THE ISTERACTION OF ZINC AND SULPHURIC ACID. 47 111.-CONTRIBUTIONS FROM THE LABORATORY OF GONVILLE AND CAIUS COLL'EGE CAMBRIDGE. No. X.-The hteraction of Zinc and Rulphuric Acid. By M. M. PATTISON MUIR M.A. and R. H. ADIE B.A. Scholar of Trinity College Cambridge. 1. I N 1830 De la Rive examined the action which occurs between zinc and aqueous solutions of sulphuric acid (Pogg. Ann. 19 221). He employed both commercial and redistilled zinc and measured the times required for t h e production of equal volumes of hydrogen from acids of different concentrations. De la Rive found that the change proceeded much more rapidly with commercial than with redistille 48 MUIR AKD ADIE THE INTERLICTION zinc ; he also found that the most rapid action occurred when coin-mercial zinc interacted with an acid solution containing about 30 per cent.H,SO,. Ue la Rive added known quantities of tin leatl, copper and iron respectively to molten redistilled zinc and found that the rates of action of the products with sulphuric acid solutions were greater than the rate of action of reclistilled zinc. Finally, De la Rive proved that the solution of sulphuric acid which showed maximum electrical conductivity interacted most rapidly with com-mercial zinc and with redistilled zinc to which a few per cent. of iron had been added. De la Rive concluded that the interaction between pure zinc and dilute sulphuric acid is probably a direct chemical change but that with ordinary comniercial zinc the action is chiefly electrochemical. In the seventh series of his Experrimei7tal Researches (pars.863 et seq.) Faraday proved that a plate of amalgamated zinc remained unchanged in sulphuric acid to which 30 parts of water had been added while a similar plate in the same acid put in contact wit11 a plate of platinum lost 5.2 per cent. of its weight. These experiments conducted by De la Rive and Faraday showed that the interaction which occurs between zinc and dilute sulphuric acid is to some extent a t least an electrolytic action. 2. It is well known that hydrogen is not the only gaseous pro-duct of the interaction of dilute sulphuric acid and ordinary zinc. Everyday experience in the laboratory proves that both sulphur dioxide and sulphuretted hydrogen are often produced in the reaction in question. We have made a fairly complete qualitative examination of this interaction and have found it to be one of great complexity.About 10 grams of zinc were used in each experiment and usually about 50 C.C. of the acid. The reactions proceeded in small flasks, each of which was connected by a T-tube with a flask containing a dilute aqueous solution of iodic acid mixed with a very little starch-paste and also with a flask containing a dilute solution of silver nitrate to which excess of amEonia had been added. Exit-tubes from these flasks communicated with a water-pump ; a current of air could thus be sucked through the whole apparatus and traces of gases pro-duced in the interactions could be swept out from the reaction-flask into the testing-flasks. Those experiments wherein the reaction was allowed to proceed for long periods sometimes extending to three weeks were conducted in small flasks which were not connected with testing-flasks ; a t the end of the allotted time a rapid current of well-washed carbon dioxide was swept through the liquids in the flasks and the escaping gas was bubbled through the testiiig solutions.The higher temperatures were maintained by heating the flasks i OF zrsc AND SULPHURIC ACID. 49 baths of zinc chloride for our purposes it was unnecessary to attempt to regulate the temperatures within a few degrees. The specimens of zinc employed were (1) “ redistilled zinc,” (2) “pure rod zinc,” (3) “ pure zinc foil,” (4) “ commercial granulated zinc,” the chief im-purities in which were lead iron arsenic and traces of cadmium, (5) “ platinised zinc fd,” and (6) “ approximately pure zinc,” prepared by us by dissolving ‘‘ pure redistilled zinc ” in pure very dilute sul-phuric acid saturating with sulphuretted hydrogen boiling filtering, electrolysing (after complete removal of H2S) digesting the zinc thus obtained with a slightly warm aqueous solution of recrystallised “ pure zinc sulphatle,” redissolving i n pure dilute sulphuric acid, electrolysing &c.and repeating