Open Collections

ft} THE SULFUR DIOXIDE, OXYGEN, SULFURIC ACID CELL A t h e s i s submit ted i n p a r t i a l f u l f i l l m e n t o f the requi rements f o r the Degree o f Mas ter o f A p p l i e d Sc ience i n the Department o f Chemis t ry 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 Columbia J u n e , 1946. ACKNOWLEDGEMENT I w i sh to acknowledge the h e l p f u l sugges t ions and a s s i s t a n c e o f D r . W.F. Seyer , under whose d i r e c t i o n the presen t work was c a r r i e d o u t . I would a l s o l i k e to thank D r . B . P . Su the r l and o f the C o n s o l i d a t e d M i n i n g and Sme l t i ng Company f o r s u p p l y i n g some much needed equipment which c o u l d no t be ob ta ined from the r e g u l a r sources . TABLE OF CONTENTS, Page INTRODUCTION 1 OUTLINE OF THE PROBLEM 2 HISTORICAL 3 GENERAL THEORY 6 Gas Electrodes • • • 6 Hydrates i n Aqueous Solution 7 PROBABLE MECHANISM OF CELL. REACTION 8 Anode Reactions 8 Cathode Reactions 8 Overall C e l l Reaction 9 DESIGN OF THE CELL 12 OPERATION OF THE CELL ' 14 Plat in iz ing the Electrodes 14 Degassing the Electrodes • • • 14 Changing the C e l l 14 RESULTS 15 Variat ion of C e l l Potential with Cone. . . . . 15 Effect of the Rate of Sulfur Dioxide into the C e l l on the E.M.F 24 CONCLUSION 25 REFERENCES 26 BIBLIOGRAPHY 27 LIST OF ILLUSTRATIONS Page 1. The V a r i a t i o n o f C e l l Vo l t age w i t h Time U s i n g 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 4 2 . T h e o r e t i c a l E . M . F . o f 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 V o l t a g e v s . 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 , . 17 5 . F r e e z i n g P o i n t Char t o f S u l f u r i c A c i d H y d r a t e s . 20 6 . The V a r i a t i o n o f C e l l Vo l t age w i t h Time 23 KEY TO SYMBOLS - f ree energy change, c a l o r i e s f - Faraday 96,500 coulombs M - mol f r a c t i o n N - n o r m a l i t y n - va lence T = temperature , degrees Cent ig rade t - t i m e , hours X = per cent a c i d 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 t ime "t" J = conve r s ion f a c t o r : 1 c a l . - 4 . 1 8 3 j o u l e s 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 resen t s tandard methods l e aves l i t t l e to be d e s i r e d , e s p e c i a l l y when e lementa l s u l f u r , s u l f i d e ores and some by-p roduc t gases are u sed . However, the u t i l i z a t i o n o f the f ree 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 f o r 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 o f f e r s i n t e r e s t i n g p o s s i b i l i t i e s . The c e l l r e a c t i o n o f such a process i s g i v e n b y : SO., +- $Q% + H t 0 - H 4 SO^ The f ree energy change i n v o l v e d i n t h i s r e a c t i o n i s made up o f the f ree energy o f fo rma t ion o f s u l f u r i c a c i d p l u s the f ree 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 . The A F change i n forming pure s u l f u r i c a c i d ' i s •49 ,100 c a l o r i e s per mole , and f o r a more d i l u t e a c i d i t 1 2 would be even g r e a t e r . Any r e a c t i o n which i s accompanied by a l a r g e nega t ive f ree energy change i s spontaneous, hence from the above f ree 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 to 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 ime draw o f f e l e c t r i c a l energy. By the 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 ach i eved . 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 uses such as p i c k l i n g s t e e l or l e a c h i n g copper o r o ther o r e s ; p r o d u c t i o n o f concent ra ted a c i d where economic ( rondi t ions war ran t , and abatement o f atmospheric 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 experiments i t was proposed to determine the e l ec t romo t ive fo rce o f a c e l l s i m i l a r to P t , SO,,: H^SO,,.: Oj, , P t a t v a r i o u s concen t r a t i ons o f s u l f u r i c a c i d . The above c e l l was s t u d i e d i n 1945 by T . C . A s s a l y , and from h i s work i t was shown t h a t four hydra tes o f s u l f u r i c a c i d were s t a b l e i n s o l u t i o n . The w r i t e r proposed to s tudy 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 to t e s t the v a l i d i t y o f A s s a l y ' s r e s u l t s . 