112 PICICERING ON THE ACTION OF XVII. The Action of Su@liurir Acid on Copper. By SPENCER PICKERIXG. Brackenbury Scholar of Balliol College Oxford. $ I. Although Maurnen6 in 1846 made many analyses of the black substance which is generally formed during the action of sulphuric acid on copper no one up to the present time has made a thorough investigation of the subject it was therefore with a view to throw some light on tlhe nature of this reaction by studying the primary and ultimate products and the circumstances which influence their forma-tion that the experiments detailed below were performed. 8 11. Nature of the Reaction. There appear to be two and only two primary reactions the one resulting in the formation of copper sul phatc sulphur dioxide and water according to the following equation :-1.CU + 2HzSO4 = CUSO~ -+- SO2 + 2Hz0, a reaction which may be regarded as consisting of two stages the first being the formation of copper sulphate and nascent hydrogen, thus-() CU + H2S04 = CUSO~ + 2H, the second being the immediate action of the nascent hydrogen on another portion of the acid forming sulphur dioxide and water, thus-(6.) 2H + HZSO = SO2 + 2EE2O SULPHURIC ACID ON COPPER. 113 The other primary reaction is expressed by the equation-2. ~ C U + 4HzSO4 = CUZS + 3C~S04 + 4H20, and this reaction unlike the former one does not necessitate the previous formation of nascent hydrogen. Of these two reactions either may take place alone or they may both occur together and in various proportions. The other products which are found in the residue insoluble in water and which are not accounted for by these equations are the results of the decomposition of the subsulphide by the hydrogen sulphate thus :-CU~S + 2HZS04 = CUS + CUSO~ + SO + 2Hz0 CUS + 2HzSO4 = S + CUSO~ + SO + 2HzO.It was found that this residue invariably consisted of copper and sulphur only and never contained oxygen as stated by Berzelius and Maumen6 (comp. p. 135). § 111. Mode of Experimenting. The experiments were performed in a small flask of about 100 C.C. capacity the gas liberated being passed through a short wide delivery tube into an ammoniacal solution of silver nitrate as a means of ascer-taining whether any hydrogen sulphide was liberated o r not the temperature of the flask being kept constant by means of a water or oil bath.The acid employed was pure and strong-density 1.843-and it was always raised to the required temperature before the intro-duction of the copper which latter was pure electrotype foil 0.15 mm. in thickness 3 grams exposing in all a surface of 65 square cm. The proportion of the metal and acid employed was 1 part of copper to 10 parts of hydrogen sulphate by weight ; the actual quantities generally used being 3 grams of copper to 16.3 C.C. of acid. The time allowed in the majority of cases was 30 minutes. For the sake of obtaining more strictly comparable results in many instances the same piece of copper was used in several experiments due allowance being made for the relative increase of surface thus exposed ; and for the same reason wherever possible the tempeyature chosen was 100" C.; this tempera-ture being more easily maintained constant than higher or lower ones. § IV. General Appearance of the Action. Sulphuric acid attacks copper at all temperatures from 19' C. (and probably even still lower) upwards to an extent increasing rapidly with the increase of temperature ; with the exception of a few minute and occasional bubbles of sulphur dioxide no gas is evolved from th 114 PICKERINCI ON THE ACTION OF liquid till about 130' as soon as the copper begins to be attacked its surface becomes dull and covered with a film of black subsulphide; when the amount of subsulphide floating in the liquid is small it exhibits a brownish colour similar to that exhibited by small quantities of the protosulphide when suspended in a liquid; but as the quantity of subsnlphide increases the colour deepens and soon appears perfectly black.If the temperature be allowed to rise the liquid generally begins to boil below 300" C. the boiling point of pure strong hydrogen sulphate being 327" ; when the source of heat is withdrawn the sus-pended sulphide subsides leaving a bluish-green solution containing copper sulphate which on cooling crystallises out fi*om the acid in white acicular anhydrous crystals leaving the hydrogen sulphate free from dissolved copper and when the proportion of metal and acid above mentioned is used but little diminished in density. After the copper is entirely dissolved and sometimes even before this a deposit of sulphur appears in the neck of the flixsk.On leading the gas evolved through water collecting that portion of it which remains iindissolved (the remainder being of courbe the air contained originally in the flask &c.) and analysing it it was proved that no gas insoluble in water such as oxygen or hydrogen, was given off from the liquid during the action. A small black precipitate deposited in the silver nitrate solution was at first mistaken for silver sulphide precipitated by evolved hydrogen sulphide but this was afterwards found to be reduced silver sulphite, which latter is precipitated as soon as the ammonia becomes neutra-lised by the sulphur dioxide and it was proved beyond doubt that in no case was any hydrogen sulphide liberated. (a.) By replacing the test tube containing the silver nitrate by another similar tube with a fresh solution whenever the white silver sulphide made its appearance in the previous tube and (6.) By placing a roll of filter paper moistened with a solution of lead acetate in the delivery tube itself and which never became darkened by the formation of any lead sulphide.The copper sulphide appears to be deposited on the metal itself and not in the liquid the coating thus formed on the copper being com-pact and when the metal used is impure difficult to remove. The sulphide shows no crystalline structure but is quite amorphous. In determining its amount it was transferred as quickly as possible by washing and other means to a weighed filter and then carefully dried at 100" C. ; after treating it with carbon disulphide to ascertiain the presence and if present, the amount of free sulphur which it contained, the whole or a portion of it was oxidised by hydrogen nitrate the copper being generally determined by reduction with pure zinc or else by Parke's potassium cyanide method o r as oxide by precipitation SULPHUMC ACID ON COPPER.115 with potassium hydrate ; and the sulphur partially in the free state, and partially as barium sulphate. (From the fact that in all cases where free sulphur was present and estimated as above the snlphide left contained the amount of copper theoretically present in the proto-sulphide I conclude that the sulphur was entirely soluble in carbon disnlphide.) Maurnen6 mentions that the sulphide obtained in this way is “ trks altthble A