the series of operations a third time. The snlphuric acid employed was that sold as “ pure redistilled,” diluted with measured quantities of water. What is hereafter called “ approximately pure H,SO ’’ was prepared by redistilling the “ pure redistilled ” acid collecting the middle portion of the distillate, repeating this process then freezing out the H2S04 and pouring off the remaining liquid.The concentrations of the various acids used are represented by formulq but these are to be regarded merely as approximate represen-tations of the ratio of H,S04 to H20 in the liquids. The concentration varied from H,SO4 to H2SO4,100H2O. No attempts were made to measure the quantities of the different products of the reactions. When a blue colour in the iodic acid and starch test-liquid or a brown colour in the silver nitrate test-liquid, appeared only after the current of air o r carbon dioxide had swept through the apparatus for 30 minutes we note the result as “ a trace” of SOz or SH, as the case may be. Production of a deep-blue coloiir o r a brown-black precipiLate immediately the first bubbles of gas came into contact with the testing liquid is noted as ‘‘ abundance ” of SO (or SH,).To save space and frequent repeti-tion we have adopted the following letters for indicating (very roughly it is to be remembered) the results of the experiments :-t. 1. m. a. Trace. A little. Moderate quantity. Abundance. Statements are made in the sequel regarding the appearance of hydrogen when the acid used was concentrated and the changes proceeded very slowly the presence of hydrogen among the products was proved by continualIy passing a rapid current of carbon dioxide through the reaction-flask and allowing the escaping gas to bubble through potash solution into a tube filled with the same solution, and testing the gas which collected in this tube.Statements are also made in the sequel regarding the solid matter VOL. LIII. 50 MUIR AND ADIE THE INTERACTION formed in the reaction-flasks this solid was proved to be free from zinc sulphide by collecting it on glass-wool filters washing with sulphuric acid thoroughly drying on a porous tile and then decom-posing it by fairly concentrated warm sulphuric acid in a flask con-nected with testing-flasks as already described. Direct experiments proved that zinc sulphide is decomposed by moderately dilute warm sulphuric acid with formation of sulphur dioxide sulphuretted hydrogen and sulphur ; the absence of these three substances was taken as evidence that the solid examined was free from zinc sul-phide. Traces of sulphur were detected by warming the solid pro-ducts of the reaction with carbon bisulphide decanting and evaporating Ihe bisulphide.The experiments which were performed in duplicate were always stopped when some zinc remained undissolved. The following formulse approximately represent the concentrations of the acids used ; each acid is distinguished by a letter or number. Concentration of acid. . HzSOI 9H2SO4,HZO 7H,SOI,3H,0 Concentration of acid. . 7HzS04,4H20 7H2S04,6Hz0. Letter AA. A. B. Letter C. D. Number 1. 2. 3. 4. Concen. of } H2S04,H20 H,S04,BH20 HZS04,3Hz0 H z S ~ ~ ~ H Z ~ ; acid and so on the number ?L representing an acid of concentration The following letters indicate roughly the quantity of the gas produced t trace; 7 a little; m moderate; a abundance.The lines - - - indicate the absence of SO, SH, or S; these three bodies are alwa-j+s enumerated in this order; thus t - - means ‘( a trace of sulphur dioxide sulphuretted hydrogen and sulphur absent.” 3. The following tables embody the greater number of our results :-H,SO4,nnH?O OF ZINC AND SULPHURIC ACID. Temperature . . . . . . OD. 1 20". 51 looo. 160'. SOZ SH, S. _ 1 -1 I -1 1 -1 1 -m I ~ L -m tn -I ?n -t l i b - - I -t 1 -2 1 -Concentr. of acid. B B SO? SH? S. t - -1 1 -1 1 -1 1 -- t -111 111 -a n -C D 1 2 Temperature . . . . . . . . . . . I OO. -___ ----4 6 7 8 lloo. 1 160'. I- -9 10 11 12 13 16 25 BO 100 SO, SH, S. a t t 1 I t m I t r)L - -- _ -- f -- - I n -1 a -t a -SO, SH, 8.a I t f a -I t -Pyoducts obtained by using Redistilled Granulated Z i n c and Acids gf zarious concentyations. Concentr. of Acid. A 1 2 3 4 5 7 9 11 12 13 16 -"j 50 SO, SH S. ' SO, SH3 S. 1 1 -t I -t t -802 SH, S. a t t a a Products obtained by using Ylatinised Z i n c F o i l and Acids of carious Temperature . . . . . . . . . . . 