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 below 50 per cent over the g r e a t e r p a r t o f the a c i d range, so a p r e l i m i n a r y s tudy was made to f i n d an e l e c t r o d e combinat ion which would increase 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 used f i r s t but were soon d i s c a r d e d s ince an ox ide f i l m formed immediate ly on bo th the anode and cathode p roduc ing a v e r y low v o l t a g e . Tes t s were then c a r r i e d out u s i n g tan ta lum meta l a t bo th e l e c t r o d e s and a cons tan t v o l t a g e was ob ta ined a f t e r about 24 hour s . The e f f i c i e n c y o f t h i s c e l l was below t h a t o f A s s a l y ' s bu t i t appeared t h a t a combinat ion o f p l a t i n u m and tan ta lum might g i v e the d e s i r e d r e s u l t s . A c e l l made up o f a p l a t i n u m anode and a tan ta lum cathode was then t e s t e d bu t t h i s s t i l l was not s a t i s f a c t o r y . The e l e c t r o d e combinat ion was then r e v e r s e d , and w i t h tan ta lum as anode and p l a t i n u m as cathode a g r ea t inc rease 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 these r e s u l t s g r a p h i c a l l y . As a r e s u l t o f these experiments i t was dec ided to determine the a c t u a l e l e c t r o m o t i v e fo rce o f the c e l l : T a , S 0 4 : H^SO^: 0,,, P t a t v a r i o u s concen t r a t i ons o f s u l f u r i c a c i d . \. 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 su l fu rous a c i d and they showed t h a t t h i s o x i d a t i o n t akes p l ace 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 s t rong s u l f u r i c a c i d s o l u t i o n s . A l s o f o r a g i v e n 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 decreases w i t h i n c r e a s i n g cu r r en t d e n s i t y . M e s s r s . M . De Kay Thompson and A . P . S u l l i v a n 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 decrease the p o l a r i z a t i o n a t the anode and thus decrease 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 by 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 experiments t h a t the p o l a r i z a t i o n was due to the accumula t ion o f f ree 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 to the ano ly te a r e a c t i o n occur red between t h i s gas and the f ree 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 fo r ce o f the c e l l : P t , S 0 l : H^SO A : 0 X , P t a t v a r i o u s concen t r a t i ons o f s u l f u r i c a c i d and a t d i f f e r e n t temperatures . 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 ve r sus 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 tep-wise curve was formed; each o f the four s teps cor responding to a hydra te o f s u l f u r i c a c i d . Other exper imenters have shown evidence f o r the ex i s t ence o f three o f these hydra tes by 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 hydra te s are s t ab l e a t 25 C as w e l l as a t t h e i r f r e e z i n g p o i n t s . The e l ec t romo t ive fo rce o f the above c e l l was shown to be dependent on the temperature o f the e l e c t r o l y t e . - 6 The v o l t a g e i nc rea sed s l i g h t l y w i t h a decrease i n temperature and approached a cons tant va lue a t low temperatures , 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 f i r s t -c l a s s conductor , i n which a gas has been absorbed. Recent i n v e s t i g a t o r s have shown t h a t the emf developed by 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 which the gas i s con ta ined as w e l l as o f the gas i t s e l f . I n o ther words, gas e l e c t r o d e s are r e a l l y gas -meta l e l e c t r o d e s s ince meta ls are commonly used to absorb these gases . The p o t e n t i a l s developed by such gas -meta l e l e c t r o d e s are s p e c i f i c f o r the p a r t i c u l a r meta l and gas , and are r e l a t e d to the absorb ing power o f the meta l f o r the gas . 