l’air,” and consequently in washing and drying it used many precautions which I found to be quite unnecessary.This sub-sulphide is not oxidised to the smallest appreciable extent either by washing with water not freed from air or by exposure to air for a considerable time when moist or dry or by drying in a steam bath at 100” C.; though in some cases it was dried over sulphuric acid i 7 b uacuo then washed and dried again at 100” in air to ascertain whether any increase or diminution in weight was occasioned by employing this latter method but none was found to have occurred. The protosul-phide though more oxidisable than the subsulphide is not too unstable to admit of being accurately estimated and analysed in the same way as the subsulphide ; and when formed by the decomposition of this latter, as in the action of sulphuric acid on copper it is more stable than when precipitated from a solution of a copper salt by hydrogen sulphide.Weighed portions of the subsulphide and of the protosulphide ob-tained in various experiments and of the persulphide precipitated by hydrogen sulphide were each exposed moist on filters for 14 days, after which-None of the subsulphide had been oxidised. 1.8 per cent. of the protosulphide (from experiments) had been 9.5 per cent. of the protosulphide (precipitated by H2S) had been Again some of the subsulphide which had been kept in a small bottle for several months on being washed did not yield sufEcient copper sul-phate to give the slightest trace of colour when tested by potassium ferrocyanide although some precipitated copper protosulphide wliich had been kept for the same length of time in a similar bottle showed that 43 per cent.of it had been oxidised. Many other experiments on the oxidisability of the different sulphides gave similar evidence.# f That copper protosulphide varies in the facility with which it is oxidised, according to the method by which it is prepared has been shown by Rose who found that when this body is precipitated from solutions containing copper by am-monium sulphide it is more oxidisable than when precipitated by hydrogen sul-phide. It is generally stated that copper protosulphide is green owing to the pre-sence of copper sulphate but this was shown to be incorrect by preparing solxe copper protosulphide washing it with solution of hydrogen sulphide and drying it oxidised.oxidised 116 PICKERING ON TEE ACTION OF 6j V. Influence of Temperature. After a number of preliminary experiments a series mas made at constant temperatures ranging from 100" to 220" C. ; the time allowed for each being 30 minutes. It was however soon apparent that those performed a t the temperatures 170" 195" 220" C. in which cases the insoluble residue did not consist of copper subsulphide were not com-parable with the others because a t tbese higher temperatures the copper had entirely dissolved before the time allowed had expired and consequently secondary reactions might have taken place. The last ekperiments were therefore repeated diminishing the time of action, so that in each case some of the copper taken might be left undissolved, and the results thus obtained together with other experiments per-formed at lower temperatures are given in Table I.It will be seen on examining experiments 3 t o 9 in this table each of which is the mean of a great number that the amount of copper dissolved in eqnal times (given for one minute in the last column) in-creases rapidly with the temperature and also that the proportion of copper converted into sulphide to that converted into sulphatc dimi-nishes as the temperature is higher till finally at 27'0" C. we get no sulphide a t all formed the action consisting solely of the first of the primary actions given on page 112 namely-1. CU + 2H2S04 = CUSOA + SO2 + 2HZO. It will be seen on examining the second primary action-2.~ C U + 4HzSOJ = C U ~ S + 3CuSO4 + 4H20, that in this the proportion which the copper converted into sulphide bears to that converted into sulphate is two atoms to three atoms and that this is the greatest proportion which could be formed if these two reactions are correct; and also as will be shown below the greatest ratio which could exist according to any theory of the formation of the sulphide. After performing experiments at temperatures from 100" upwards and thus ascertaining that the proportion of sulphide in a current of that gas when it was proved to be green in colour though no sul-phate could possibly have been present in it The protosulphide when obtained in the copper action is rather darker in colour than when precipitated by hydrogen sulphide ; probably owing to the fact that the former is in a more compact state than the latter.It is also stated that copper protoeulphide is not attacked by cold hydrogen nitrate and that the subsulphide is converted into the protosulphide j but this latter is undoubtedly attacked to a considerable extent when dry by cold hydrogen nitrate, and therefore also the subsulphide is in all probability not conrertcd into the proto-sulphide by the action of this acid but both sulphides are attacked to varying and indefinite extents SULPHURIC ACID ON COPPER. 117 to sulphate increased as the temperature was lower it was thought probable that at still lower temperatures the proportion of 2 3 might be reached i.e. that the action (§ I p.112) might be obtained alone ; and although in most cases owing to circumstances not entirely evi-dent this ratio was not obtained still in three instances it was obtained within the limits of possible error; once at 80" C. once at a lower temperatu?e and once at 130" C. with dilute acid. The actual ratios found being-2 2.914 2 2964, besides other instances in which this ratio was nearly attained but in no case was a higher ratio for copper as sulphide to that of sulphate than 2 3 observed. TABLE I. Showing the InjlueiLce of Temperature.* Percentage pro-portion of Copper converted into Percentage cOm-position of insol uble residue. Percent -age of cop-per clis-solved in one minute. Percent-age of Copper dissolved. 5.7 2 -532 1 503 Tempera-ture.Time allowed. 14 days 120 min. 30 min. 30 min. 30 min. 30 min. 30 min. 30 min. 10 min. 2 min. 0.5 min. f afem 1 seconds -cu2s. Cu28 -100 100 100 c u s S. cuso4. 17 '33 5 *25 30 *20 83 -67 94 -75 69 -80 0 0 0 0 0 0 1. 19°C. 2. 60 3. 80 4. loo 5. 124 6. 130 7. 137 8. 150 9. 170 10. 195 11. 220 12. 270 13. 170 14. 195 15. 