0". 20". Concentr. of acid. SO, SH, S. SO, SH, 8. A m - -B C 1 t(?) t(?) -2 4 5 7 9 t 1 -11 t l -13 - 1 -16 t t -3 I _ - -- - - - - -t - - t(?) - -100". 150". SO, BH2 S. SO, SH2, m - -l t t m - - a m - - 1 I l - m t(?) - -- - -t - 25 -50 - - c 100 --Pyoducts obtained by using " Approximately P u r e " Z i n c and Acids Temperature .. . . . . . . 15-18". Concentr. of acid. A B 2 12 110-112'. 1 135'. 1 145". OF ZINO AND SULPHURIU ACID. 53 Temperature . . . . . . . . . . . 110". Products obtained by using Pure Rod Zinc and Acids of various con-centrations. 160". 13. p. of acid. SO, SH, S. a t m a a a a a a Concentr. of acid. A B C D 1 2 3 4 8 12* SO, SH, S. I t -S02 SHS S. loo". j 160". t t -- 1 -B. p. of acid. * Acid of this strength gives only hydrogen at 85". Products obtained by using Pure Zinc T o i l and Acids of various concen-trations. Temperature . . . . . . I 20". Concent,r. of acid. A B 1 2 3 4 5 1 - - I 1 1 m t 1 1 -4. We think it may be safely asserted that hydrogen was produced in all the reactions.Direct proof of the presence of hydrogen was sought for and found in the cases of the acids HzS04,Hz0 at the ordinary temperature 9H2S0,,H20 at looo 7H2S04,ZH20 at loo", and platinised zinc with 7H,S04,2Hz0 at 150-BOO at which temperature sulphur was freely produced. The only solid product of the interaction is zinc sulphate. This was proved for acid of very varying concentrations and at different temperatures ; for 9HZSO4,I-I20 at 180" ?H2SO4,2HzO at loo", and H2S04,3H20 HzS04,4H20 and H2S04,BHz0 at ordinary tem-peratures. 5. The results of our experiments when regarded broadly esta-blish a similarity between the interaction of sulphuric acid with approximately pure zinc and with commercial zinc. But at the sam 54 MUIR AND ADIE THE INTERACTION time there are differences.Thus the results show that as the zinc becomes purer the quantities of sulphur dioxide and sulphuretted' hydrogen particularly the latter produced a t the ordinary temperature (12-18") become markedly less whether the acid be concentrated or dilute ; when the acid is so dilute as H2S04,lO or 12H20 hydrogen is almost the sole gaseous product even a t temperatures up to the boiling point of the acid used. When approximately pure zinc is employed and the temperature is high (160" o r so) dilution of the acid is accom-panied by diminution both of sulphur dioxide and sulphuretted hydrogen; bnt when commercial zinc is used the sulphur dioxide diminishes more than the sulphuretted hydrogen ; when commercial zinc was used however small quantities of sulphuretted hydrogen were indeed obtained with every acid examined even with HzS04,100H20, and a t temperatures varying from the boiling point of the acid to 0".Platinised zinc foil behaves on the whole similarly to commercial zinc ; sulphur dioxide and sulphuretted hydrogen are produced a t very varying temperatures and with almost every acid examined. When the acid is fairly concentrated sulphur dioxide begins to be produced a t a temperature lower than that a t which sulphuretted hydrogen appears both in the case of platinised foil and commercial zinc ; when the acids are dilute the tendency in the case of platinised zinc foil is to produce sulphuretted hydrogen more freely than sulphur dioxide. It is somewhat remarkable that scarcely any sulphur dioxide or sulphuretted hydrogen should be produced when comniercial zinc and acid approximately of the concentration HzSO4,2H20 interact a t lo@" but that at 165" abundance of sulphuretted hydrogen accom-panied by a little sulphur dioxide should be formed and that a t 180" torrents of the former gas nearly free from sulphur dioxide should be evolved.I n connection with this result it is of interest to note that " approximately pure " zinc interacting with acid of the same con-centration as the above (H2S04,2H20) a t 160" produces quantities of both the gases i n question. The conditions under which sulphur appeared are noted in the table on In the experiments with platinised foil sulphur disappeared again a t 210-2220" and SHz increased very much.Sulphur did not appeal- in any other experiment with " approxi-mately pure zinc;" the other acids examined were HzSO1 and Sulphur is produced neither at low temperatures nor with acids less concentrated than HzS04,2H20 ; the formation of sulphur is always accompanied by the evolution of sulphur dioxide and sulphuretted hydrogen but the quantity of the latter gas is sometimes extremely 6. The production of sulphur requires a little examination. p. 55. HZS 04,2H20 OF ZINC AND SULPHURIC ACID. 55 , , , . . . . . . . . . . . . , . Concentration of acid. Kind of zinc. gHySO,,H,O 7H,SO4,2H,O 7H2S04,4H,0 7H,S04,6H,0 H,SO,,2II,O 0 zinc" 9 1 7, Y, " Platinised foil " 7 . . . . . . 