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 the nature o f the e l e c t r o l y t e i n which the e l e c t r o d e s are immersed. I t has a lso been found t h a t there i s a somewhat d i f f e r e n t p o t e n t i a l developed when the e l e c t r o d e i s s t a t i o n a r y than when i t i s i n m o t i o n . T h i s l a t t e r e f f e c t may be shown by moving the e l e c t r o d e or by keep ing 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 . The e l e c t r o l y t e may be moved by s t i r r i n g , caus ing the e l e c t r o l y t e to f l o w pas t the e l e c t r o d e , o r by b u b b l i n g the gas through the e l e c t r o l y t e . I t has been observed 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 meta l , i s v e r y 1 s m a l l indeed . I n f a c t , the p ressure 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 i s b e i n g pumped out o f the s o l u t i o n l i t t l e e f f e c t i s observed on the p o t e n t i a l u n t i l the b u b b l i n g out o f the gas causes a s t i r r i n g e f f e c t . HYDRATES IN AQUEOUS SOLUTION The number o f molecules o f water i n combinat ion w i t h one molecule o f the d i s s o l v e d substance , f r e q u e n t l y i nc r ea se s from the most concent ra ted to the 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 . Wi th some substances the number o f molecules o f water h e l d i n combinat ion by one molecule o f the d i s s o l v e d substance may pass through a w e l l - d e f i n e d maximum as the d i l u t i o n i s i n c r e a s e d . I n o ther cases , the number o f molecules o f water h e l d i n combinat ion by one molecule o f the d i s s o l v e d substance may reach a maximum v a l u e as the d i l u t i o n i s i n c r e a s e d ; t h i s maximum va lue may then remain p r a c t i c a l l y cons tant w i t h f u r t h e r i nc reases i n the d i l u t i o n . The q u e s t i o n a r i s e s whether these hydra tes are t rue c h e m i c a l compounds o r whether they r ep resen t some l e s s s t a b l e form o f combina t ion . That they are uns t ab l e i s shown by the ease w i t h which they are broken down by h e a t . Most o f the water 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 hydra tes a r e , t hen , decomposed i n s o l u t i o n a t a compara t ive ly low temperature and the water i s g i v e n o f f i n the form o f vapour . I n the l i g h t o f these f a c t s the hydra tes can s c a r c e l y be regarded as t r ue chemica l compounds. I f however, we i n s i s t on c a l l i n g them chemica l compounds, we must admit t h a t they r ep resen t a v e r y low order o f s t a b i l i t y . I t i s the gene ra l o p i n i o n t h a t bo th molecu les and ions combine w i t h wate r , forming h y d r a t e s . I t seems t h a t the molecu les are c e r t a i n l y capable o f fo rming h y d r a t e s , because i n v e r y concent ra ted 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 en have cons ide rab le h y d r a t i o n . That ions are capable o f combining w i t h water 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 s o l u t i o n s , where c h i e f l y i ons and o n l y a few molecules are p r e sen 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 s u l f u r i c a c i d e l e c t r o l y t e and then adsorbs on the sur face o f the e l e c t r o d e : SO t (gas ) = SO^(so lu t i on ) (1) The s u l f u r d i o x i d e then combines w i t h water as i n equa t ion (2) ' S O z + H J L 0= S0~ + 2 H % ^ ^ (2) CATHODE 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 e l e c t r o l y t e and then adsorbed on the surface o f the e l e c t r o d e . • 0 4 ( g a s ) '=:" O^Csolu t ion) (3) The oxygen then combines w i t h water as i n equa t ion £ O F C * H .0 +• 2 ( - ) - 20H (4) OVERALL CELL REACTION The o v e r a l l c e l l r e a c t i o n i s g i v e n by : S 0 2 + £ O i mH^O - Hj,S0 4 (N = X) (5) The 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 