220 -0 -0003 0 '0211 0 '0501 0 .lo41 0.76 1 -09 1 *17 2 *31 5 -19 26 -75 70 -57 clis-solred in + minute ----75 '00 78 '47 82 '40 83 -00 86 '73 89.18 98 .OO 92 '84 00 .oo 100 100 100 100 100 100 100 100 100 25 '00 21 -53 17 -60 17 *oo 13 -27 10 '82 8 '00 7 -16 to 11 *53 7 *20 2 -136 3 '123 22 *7 32 -6 35 .o 69 -2 51 *92 53.5 70 -57 30 min.30 min. 30 min. 88 '47 92 .SO 97 *S64 0 G 0 -4.17) 100.0 .o*oo 100.0 3.001 100.0 * Columns 3 and 4 in this Table 4 and 5 in Table 11 and corresponding columns in other tables showing the proportion of copper converted into sulplde to that converted into suiphate are made out so as to show the amount of copper originally as subsulphide calculated from the amount of subsulphide protosulphide and free sulphur (see secondary actions) found in the liquid a t the conclusion of the experi-ments 118 PICKERING ON THE ACTION OF The combination of these two primary reactions of which the one takes place alone at high temperatures only the other only at low tem-peratures explains all the results obtained at various interniediate temperatures as given in the first twelve analyses of Table I and equations can easily be formed representing the action in any given case thus the following equations represent approximately the actions which take place at loo" 130" and 170" C.respectively. l l C u + 16H,SO = Cu2S + 9cuso4 + 6S0 + 16H20, ~ C U + 10HZSO4 = CUZS + ~ C U S O ~ + 3502 + 10Hz0, 2 4 C ~ + 42HzSO4 = CU~S + 22CuSO4 + 19SO2 + 42H2O. § VI. Xecondary Actions. The1 ast three results given in Table I were those obtained originally at the temperatures 17O" 195" and 220" C. in all of which ca8es the whole of the copper had been attacked before the time allowed for the experiment had expired thus better opportunity for secondary re-actions was afforded.It will be seen that in none of these cases was there any of the subsulphide left which had been proved by experi-ments given higher up in the table to have been first formed in each case but in its stead a mixture of copper protosulphide and free sul-phur and in other cases not given in the table protosulphide only or a mixture of the protosulphide and subsulphide was found. I n order to ascertain whether the formation of tbese products was explicable by the decomposition of the subsulphide at first formed by the excess of hydrogen sulphate present some of the sulphides were prepared by the ordinary methods analysed and then decomposed by beating with strong sulphuric acid. The protosulphide was attacked by the acid at a temperature as low as lOO"C.and was found by quantitative experiments to break up in the following manner :-GUS + 2H2SO4 = S + G U S ~ ~ + 2Ha0 + SO,. This may be regarded as consisting of the following stages-(1.) GUS + HzS04 = CuSOI + S + 2H, (2,) S + 2H = H,S, ( 3 . ) H2S + HzSOd= SO2 + 2HzO + S, but it seems to be more probable that the second molecule of hydrogen sulphate is decomposed directly by the nascent hydrogen as in the main action of copper on the acid thus-(2.) H2SO4 + 2H = SO + 2H30, and not by the intervening formation of hydrogen sulphide becaus SULPHURIC ACID ON COPPER. 119 (n.) The hydrogen sulphide if formed would probably react on the copper sulphate instead of on the acid. ( b . ) Hydrogen sulphide does not react on hydrogen sulphate at moderately high temperatures with great energy and therefore a con-siderable portion of it would probably be evolved free from the liquid, whereas in reality no traces of it could be detected.For these reasons it is therefore more rational to regard the steps constituting this reaction to be-(1.) CUS + H2so4 = CUSO + S + 2H, (2.) 2H + HzS04 = SO + 2H20. The subsulphide was attacked by hydrogen sulphate at temperatures below loo" breaking up first into the protosulphide and forming at the same time copper snlphate sulphur dioxide and water thus :-(1.) CU~S + HZSO, (2.) 2H + HzSO4 = 2HzO + SO,, GUS + CUSO~ + 2H, and then after the whole of the subsulphide had been decomposed in this manner the protosulphide formed by its decomposition was further broken up with formation of snlphate and liberation of sulphur dioxide and free sulphur as shown above.These decompositions of the protosulphide by the excess of acid present explain all the results obtained in every experiment ; and in various cases every step in the decomposition was traced some of which will be found in the tables below namely cases in which The copper was not entirely dissolved and the black residue con-sisted entirely of subsulphide. The copper was onlyjust dissolved and none of the subsulphide had been attacked. The copper was entirely dissolved and varying proportions of the subsulphide also leaving a mixture of the two sulphides. The copper was entirely dissolved and the subsulphide just con-verted altogether into protosulphide.The copper and subsulphide were entirely dissolved and also the protosulphide to varying extents. The copper and both the sulphides were dissolved and nothing but sulphur left in the liquid. 8 VII. Formation of the Szclphide. The following are arguments :-(a.) That the sulphide is not formed by the action of nascent (1.) I f the subsulphide was produced by the action of nascent hy-1Lydropn 120 PICKERIKG ON THE ACTION OF drogen on the copper sulphate it would be according to the following equations-(1.) ~ C U + 6H:,SOA = GCUSOI + 12H, (2.) 12H + 2CuS04 = CuZS + SO + 6H0, in which case the greatest possible ratio which the copper converted into subsulphide could bear t o that converted into sulphate would be 2 4 whereas the ratio 2 3 is obtainable.(2.) I f nascent hydrogen could react simultaneously on copper and copper sulphate it would be according to the equation-~ C U + 4HZSOI = 3CuS04 + CU~S + 4Hz0, this equation being then composed of two steps. (1.) ~ C U + 4H,SO = ~ C U S O ~ + 8H, (2.) 8H + CU + CUSO~ = CUZS + 4HZO. But that nascent hydrogen cannot react in this way ma'y be proved by treating a piece of zinc with dilute hydrogen sulphate mixed with a solution of copper sulphate in which case no trace of copper subsul-phide is formed although hydrogen is here liberated in the nascent state in presence of metallic copper and copper sulphate. (6.) That the sulphide is not formed by the action of hydrogen sdphide :-(1.) Some hydrogen sulphide would in all probability be given off as such from the liquid and its presence would ha've been easily recog-nised by the solution of silver nitrate.(2.) The hydrogen sulphide would be formed by the previous liber-ation of nascent hydrogen and the subsequent reaction of this body either on the hydrogen sulphate or on the copper sulphate ; if on the hydrogen sulphate the reactions would be represented by the follow-ing formuke :-(1,) ~ C U + 8CuS04 = ~ C U S O ~ + 16H, ( 2 . ) 12H + 3HZS04= HZS + 2SOZ + 8HZ0, (3.) H,S + 4H + 2CuSOa = CuZS + HZSOa + SO2 + 2HZ0, and if on the copper sulphate by the formula+-(1.) 