9 , 7, > 9 . . . .. '' Approximately pwe zinc " 7H,S04,2H20 7 II,S04,6H20 H,S04,H20 7H2S G4,2H,O 7H,S04,4H,0 7H2S 0,,6H20 H2S0,,2H,0 H,SO4,HzO 7HzS 04,2H30 Temp. 165" 165 165 165 165 165 190 165 100 100 100 175-185 175-180 125 170 130 160^ Bemarks. Very little S ; plenty of SO ; Decided S ; plenty of SO,; Decided S; plenty of SO, Decided S ; much SO ; little Little S ; much SO and merest trace of SH,. merest trace of SH,. and SH,. SH,. SH2. Trace of S ; much SO and SH,. Distinct S ; fair quantities Little 8 ; much SO,; little Little S; much SO,; very Little S; little SO2 and SH-. Trace of S; fair quantity of Much S; very inucli SO,; Much S; much SO2; little of SO and SH,. SH2. little SH,. SO,; little SH,.little SH,. SH,. 7 J , 3 3 , Trace of S ; much SO,; fair quantity of SH,. Sinall quantity of S; quan-tity of SO,; trace of SH,. small. Wben " approximately pure " zinc aiicl sulphuric acid are used there is scarcely any separation of sulphur ; with platinised foil, on the other hand sulphur is formed in large quant,ities. The sulphur can scarcely be a product of the interaction of sulphur dioxide and sulphuretted hydrogen else we should expect to find it gencrally accompanying these gases; nor can i t always be produced by the reducing action of the nascent hydrogen on either the sulphur dioxide or sulphuretted hydrogen because these gases are so frequently found at moderate temperatures unaccompanied by sulphur. It may be that the formation of sulphur is sometimes to be traced to the mutual action of sulphuretted hydrogen and hot concentrated sulphuric acid.This supposition would account for the preponderance of sulphur dioxide over sulphuretted hydrogen when the acid is concentrated, and for the increase in the quantity of the latter gas as the acid becomes more dilute for the sulphuretted hydrogen supposed to be formed when the acid is concentrated would be decomposed b 56 MUIK AND ADIE THE INTERACTION reacting with the hot acid but would escape unchanged when the acid becomes more dilute. The fact that the cessation in the produc-tion of sulphur is accompanied by a marked increase in the quantity of sulphuretted hydrogen in the reaction of platinised foil with con-centrated acids is also in keeping wit,h this supposition.The non-production of sulphur when “ approximately pure ” zinc is used supports the view that some of the snlphur is formed in secondary reactions between the gaseous products or between these and the acid, rather than in the direct interaction of the metal and acid. The sulphur produced was soluble in carbon bisulphide. 7. Increase of the mass of the zinc relatively t o that of the acid does not produce any regular and definite effect on the nature or quantities of the gaseous products of the change. This is shown by the tabu-lated results given on p. 57. 8. The interaction between zinc and sulphuric acid evidently differs considerably from the interaction between copper and sulphuric acid (see Pickering Trans. 1878 112).The latter change proceeds in two definitely marked stages expressible by the equations-(1.) 5cu + 4HaSO4 = Cu,s + 3CnSO4 + 4H,O (2.) CU + 2HZS04 = CUSO~ + SO + 2HzO. Hydrogen is not obtained in the reaction between copper and sulphuric acid nor is sulphuretted hydrogen evolved ; the sulphur dioxide produced is most probably the result of secondary reactions occurring between the acid and the cuprous sulphide formed. In the interaction between zinc and sulphuric acid on the other hand, hydrogen appears to be always produced ; both sulphuretted hydrogen and sulphur dioxide are very frequently formed and the production of these gases cannot be traced t o a secondary reaction between the acid and zinc sulphide formed in the primary change ; the entire interaction is more irregular than