such: SO 2. +• J 0 a +- H t O = H^SO^. (6) 2 H % SO^ mHaO - H^SO^. (N - X ) (7) SO z +• io^ + mHtO = H z SO^ ( N =• X) (8) A d d i t i o n o f equat ions (6) and (7) g i v e s the o v e r a l l c e l l r e a c t i o n . Hence a d d i t i o n o f the f ree energy o f r e a c t i o n s o f equat ions (6) and (7) w i l l g i v e the o v e r a l l f ree 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 equa t ion (6) i s ob ta ined from the d i f f e r e n c e o f the f ree energy o f fo rma t ion o f s u l f u r i c a c i d and the sum o f the f ree energ ies o f fo rmat ion o f s u l f u r . d i o x i d e , water and oxygen. A is* H z s o + - ~ 176,000 c a l o r i e s ^is" \s.o£, - - 71,740 c a l o r i e s ^ F * * ° u » o - - 56,690 c a l o r i e s O j . - zero Therefore A F change = - 49,100 c a l o r i e s . The number o f accura te measurements from which the f ree 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 c a l c u l a t e d i s l i m i t e d . Tables are a v a i l a b l e on the f ree energy o f d i l u t i o n o f the 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 Brons ted s t u d i e d the f ree energy o f d i l u t i o n over a r a t h e r wide range o f concen t r a t i ons o f s u l f u r i c a c i d bu t a t • 9 temperatures r a n g i n g o n l y up to 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 ree energy o f t h i s a c i d , bu t o n l y over a s m a l l range o f c o n c e n t r a t i o n . A s a t i s f a c t o r y t a b l e o f the f ree energy o f d i l u t i o n a t 25°C f o r concen t r a t i ons up to 0 .2 mol f r a c t i o n s u l f u r i c a c i d has been worked out by R a n d a l l and Cushman.* T h e i r r e s u l t s which are the f ree energ ies * F , o f the r e a c t i o n : 2H + + S 0 ^ + mH z0 = R \ S 0 4 (N-X) (7) are g i v e n i n the t h i r d column o f Table I , and the cor respond-i n g v a l u e s o f mol f r a c t i o n M and per cent a c i d X i n the f i r s t and second columns r e s p e c t i v e l y . TABLE I M X(%) A F . Ccal.") A F , (k c a l . ) E ( v o l t s ) 1.00 100.00 0 - 4 9 . 1 1.065 .20 57.70 5270 - 4 3 . 8 .950 .13 44 .90 2915 - 4 6 . 2 1.000 .10 37.70 1645 - 4 7 . 5 1.030 .08 31.60 702 - 4 8 . 4 1.048 .065 27.55 -85 - 4 9 . 2 1.067 .05 22.28 -865 - 5 0 . 0 1.084 .03 14.42 -2048 - - 5 1 . 1 1.108 .02 10.00 -2735 -51 .8 1.122 .01 5.21 -3702 - 5 2 . 8 1.143 .002 1.08 -5613 - 5 4 . 7 1.185 .0009 .487 -6547 - 5 5 . 6 1.204 A F A r ep re sen t s the o v e r a l l c e l l r e a c t i o n £x Fx r ep re sen t s the f ree 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 the fo rmu la , E - - & F ) ( J ) a c a l o r i e s n f J = 4 .183 j o u l e s / c a l o r i e © assuming a temperature o f 25 C , the t h e o r e t i c a l v a l u e s o f the emf t h a t would be ob ta ined i n a comple t e ly r e v e r s i b l e c e l l are o b t a i n e d . The v a l u e s are g i v e n i n column 5 o f Table I . I n F i g , 2 the v a l u e s o f the emf g i v e n i n Table 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 curve was drawn from 0 .2 M to 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 ree 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 ske tch 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 toppered 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 these v e s s e l s had a g l a s s tube ex tend ing from i t , about two inches above the f r i t t e d d i s c . These v e s s e l s were connected by means o f a rubber tube c o n t a i n i n g a c l a y diaphragm. E l e c t r i c a l con tac t was made by two mercury f i l l e d g l a s s l e a d s . One end o f each l e a d was sea led to the 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 an ta lum and the cathode a p l a t i n i z e d p l a t i n u m gauze. The p o t e n t i a l was measured by a po ten t iome te r . The whole c e l l was s e t i n a cons tant temperature o ba th which main ta ined the temperature w i t h i n 0 .05 C o f the d e s i r e d v a l u e . 