11 CU + 11H,SO = llCuSOi + 22H, ( 2 . ) 18H + ~ C U S O = HZS + CUZS + 8H20, (3.) H,S + 4H + ~ C U S O ~ CUZS + HZS04 + SO2 + 2H20. In the former of these cases the grestest proportion which the copper as subsulphide conld bear to that as sulphate would'be 4 12, and in the latter 4 7 whereas in reality the proportion 4 6 is obtainable SULPHURIC ACID ON COPPER.121 (3.) Hydrogen sulphide reacts on copper sulphate forming copper protosulphide and not subsulphide which latter is the product obtained in the action in question. (4.) The subsulphide is apparently formed on the surface of the copper and not by the passage of bubbles of gas through the liquid. (c.) That the sulphide is not formed by the union of the metallic copper with free sdpuhur previously liberated during the action as stated by Calvert and Johnson. (1.) Since t'he formation of hydrogen sulphide is untenable the sulphur could only be liberated by the actiou of nascent hydrogen on the hydrogen sulphate o r sulphur dioxide present both of which re-actions are improbable and unsupported by fact.(2.) No sulphur is ever found in the insoluble residue produced during the action of hydrogen sulphate on copper till after the decom-position of the copper sulphides and all the experiments performed on the subject prove t'hat this sulphur so far from being the cause of their formation is in reality the product of their decomposition. (3.) If the sulphide was formed by the combination of the copper with free sulphur the amount of it produced would be increased by increasing the quantity of that sulphur ; but this was proved not to he the case by adding a weighed quantity of sulphur to the acid used in one of the experiments; after the conclusion of the experiment the sulphur which had been added was not appreciably diminished in weight nor was the quantity of copper sulphide formed greater than in other similar experiments where no sulphur had been added to the liquid.(It is sometimes stated in t e s t books that sulphur when sus-pended in a liquid as obtained in the preparation of pentathioiiic acid, may be precipitated as copper sulphide by agitation with copper turnings as if the metal combined directly with the sulphur. But the sulphur is in reality carried down with copper sulphide formed by the decomposition of copper pentathionate as shown by W a c k e n r o d e r, who proposed this method.) We may therefore conclude that the snlphide cannot be formed in the way supposed by C a l v e r t and Johnson nor by the action of nascent hydrogen or hydrogen sulphide but that the hydrogen snl-phate is decomposed by the copper directly forming copper subsul-phide &c.I n several experiments where only a mere trace of copper was left unattacked none of the subsulphide was decomposed the reason being obvious namely that the acid would continue to act on the copper as long RS any of this latter remained in the free state and that not till after all the free metal had been dissolved would it attack the sulphide it being much easier for the acid to combine with the VOL. XXXIII. 122 PICmRING ON THE ACTIOX OF Percent,agc composi-t.ion of the insoluble residue. copper in the free state than to decompose a compound body contain-ing it and then combine with the liberated metal.The temperature at which these experiments were performed was above 170" C. i.e. a t a temperature when the brisk evolution of gas would prevent the metallic surface from becoming so densely coated by the deposition of subsnlphide a i d sulphate as to protect it from the action of the acid ; it was therefore anticipated that at lower temperatures when the evolution of gas was slow the coating of subsnlphide and sulphate might be deposited so compactly on the copper that it would protect it entirely or to a great extent from the action of the acid which would then be a t liberty to attack the subsulphide. This anticipation was realised by a number of experiments perfoymeci at various tem-peratures and forvarious lengths of time some of which are shown in Table 11.The protective effect of the coating of sulphide is shown in columns 6 7 and 8 where it will be seen that the amount of subsul-phidc decomposed increases with the duration of the experiment and it is also shown in the last four experiments in column 9 where the amount of copper dissolved during successive intervals of thirty minutes is given. (In this column the first three experiments appear to give evidence contradictory t,o that afforded by the last four but this will be satisfactorily explained below p. 128). TABLE 11. COPP~ dis-solred in Tem-perature. 1. 100OC. 2. 100 3. 100 4. 137 5. 137 6. 137 7. 137 Percent-age of Copper dis-solTed. 3 -336 13 '31 18 *94 40 '20 65 '82 85 -67 99 '50 Time allowed.30 inin. 90 min. 120 min. 30 iiiiii. 60 min. 90 min. 120 min. Proportion of Colqwr con-verted into s. 0 0 0 0 0 0 0 corisecut ive intervals of 30 minutes. 3 -336 4 *987 5 '630 40.20 25.62 19 3 5 13 *83 cu.,s. 18-60 18.14 18.64 16.00 8.60 cuso4 -~ - -81.4 81 -86 81.36 84.00 91.4 - -CUZS. -LOO 6'3.0 51.0 100.0 90-6 72.1 31-16 8 VIII. Proportion of Acid to Metal. The amount of copper dissolved and the relative amount of sul-phide formed are apparently not influenced by the proportion of acid and metal used when these are varied. Thus experiments were made with 1 part of copper to 0.6 5.0 10.0 and 20.0 parts of acid but the means of many results obtained with these several proportions did not vary in any (lofinite direction or differ from each to a greater extent cus.0 4,l.O 49.0 0 9.4 27'9 68-8 SULPHURIC AUID ON COPPER. 123 than did the results of single experiments in which the same propor-tions of metal and acid were employed. § IX. The Water formed. After the action is over the acid is found to contain no copper sul-phate in solution though water being formed during the action it might be thought at first that some of the sulphate would certainly be dissolved since this body is soluble in dilute though not in strong acid. But the fact is that with the proportion of metal and acid used in most of the experiments the water given off would be too insignificant to alter the density of the acid to any considerable degree.Thus supposing that the action in which most water is formed with a given weight of copper namely, CU + 2HZSO4 = CUSO~ + SO2 + 2Hz0, to take place only and that the whole of the copper in the experiment were dissolved the diminution in the density of the acid as found experimentally would only amount to about 0.008. Even a very small proportion of this water is given off at temperatures below 150" C. as mas found by connecting a weighed calcium chloride drying tube with the flask taking care after the cxperirnent to expel the sulphur dioxide by a current of dried air. This increase in weight of the drying tube might possibly have been due to the spray of the acid carried off in the current of gas ; but as the evolution of gas was very gentle and as in other experiments the acid was not found to have experienced quite the theoretical diminution in density it is probable that it was principally due to the water formed during the action.At higher temperatures more water is given oE and it makes its appearance in minute drops in the neck of the flask and in the delivery tube. I n addition to there being more action and therefore more water formed at higher temperatures ibs volatilisation would be augmented by the increased evolution of gas. It was a t first thought possible that owing t o the great hygroscopic powers of anhydrous copper sulphate that this salt might retain the water and form partially hydrated crystals, but this does not appear to be the case because-(1.) The crystals which separat.e out on cooling are perfectly white, like anhydrous copper sulphate and do not exhibit any trace of a blue colour as do all its hydrated salts.