that between copper and sulphuric acid.The dependence of the production of large quantities of sulphur dioxide and sulphuretted hydrogen in the interaction between zinc and sulphuric acid on the temperature and the concentration of the acid indicates we think that the formation of these gases is not t o be altogether ascribed t o the reducing action of nascent hydrogen on the acid. This view is confirmed by the fact that variations in the mass of the zinc employed and hence in the quantity of hydrogen produced in a specified time do not exert any marked or regular effect on the nature of the change. The behaviour of commercial zinc with acid of the concentration HzSOa,2Hz0 and of approximately pure zinc with the same acid also confirms this supposition in the case of com-mercial zinc very little sulphur dioxide or sulphuretted hydrogen i ON' ZINC AND SULPHURIC ACID.57 Vtwious Z i n c s w i t h Acids of d i f e r e n t concentrations; 50 C.C. acid used in each case. ~~ Acid. I. Pure rod zinc-7H&304,2Hz0 H2SO4,2H,O 11. Pwre zinc fod-7H,S04,2H,O 7H$404,6H,O H+30,,2H,O H,S04,4H,O 111. Redistilled zinc-H2S04,2H,0 IV. Commercial granzc-H,S04,2H20 lated zinc-V. Platinised zinc foil-H9S04,2H20 Weight of zinc used. Approximate temp. of appearance of so,. 0 144-146 150 150-155 135-140 120-125 105-110 105 -110 125-130 130-135 115-120 140-145 145-150 135-140 140 135-140 150 150 130-135 140-145 140 135 140-145 140-145 7 10 105 SHp 0 135-137 137 145 130-135 90-95 70 -75 70-75 105-110 100 85 130 135 125 120-125 120-1 25 120 130-135 130-135 125-130 135-140 125-130 135-140 125-135 '70-80 85-90 formed at 100" ; at 165" both gases are produced Remarks.Little SH even at 150". 1) , Plentiof both-gases at 155'. SO rapidly at once; SH2 slowly increas-ing. Not much SH till 145". Plenty SH2 at 150". Not muchSO till 130°, and SH till 90". reely; and at 180" the latter gas is evolved in very large quantity; in the case of approximately pure zinc both gases are produced in quantity at 165'. Moreover if the gases in question were altogether secondary products of the reduction of snlphuric acid by the hydrogen evolved in the primary change we should expect to find them formed as freely whe 58 THE INTERACTION OF ZINC AND SULPHURIC ACID.the former zinc is used as when commercial zinc is emplored; but this supposition is not confirmed by experiment. We are inclined to think that the interaction between approximately pure zinc and acid is chiefly a direct chemical interaction and that the products of the reactions with the less pure zincs are largely due to the occurrence of secondary electrolytic changes. With pure or almost pure zinc and dilute acid (say H2S04,12 to 15H20) hydrogen and zinc sulphate are the only products of the reaction; with the same zinc and con-centrated acids (say H2SOI,H20 to H,SOJ the chief gaseous product is sulphur dioxide and sulphuretted hgdrogen is also produced at higher temperatures ; with an acid of intermediate concentration (in our experiments with H2S04,'LH20) both compounds of sulphur are formed in considerable quantities a t fairly high temperatures (160").But the results of our experimeiits make i t certain that whether the zinc be pure or commercial the concentration of the acid used plays a most important part in the chemical change ; hence it is probable that the bodies actually interacting are numerous and that the reactions are much more complex than can well be represented in any series of equations which can a t present be devised. It seems to us that the chemical change which occurs is to be ascribed to the interaction of the zinc with various molecular aggregates of H,SO and H,O, probably also of SO and H20 or of H2S04 SO, and H,O; and that the compositions of these aggregates probably vary with variations in the concentration of the acid used and with variations of tem-pera t ure . The results of experiments on the electrolysis and electrolytic conductivity of sulphuric acidX establish a close resemblance between the electrochemical behaviour of this acid and its chemical behaviour as described in this paper. * Wiedemann Die L e h e volz de?. Elektricitat 2 535; also 3'. Kohlrausch, Ann. Ph,ys. Chem. 26 206 ; also Ostwald Phil. Mag. August 1886 and other places ; also Gladstone and Tribe this Journal Trans. for 1879 177