13 OPERATION OF THE CELL 14 PLATINIZING THE ELECTRODES The p l a t i n u m e l e c t r o d e s were coated w i t h a l a y e r o f p l a t i n u m "black depos i t ed e l e c t r o l y t i c a l l y from a three 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 leaned i n warm chromic a c i d and then lowered 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 used and a commutator a l lowed the cu r r en t to be r e v e r s e d a t d e s i r e d i n t e r v a l s . 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 to produce a moderate e v o l u t i o n o f gas . The d i r e c t i o n o f the c u r r e n t was r eve r sed every 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 surface o f the e l e c t r o d e s . DEGASSING THE ELECTRODES A f t e r each r u n the 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 wa te r , and degassed by 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 approx imate ly one hour . 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 every t e n minutes f o r the f i r s t f i f t y minutes , then every minute f o r the l a s t t e n minutes . The p l a t i n u m e l e c t r o d e s were then 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 minutes forremove any p o i s o n i n g agents . F i n a l l y a l l the 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 15 connec t ions to the c e l l were opened and the two h a l f - c e l l s were removed from the cons tant temperature b a t h . A f t e r f l u s h i n g w i t h warm d i s t i l l e d wa te r , the h a l f - c e l l s were d r i e d i n a warm oven then 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 then se t up aga in and connec t ions made to the s u l f u r d i o x i d e and oxygen t a n k s . S e v e n t y - f i v e c u b i c cen t ime te r s o f t e s t s u l f u r i c a c i d was then added to 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 h a l f - c e l l s . The s u l f u r d i o x i d e and oxygen were ob t a ined from pressure t a n k s . 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 f o r each r u n was ob ta ined by d i l u t i n g C P . a c i d w i t h d i s t i l l e d wate r . The exac t s t r e n g t h o f the a c i d was determined by t i t r a t i n g a g a i n s t a s tandard 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? mgrfT.- POTENTIAL WITH CONCENTRATION 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 these are shown i n Table I I . The f i r s t and second columns g i v e the n o r m a l i t y N and the per cent a c i d X and the 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 the emf g i v e n i n Table I I are the v a l u e s ob ta ined a f t e r the s u l f u r d i o x i d e and oxygen had bubbled through the c e l l f o r n e a r l y 24 hours and the c e l l p o t e n t i a l approached a cons tan t v a l u e . 16 TABLE I I I X 1 I E ( v o i t s ) 0 .98 4 . 6 9 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 Table I I are p l o t t e d 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 the a c i d as a b s c i s s a . The exper imen ta l r e s u l t s f a l l on a s t ep -wise curve w i t h each o f the four s teps cor respond ing to a hydra te o f s u l f u r i c a c i d . The f i r s t s tep a t 30 .9 N corresponds to H 2S0^- H a 0 ; the second a t 24.4 N to H a S 0 ^ 2 H t 0 ; the t h i r d a t • 18 17 .1 N to H 4 S 0 4 - 4 H Z 0 ; the f o u r t h a t 5 .01 If to H 2 S 0 4 - 20 R \ 0 . I t should no t be a t a l l s u r p r i s i n g t h a t hydra tes 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 . Hydra tes o f s u l f u r i c a c i d are known to e x i s t i n s o l u t i o n . S ince the fo rmat ion o f a hydra te 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 cons ide rab le f ree energy change, i t should 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 c e l l . The p o t e n t i a l should v a r y w i t h the nature o f the hydra te formed, hence i t should be p o s s i b l e to d e t e c t the presence o f hydra tes 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 us cons ide r the curve shown i n F i g . 4 . We w i l l f i r s t cons ide r the c e l l v o l t a g e a t a v e r y low a c i d concen t r a -t i o n . 