(2.) It was found by experiment that when copper sulphate con-taining even 1 molecule of water of crystallisation onl7 was heated with strong hydrogen sulphate that it entirely gave up its water to the acid this latter becoming diluted t o a corresponding degree and the copper sulphate crystallising out in white anhydrous crystals. L 124 PICKERING ON THE ACTION OF That the acid retained most of the water liberated in the majority of my experiments is proved not only by the slight diminution of density observable a t the conclusion of the experiment but also by the fact already noticed that the acid generally boils at a temperature a little below the boiling point of perfectly pure hydrogen sulphate.tj X. The Hydrogen Szclphate used. Some experiments were made to see if the acid actually entering into the reaction agreed with the amount required by the equation. The percentage of true acid in the hydrogen sulphate employed was first determined both volumetrically by a freshly-prepared standard solution of ammonia and gravimetrically as barium sulphate. A weighed quantity of the acid was then heated as in other cases with a weighed quantity of copper. After the time allowed had expired, the acid was diluted and boiled in a current of carbon dioxide to free it from sulphur dioxide. I n order to prevent any acid from being carried off during boiling the delivery tube dipped into another flask containing a little water and this second flask was similarly con-nected with a third flask the contents of both of which were boiled in turn to free them from sulphur dioxide.* The loss in weight ex-perienced by the copper and the amount of copper sulphide formed were then ascertained from which data the equation representing the reaction which had taken place was constructed.The residual hy-drogen sulpbate was afterwards determined and found to agree within experimental error with that required by this equation. Several determinations were made all of which gave satisfactory results. § XI. Determination of the Xulphzcr Dioxide. The equation was first constructed from the amount of copper attacked and the amount of sulphide formed; the gas eliminated from the liquid by boiling as in the determination of the hydrogen sulphate was collected in a large volume of water carefully freed from air by long boiling a, * Beforc the determination of the hydrogen sulphate preliminary experiments were made by w-hich it was ascertained-(1.) That boiling about 100 C.C.of a saturated solution of sulphur dioxide for 30 minutes in a current of carbon dioxide was amply sufficient to free it from all traces of the former gas. (2.) That the two washing flasks were nccessary but also sufficient t o prevent any loss of acid from the experimental flask. (3.) That the copper sulphides were not appreciably decomposed by boiling in weak hydrogen sulphate (1 V O ~ . of acid to 10 vols. of water being the proportion used) for 30 minutes the liquid and apparatus being freed from air and filled with carbon dioxide.The sulphur dioxide evolved was then determined SULPHURIC ACID ON COPPER. 125 current of carbon dioxide being drawn through the a.ppitratus during the experiment. The sulphur dioxide thus liberated and collected was determined either gravimetrically as barium sulphate after oxida-tion by means of bromine or volumetrically by standard solutions of iodine and sodium hyposulphite. Good results were obtained in this case also two of which are given below. In the first of these the primary reactions alone took place the black residue consisting en-tJrely of copper subsulphide ; in the second all the primary secondary, and tertiary action's had occurred the residue consisting of copper protosulphide and free sulphur.I. SOz required (as determined by copper dissolved and copper sub-sulphide formed) = 0.06517 gram. SO2 found = 0.06513 gram. = (39.94 per cent. 11. SO2 required (as determined by the copper dissolved and the persul-phide and sulphur formed) = 0.3419 gram. SO found = 0.3364 gram. = 98.363 per cent.* Thus having determined accurately every factor (with the exception of the water and that necessarily only approximately) on both sides of the equation representing the action of hydrogen sulphate on copper, and the eecondarg action entailed by it and finding these determina-tions to agree perfectly with the explanation above given of this action, there can be little doubt that this explanation is correct and that the explanations given by others to be discussed below are incorrect, since if otherwise in none of these cases could the factors be what experiment shows they really are.§ XII. The Sublimate of Sulphur. The appearance of a small sublimate of sulphur in the neck of the flask and in some cases in the delivery tube occurred as already men-tioned only after the complete solution of the copper ; and since its formation was also observed on decomposing the sulphides of copper by hydrogen sulphate we may conclude that it owes its origin to this latter action and not to either of the primary reactions. Now since no hydrogen sulphide was detected by the silver nitrate solution as being liberated during the decomposition of the sulphides this sulphur In the second one owing to the necessarily small quantity of copper taken and small quantity of the insoluble residue obtained a very sinall error in the determination of the free sulphur would account for the difference of 1-64 per cent.obtained. # Both of these results are within the limits of experimental error 12 6 PICKERING ON THE ACTION OF could not have been formed by the mutud reaction of the hydrogen sulphidc on the sulphur dioxide. This may a t first sight appear a false argument but in observing other similar reactions such as the decomposition of tetrathionates by hydrogen chloride in which the main gnseous product is sulphur dioxide a sinall quantity of hydrogen sulphide being liberated together with it it is seen that a comparatively small deposition of sulphur is accompanied by a comparatively large indication of hydrogen sulphide in a silT-er solution into which the delivery tube is made to dip.Therefore the formztion of the sulphur sublimate in the copper action may be more reasonably attributed to a cause which must inevitably come into play namely the volatility and tendency to creep up the sides of the containing vessel exhibited by sul-phur wheri in a state of fine division. This is supported by the fact, that the