  3  30  0.8770  0.7685  0.5530  4  00  0.8650  0.7552  0.5270  5  00  0.8550  0.7490  0.4930  7  00  0.8500  0.7355  0.4730  10  00  0.8308  0.7185  0.4710  15  00  0.8270  0.6978  0.4680  20  00  0.8260  23  30  0.8250  0.6740  0.4635  25  30  0.8249  0.6738  0.4634  urs  volts  0.6750  . 0.4637  24 EFFECT OF THE RATE OF SULFUR DIOXIDE AND OXYGEN INTO THE CELL ON THE EMF I n t h e p r e s e n t work a r a t e o f f l o w o f 60 b u b b l e s p e r minute was u s e d f o r b o t h t h e s u l f u r d i o x i d e and o x y g e n . A l t h o u g h t h i s method o f c o n t r o l was o n l y q u a l i t a t i v e i t s e r v e d i t s purpose s i n c e o n l y v o l t a g e was b e i n g s t u d i e d and hot rates o f formation o f a c i d . On i n c r e a s i n g t h e r a t e o f f l o w o f s u l f u r d i o x i d e i n t o t h e c e l l from 60 b u b b l e s p e r minute t o 120 b u b b l e s p e r minute the v o l t a g e i n c r e a s e d from 0.8000 t o 0.8007 several hours.  after  T h i s v a l u e r e t u r n e d t o 0.8002 on d e c r e a s i n g  t h e r a t e b a c k t o 60 b u b b l e s p e r m i n u t e .  Decreasing the r a t e  t o 30 b u b b l e s p e r minute c u t the emf t o 0.7991 w h i c h r e t u r n e d t o 0.8001 on i n c r e a s i n g the r a t e t o 60 b u b b l e s p e r m i n u t e . On i n c r e a s i n g t h e r a t e o f f l o w o f oxygen i n t o the c e l l from 60 b u b b l e s p e r minute t o 120 b u b b l e s p e r minute t h e v o l t a g e d e c r e a s e d from 0.8000 t o 0 . 7 9 9 0 .  On d e c r e a s i n g  t h e r a t e t o 60 b u b b l e s p e r minute t h e emf went up t o 0.7997 w h i l e s h u t t i n g o f f the oxygen c o m p l e t e l y , caused t h e emf t o increase slowly to 0.8015.  25  CONCLUSION  From the work o f T, C . A s s a l y and the w r i t e r  it  has been shown t h a t the u t i l i z a t i o n o f the f r e e energy o f s u l f u r d i o x i d e i n the form o f e l e c t r i c a l energy has  consider-  able p o s s i b i l i t i e s . The p r e s e n t work has been c o n c e n t r a t e d e n t i r e l y on the v o l t a g e d e v e l o p e d  i n a s u l f u r dioxide-oxygen c e l l but  in  f u t u r e the r a t e o f f o r m a t i o n o f s u l f u r i c a c i d i n such a c e l l s h o u l d be s t u d i e d .  I f t h i s r a t e c o u l d be made  t h e n t h i s c e l l would have d e f i n i t e  appreciable  p o s s i b i l i t i e s for  the  p r o d u c t i o n o f s u l f u r i c a c i d and e l e c t r i c a l energy on a commercial  scale.  26  REFERENCES..  1.  Chem. and M e t . E n g . , 15,677  (1916).  2.  Chem. and M e t . E n g . , 18,178  (1918).  3.  T . C . A s s a l y , The B e h a v i o u r o f S u l f u r D i o x i d e , Oxygen, S u l f u r i c A c i d and Water i n an E l e c t r o l y t i c Cell.  4.  T r a n s . Am. E l e c t r o c h e m . S o c . , 5 6 , 2 0 1 (1929.  5.  L e w i s and R a n d a l l , Thermodynamics, 554 ( 1 9 2 3 ) .  6.  P e r r y , Handbook o f Chem. E n g . E d . I I , ' 5 6 3  7.  P e r r y , Handbook o f Chem. E n g . E d . I I , 5 5 3 ,  8.  Z . P h y s i k . Chem., 68,693  9.  J . A . C h . S . , 47,945  (1925).  10.  J . A . C h . S . , 36,804  (1914).  11.  J . A . C h . S . , 40,393  (1918).  12.  F i n d l a y , P r a c t i c a l P h y s i c a l C h e m i s t r y , 152 ( 1 9 2 3 ) .  13.  Chem. News, 60 ( 6 8 ) .  14.  B u l l . S o c . C h i m . , 1913, 1 3 , 1 0 4 9 .  15.  Chem. Weekblad, 1 0 , 9 5 6 , A b s t . Am. Chem. S o c . , 1 9 1 4 , 2 , 1 9 2 6 .  (1941). (1941).  (1910).  27  BIBLIOGRAPHY.  1.  A b e g g s Handbuch d e r A n o r g a n i s c h e n Chemie, I V ,  2.  Adam, N . K . , P h y s i c s and C h e m i s t r y o f  3.  C r e i g h t o n and K o c h l e r , E l e c t r o c h e m i s t r y , E d . I I , V o l . 1 1 (Applications).  4.  F r e n c h , S . J . , and K a h l e n b e r g , L . , T r a n s , o f Am. E l e c t r o chem. S o c . , 5 4 , 1 6 3 - 1 9 9 ( 1 9 2 8 ) .  5.  Getman and D a n i e l s , O u t l i n e s o f P h y s i c a l C h e m i s t r y ,  1  (1927),  Surfaces,  Ed.VII. 6.  Glasstone, P h y s i c a l Chemistry.  7.  G r e g g , S. J . , A d s o r p t i o n o f Gases b y S o l i d s .  8.  J o n e s , H . C , H y d r a t e s i n Aqueous S o l u t i o n .  9.  Kreuger, A . C ,  and K a h l e n b e r g , L . , T r a n s . Am. E l e c t r o chem. S o c , 58,107-152  (1930).  10.  Latimer, Oxidation Potentials.  11.  L e w i s and R a n d a l l , Thermodynamics and the F r e e E n e r g y o f Chemical Substances.  12.  M c B a i n , S. W . , S o r p t i o n o f G a s e s .  13.  P e r r y , Handbook o f C h e m i c a l E n g i n e e r i n g , E d . I I .  14.  W y l d . W . , S u l f u r i c A c i d and S u l f u r D i o x i d e .