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 inc rease 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 ree energy change caus ing the p o t e n t i a l to d rop . The drop i s f a i r l y cons tan t up to a c o n c e n t r a t i o n o f -5 N and over t h i s range we have the hydra te H z SO^ 2 0 H a 0 . Beyond t h i s c o n c e n t r a t i o n we do no t have enough water p resen t to form t h i s hydra te so we have another hydra te fo rming , namely ^ 3 0 ^ - 4 ^ 0 . From 5 N to 17 .1 N the s lope o f our curve i s cons tant and over t h i s range we have the hydra te H^S0^4H^0. Beyond 17 .1 N we do no t have enough water p resen t to form t h i s hydra te so t h a t we have a change over t o H t S O ^ 2 H i 0 accompanied by a marked change i n p o t e n t i a l . The o the r s teps can be e x p l a i n e d s i m i l a r l y . The p o r t i o n o f the t an t a lum-p la t i num curve A-B shown i n F i g . 4 was p l o t t e d by measuring the c e l l p o t e n t i a l s t a r t i n g 19 a t a low c o n c e n t r a t i o n o f a c i d and work ing up to a c o n c e n t r a t i o n o f 17 IT. The o ther end o f the curve was p l o t t e d by measuring 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 then work ing back to 18.3 N . T h i s gave the curve C - D - E . An apparent d i s c r epancy i n the curve i s shown do t t ed between D and E . To check t h i s p a r t o f the curve t e s t s were c a r r i e d out w i t h 18 .3 N , 22 .1 N and 24 .2 N a c i d . A f t e r 20 h o u r s , v a l u e s were ob ta ined 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 bu t a f t e r another 12 hours these approached the s o l i d p o r t i o n o f the curve D-E and remained cons tan t f o r over 5 h o u r s . The curve ob ta ined 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 ob ta ined by the w r i t e r . The e f f i c i e n c y o f the t an ta lumrp la t inum c e l l i s c l e a r l y shown to be much g rea t e r than t h a t o f A s s a l y 1 s over most o f the a c i d range . The g rea 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 robab ly due t o the t an ta lum 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 exper imenters have shown evidence o f th ree hydra t e s o f s u l f u r i c a c i d by f r e e z i n g methods. P i c k e r i n g ob t a ined the hydra te H a S O ^ ^ O ; Giron^found the hydra te HgSO^SHaO; and Donk ob ta ined 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 f e S O v R^O. 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 hydra tes i s shown i n F i g . 5 . B , D and G are the eryohydrate p o i n t s and C , E and H are the m e l t i n g p o i n t s o f the hydra t e s R \ S 0 4 . 45^0, Hj,S0 + . 2 H t 0 and R\S0,_- H x 0 r e s p e c t i v e l y . From B to C , D to E 10 o 10 20 3o J O to 70 8° 9o Fig.5 21 and G to H these r e s p e c t i v e hydra tes s o l i d i f y o u t . The ex i s t ence o f H^SO^- 20H t 0 i s no t i n d i c a t e d i n the graph bu t would p robab ly be de tec t ed between A and B on c l o s e r examina t ion . Measurements were made o f emf w i t h t ime f o r s e v e r a l concen t r a t i ons o f a c i d . , The measurements f o r th ree concen-t r a t i o n s are g i v e n i n Table I I I . The f i r s t column g i v e s the t i m e , the second, 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 . T ime-vol tage curves f o r these a c i d concen t r a t i ons are shown i n F i g . 6. The i n i t i a l v o l t a g e i n a l l cases was v e r y h i g h , d ropp ing o f f to a cons tant va lue a f t e r about 24 h o u r s . 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 tan ta lum meta 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 bu t as t ime goes on an ox ide f i l m forms on the tan ta lum and an e q u i l i b r i u m i s se t up between the tan ta lum ox ide and the s u l f u r d i o x i d e i i i s o l u t i o n . The i n i t i a l drop i n v o l t a g e occu r s w h i l e the ox ide f i l m i s forming then the v o l t a g e g r a d u a l l y approaches a cons tant v a l u e . 