sublimate is visible in the neck of the flask in greater quan-tities and long before it is visible in the delivery tube and in this latter only at high temperatures ; whereas owing to the comparative narrowness of the tube if formed by the reaction of two gases on each other it would as in the decomposition of the polythionates, show itself sooner and in greater quantities here where the gases are brought into closer proximity with each other.It is more probable that the free sulphur is expelled from the liquid chiefly owing to its tendency t o creep up tthe sides of the containing vessel and not to its volatility because when a compact piece of sulphur is heated with hydrogen sulphate none of it sublimes till some time after that in tbe liquid has attained its melting point ; whereas if sulphur in the finely-divided state is employed a deposit is observed in the neck of the flask as low as 110" C. Still sulphur is volatile to a certain extent, even a t 100" C. especially when its volatilisation is aided by the evolution of a gas or vapour in proximity to it as shown by the notable smell of this body perceived when drying a precipitate con-taining it in a steam bath or when boiling liquids in which it is sus-pended.$ XIII. I?juence of an Electric Current. The influence of an electric current on copper while being acted on by hydrogen sulphate was then tried chiefly with a view of ascer-taining whether the relative amount of copper sulphide and sulphate formed would thus be altered. This was done in two ways either by connecting the copper with one of the plates of a small Daniell's cell the other plate being connected by a platinum wire with a small piece of platinum foil immersed in the acid or else the copper was made electropositive by simply attaching it to a piece of platinum wire coiled up and entirely immersed with the copper in the acid.O SULPHURIC ACID ON COPPER. 127 course the more electropositive the copper was made the more of it was dissolved in a given time and vice vewd (though the poles wcre not arranged so as to get the greatest amount of action) ; but in addi-tion to this it mas found that when the copper was electropositive the proportion of siilphide to sulphste formed was increased whereas when electronegative the proportion was diminished as shown in Table 111 where the means of many experiments are given. The diminution and increase in the proportion of sulphide to sulpliate formed according as the metal is made electronegative or eleutro-positive respectively is not great but in all cases it varied in the same direction though to a greater or less extent.TABLE 111. ,Showity the Ffeect o,f aiz Electric Cuwent. Tem-perature. 1. 100°C. 2 . 100 3. 100 4. 100 5. 100 6. 100 -Time allowed. 30 min. 30 min. 30 min. 15 min. 30 min. 60 min. Condi-tion. Positive Ordinmy Negative Positive Positive Positive -Percent-age of Copper dis-solved. 14 *214 3,707 3 -2% 6 *978 13 *197 18 *184 Percentage com-position of the insoluble residue. cu,s.l cus. s. - 1 I-Proport ion of Copper as cu,s. CUSOd. l l Percentage of Copper 1 3 s -solved in consecutive intervals of 15 minutea. , I- I It will be observed that when the copper was made electropositive, the sulphide did not consist as in other cases of subsulphide only, but of a mixture of subsulphide and protosulpliide.This is easily explained thus When the copper is made electropositive as much metal is dissolved during the first 15 minutes as is dissolved in about oiie hour when it is not made electropositive or when made electro-negative ; and consequently in the former case as much or even more sulphide (since also the relative proportion of sulphide is increased) is formed as in the latter case ; but as shown above the amount of subsulphide formed during about one hour at 100" C. under ordinary conditions is sufficient to coat the copper to such an extent that it greatly protects the metal and itself become attacked ; consequently, when the copper is made electropositive and the action proceeded with about four times its ordinary rapidity me should expect as is the case? that the sulphide will begin to be decomposed after about the first 15 minutes.The protective action of the sulphide is shown by the last three experiments given in Table 111 where (in the las 128 PICRERING ON THE ACTION OF column) a relatively smaller amount of action is seen to occur as the time allowed is longer. I n order to make sure that in these cases as well as in those already considered the subsulphide was in reality the first product and that the protosulphide owed its origin to the secon-dary reaction and was not produced directly from the copper expe-riments were made with copper rendered electropositive for 5 10 and 15 minutes; always when 5 and 10 minutes and sometimes when 15 minutes were allowed the insoluble residue consisted entirely of subsulphide showing that this was in reality the first product.From the moderately good conducting power of copper sulphide and the fact that being less easily attacked by hydrogen sulphate than metallic copper it would act i n an electronegative manner towards this latter body it was thought probable that the sulphide as i t was formed by making the copper more electropositive would increa,se the actual amount of action during the first stages. That this was in reality the case was proved by performing several series of experi-ments a t the same temperature wit'h the same piece of copper (to make the several results more strictly comparable with each other due corrections being made for the relative increase of surf ace exposed) for successively increasing intervals of time.Two of these series are given in Table IV and tliese may also be taken in conjunction with those given in Table 11 &c. as illustrative of the eff'ect of time on the products of the action. The figurcs in the last column of tliis table TABLE IT. Shouhg the Actio?L of the Szdphide o n the Anxount of Copper Bissolved. Tem-perature. -1. 100°C. 2. 100 3. 100 4. 100 5. 100 6. 100 7. 100 8. 100 9. 100 1. 100 2. 100 3. 100 4. 100 5. 100 -Percent-age of Copper dis-solved. 0 *2001 0 '834 2 -300 3 -336 4 -987 7 -171 7.572 13 *310 18.94 0.513 1.310 3.574 7.206 9 '072 Percentage composi-tion of the insoluble residue.CU2S. -. 100 100 100 100 100 100 59 .o 51 '0 -----_ -c u s . 0 0 0 0 0 0 4.1 -0 49 -0 -Proportion of Copper LlS 3usoq -90 '4 87.25 86 -9 86.7 81 '4 --Percentage of Copper dissolved in consecutive intervals of 5 minutes. -0 '2001 0 %34 0 -767 0.901 0 *825 1 -093 0 -2005 0 *9565 0 *go5 0 -513 0 -797 1-132 0 '932 0 -44 SULPHURIC ACID ON COPPER. 