22 TABLE III t E(0.98N) EC27.2N) E(35.6N) u r s mins . v o l t s v o l t s v o l t s 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 1 0 00 0.8308 0.7185 0.4710 15 00 0.8270 0.6978 0.4680 20 00 0.8260 0.6750 . 0.4637 23 30 0.8250 0.6740 0.4635 25 30 0.8249 0.6738 0.4634 24 EFFECT OF THE RATE OF SULFUR DIOXIDE AND OXYGEN INTO THE CELL ON THE EMF I n the p resen t work a r a t e o f f l o w o f 60 bubbles pe r minute was used f o r bo th the s u l f u r d i o x i d e and oxygen. Al though 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 served i t s purpose s ince 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 ho t r a t e s o f fo rmat ion o f a c i d . On i n c r e a s i n g the 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 the c e l l from 60 bubbles per minute to 120 bubbles per minute the v o l t a g e inc reased from 0.8000 to 0.8007 a f t e r s e v e r a l hou r s . T h i s va lue r e tu rned to 0.8002 on dec rea s ing the r a t e back to 60 bubbles per minu te . Decreas ing the r a t e to 30 bubbles per minute cu t the emf to 0.7991 which r e t u r n e d to 0.8001 on i n c r e a s i n g the r a t e to 60 bubbles per minute . On i n c r e a s i n g the r a t e o f f l o w o f oxygen i n t o the c e l l from 60 bubbles per minute to 120 bubbles per minute the v o l t a g e decreased from 0.8000 to 0 .7990. On dec rea s ing the r a t e to 60 bubbles per minute the emf went up to 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 the emf to inc rease s l o w l y 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 i t has been shown t h a t the u t i l i z a t i o n o f the f ree 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 c o n s i d e r -ab le p o s s i b i l i t i e s . The present work has been concent ra ted e n t i r e l y on the v o l t a g e developed i n a s u l f u r d i o x i d e - o x y g e n c e l l but i n fu tu re the r a t e o f fo rmat ion o f s u l f u r i c a c i d i n such a c e l l should 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 app rec i ab l e then t h i s c e l l would have d e f i n i t e p o s s i b i l i t i e s f o r 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 s c a l e . 26 REFERENCES.. 1. Chem. and Met . E n g . , 15,677 (1916) . 2 . Chem. and Met . E n g . , 18,178 (1918) . 3 . T . C . A s s a l y , The Behaviour 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 C e l l . 4 . Trans . Am. E lec t rochem. S o c . , 56,201 (1929. 5 . Lewis and R a n d a l l , Thermodynamics, 554 (1923) . 6. P e r r y , Handbook o f Chem. Eng . E d . I I , ' 5 6 3 (1941) . 7 . P e r r y , Handbook o f Chem. Eng . E d . I I , 553, (1941) . 8 . Z . P h y s i k . Chem., 68,693 (1910) . 9 . J . A . C h . S . , 47,945 (1925) . 10 . J . A . C h . S . , 36,804 (1914) . 1 1 . J . A . C h . S . , 40,393 (1918) . 1 2 . F i n d l a y , P r a c t i c a l P h y s i c a l Chemis t ry , 152 (1923) . 13 . Chem. News, 60 ( 6 8 ) . 14 . B u l l . Soc . C h i m . , 1913, 13,1049. 1 5 . Chem. Weekblad, 10 ,956 , A b s t . Am. Chem. S o c . , 1 9 1 4 , 2 , 1 9 2 6 . 27 BIBLIOGRAPHY. 1. Abegg 1 s Handbuch der Anorganischen Chemie, I V , (1927) , 2 . Adam, N . K . , P h y s i c s and Chemis t ry o f Su r f ace s , 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 ( A p p l i c a t i o n s ) . 4 . F r e n c h , S. J . , and Kah lenbe rg , L . , Trans , o f Am. E l e c t r o -chem. Soc . ,54 ,163-199 (1928) . 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 Chemis t ry , E d . V I I . 6 . G l a s s t o n e , P h y s i c a l Chemis t ry . 7 . Gregg, S. J . , A d s o r p t i o n o f Gases by S o l i d s . 8. J o n e s , H . C , Hydra tes i n Aqueous S o l u t i o n . 9 . Kreuger , A . C , and Kah lenberg , L . , T rans . Am. E l e c t r o -chem. S o c , 58,107-152 (1930) . 10 . L a t i m e r , O x i d a t i o n P o t e n t i a l s . 1 1 . L e w i s and R a n d a l l , Thermodynamics and the Free Energy o f Chemica l Substances . 12 . M c B a i n , S. W. , S o r p t i o n o f Gases . 1 3 . P e r r y , Handbook o f Chemica l E n g i n e e r i n g , E d . I I . 14 . Wyld . 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 .