129 sh0.w the additional quantities of copper dissolved in consecutive and equal increments of time and it is thus seen that the amount dissolved increases rapidly a t first then more gradually till after reaching a maximum it begins to decrease chiefly owing as was said above in connection with other experiments,' to the protective action of the coating of sulphide and partially also to the increasing diminution in density of the acid a very small diminution of which is capable as will be shown below of having considerable influence on the amount of copper attacked.I n the series given in Table IV the amount and composition of the sulphide formed was not determined with much accuracy this being unnecessary and indeed with the small amount of action in many cases it would have been impossible t o do so j but as might have been anticipated from the effect of making the metal electropositive by other means the proportion of sulphide to sulphate increases with the amount of copper. acted on i e . with the actual amount of sulphide present. (When the copper acted on is made the negative electrode the pla-tinum which acts as the positive electrode naturally remains bright, but when the current is reversed the negative electrode ( i e .the pla-tinum) becomes slowly coated with a black deposit which as far as can be judged by qualitative analysis (since sufficient of it was not obtained to admit of a quantitative determination) consists of copper sub-sulphide. This evidently arises from the deposition of metallic copper on the platinum which acts as the negative electrode owing to the electrolysis of the copper sulphate in the liquid ; the copper thus deposited soon becomes attacked by the acid in the same way as but to a smaller extent than the copper constituting the positive electrode, thus giving rise to it film of copper sulphide covering the platinum.The prior deposition of the metal may be observed by taking a.bright piece of copper coating one half of it with mercury and then immers-ing it without connecting it with the battery in heated hydrogen sul-phate ; almost as soon as the exposed copper begins to be attacked the mercury acting as a negative electrode becomes coated with a thin film of riietallic copper and if the acid be poured off beiore the action has proceeded too far this deposit will exhibit most beautifully the colours of thin films in which owing to the natural colour of the copper rich purple and magenta tints prevail. I f left in the acid this film soon becomes reddish and finally black consisting then of copper sulphide. These colours first exhibited by the film would prove that a deposition of a layer of nietallic copper must be the origin of the coat-ing of copper sulphide on the platinum or mercury since no such colour can be seen during the deposition of sulphide or) uncoated copper and it is indeed impossible to see how any copper subsulphide could be otherwise deposited on the negative electrode.130 Proportion of copper as PICKl3RISQ ON THE ACTION OF I Percent-age of Copper 0 XIV. Iujlzience of Puw*ty. (a.) Of the MefQZ.-The influence of the purity of the metal used was then studied. The copper employed in the above experiments as already mentioned was ordinarily pure electrotype foil and the nsual tests failed to detect any mctallic impurities in it. Some commercial sheet copper was then taken containing small quantities of arsenic, tin silver iron and lead and substituted for the electrotype metal ; the mean of several experiments with t,he impure metal is given in Table V 5 comparable with 2 of the same table yesults which were obtained with the pure metal under similar circumstances.TABLE V. Showing the Inflzience of Impurity itz the Copper. ~~~~ ~ ~ Quality of Sample. 1. Purest electru-tppc wire . . . . 2. Pure electrotype foil. . . . . . . . . , 3. Less pure elec-troti-pe foil . . 4. Copp& binding wire . . . . . . . . 5. Commercial sheet copper 6. A bronze coin 7. Copper turnings (very impure) ~ ~~ Tem-p m t u r e -looo c. 100 100 100 100 100 100 ~~ ~ Time a1 lo wed, Pcrcezlt,age compo.sition of the insol-uble residue. cuzs. 100 100 100 -83 '3 --cus. The main points that will be noticed in the experiments performed with the common metal are firstly that the insoluble residue does not consist of tlie subsulphide only but of amixture of the two sulphides, and secondly that a far greater amount of the metal is attacked than when pure copper is used. The former of these two results is a con-sequence of t h e latter being explicable in the same manner as when an increase in the amount of action is produced by making the metal electropositive namely that sufficient sulphide is formed in the first part of the reaction to protect the metal considerably from the acid, and thus the subsulphide itself becomes attacked before the action has proceeded far.In this case a s in the others it was proved by making tbe experiment last only 10 or 15 minut>es that the first product was invariably copper subsnlphide only. Tlie reason why much more copper should be dissolved in a given time when im SULPHURIC ACID ON COPPER. 131 pure than when pure is not so obvious. From the near accord-ance of the amount of impure metal dissolved with the amount of pure metal dissolved when rendered electropositive I thought at first to attribute the increase of action obtained when the metal is impure to its being made electropositive by the metallic impurities present in it ; but it was found that this explanation would not hold good from the following consideration :-Far a day has shown that the metals which exist as impurities in commercial copper namely, lead tin bismuth iron antimony,* and silver are with the exception of this last-named metal electropositive towards copper in weak hydro-gen sulphate silver being electronegative towards it ; and as far as can be judged from rough experiments these metals hold nearly the same position in the elect~ochernical series towards copper when the liquid is strong hydrogen sulphate a t a temperature of 150" C.; arsenic and silver being electronegative towards copper since it is improbable that the small traces of arsenic and silver in commercial copper should so entirely counteract the effect of all the other impuri-ties present as to render the metal decidedly electropositive we should expect to find impure copper less electropositive than pure copper towards hydrogen sulphate.The following facts show that this is in reality the case When copper either pure or impure is acted on by sulphuric acid it is found to be in a more electronegative than electropositive condition for the increase in action when made electro-positive is much greater than the decrease when made electronegatire. But when pure the increase when made electropositive is 3;?$ and the decrease when made electropositive is c?.F whereas when impure the increase in action 7i&5 and the decrease f'?g the impure being obvi-ously more strongly electronegative than the pure and thercfore the increase of action caused by the presence of impurity i n the copper cannot be attributed to the galvanic influence of that impurity.In addition to the two main points noticed as above in connection with the purity of the copper it will also be seen that the relative pro-portion of sulphide formed is rather greater when the metal is impure than when pure. I can give no satisfactory explanation of this and can only state that a t the same temperature the relative amount of sulphide formed increases slowly with the amount of metal acted on ; and it would follow from this that the proportional increase of sulphide observed when pure copper is rendered electropositive and zice veysd is not due to the fact of the metal being in a certain electrical condition, but merely to the increased or diminished amount of action owing to that electrical condition. * F a r a d a y did not experiment on arsenic which is also a common impurity in copper but it seems t o be ekctronegatirc towards copper and electropositive towards d v e r in hydrogen sulphate either Btrong or weak 138 PICKERING ON THE ACTION OF The increased relative amount of sulphide formed with impure cop-per is also seen by its forming a notable amount of sulphide at 2'70" C., when the pure metal forms none.Pure copper also when rendered electropositive by being attached to a platinum wire also forms an easily distinguishable amount of sulphide a t 270" C. The best mode of observing it is to string tlie pieces of copper on to a loop of platinum wire fused into a glass rod ; this may be passed through a hole in the cork closing the tube or flask used and gradually lowered into the acid when it has attained the required temperature ; the action is thus rendered manageable and the escaping gas may be conducted away through a delivery tube and be absorbed.On examining the piece of copper after it has been attacked by the acid it is found that when impure the surface is far more rough and uneven than whea pure showing that in the former case the action has been very unequal in contiguous parts owing to tlie presence of impurities. The coating which the sulphides (afterwards becoming attacked and beihg replaced by sulphur) together with the anhydrous sulphate forms first in the rugosities and then over the whole surface, is naturally much more compact than when formed on the even sur-face retained by pure metal throughout the action so much so that after a certain amount of metal has been dissolved the remainder is almost conipletely shielded from the acid and remains unattacked, even after prolonged immersion in the acid at moderately high tempe-ratures.This protective action of the sulphide deposited on impure copper may be shown by the following experiment:-A piece of impure cop-per is immersed in acid in contact with a piece of pure metal both pieces exposing equal surfaces ; being in contact the effect is the same as if the impurity were equally divided between both pieces and each should be acted on t o the same extent but it is foucd that the pure metal is invariably most attacked showing that the impure must have been protected to a greater extent than the pure by the coating of sulphide.Other samples of copper were experimented with and an average of the results obtained with each is given in Table V.* The results obtained at higher temperatures agreed in the several instances with those obtained at 100" C. which are here given. They are arranged, beginning with ssniples which gave the least amount of action and ending with those that gavc the most and the order thus obtained is, a s far as can be judged without accurate quantitative analyses of each * The results given in the table are corrected so as to exhibit the amount of action obtaiued with equal weights esposing equal surfaces to the acid this being the o11Iy way in which eqwriments performed with samples of various thicknesses can be rendered comparable SULPHURIC ACID ON COPPER.133 sample the same on the whole as that which would be obtained by arranging the metals according to their relative purity the greater being the amount of action according as the amount of impurity is greater. (The coin probably contained a considerably smaller per-centage of copper than the turnings but it owes its position in the table to the zinc present in it which would act electronegatively in the strong acid and therefore in opposition to the other impurities.) This fact and the fact that a verysmall amount of impuritlyis capable of causing a great increase in action leads to the conjecture that if such a thing as perfectly pure copper could be obtained it might not he acted on a t all by hydrogen sulphate but from the impossibility of obtaining absolute purity this must f o r the present remain but a mere conjecture.(b.) Of the Acid.-The usual amount of impurity in commercial hydrogen sulphate is too minute to have any appreciable effect on the action ; but if the arsenic usuaWy present in it be increased in amount, it becomes deposited on the copper and being electronegative towards it increases the action.* If the amount of lead is increased no altera-tion is caused in the action since lead is electropositive towards copper in hydrogen sulphate. Ej XV. Actim with Dilute Acids. Table TI exhibits the influence of diluting the acid employed in the experiment,s; it will be seen from it that the amount of action decreases rapidly with the decrease in densit'y of the hydrogen sub phate.The amount of copper dissolved during the time allowed-30 minutes-began to be appreciable a t 130" C. with the acid H2SOa,H20 and at 165" C. with the acid H2S0d,2Hz0 ; though pro-bably if the time allowed was much prolonged an appreciable action would take place a t considerably lower temperatures since the amount of action obtained with the stronger acid at 130" C. and with the weaker one a t 165" C. is about equivalent to that obtained with pure acid at 100" C. No appreciable action was obtained with acid weaker than HzS02,2Hz0. The composition and quantit'y of sulphide formed agreed with the composition and quantity of it formed by strong acid at such temperatures when equal amounts of copper were acted on. The results given in Table VI agree with those obtained by Gal-r e r t and Johnson as near as can be expected considering what minute differences in the purity of the metal and dilution of the acid alter to a great extent the amount of action.* Reinsch's test may thus be effectually performed in hydrogen eulphate elren when weak though it is far less quick in operation than when the liquid used is hydrogen chloride 134 PICKERING ON THE ACTION OF TABLE TI. Showing the Action of Acid of Various Strengths. Percentage of Copper dissolred' Tem-perature. -1. 100OC. 2. 100 3. 100 4. 100 5. 130 6. 130 7. 130 8. 165 9. 165 10. 165 to t6at dissolved by aciod sp. gr. 1.843 at e same tempera-ture and for the same Time allowed. -Density. 1 -843 1 %295 1 *780 1 '620 1 '843 1 -780 1 '620 1 -843 1.780 1.620 1 Proportion dissolved 1 length of time.I--- -2 -380 0 *585 0 0 32 *6 1 *182 0 'Oin 15min. 16 -54 2 '744 1 l 0.246 1 0 l 0 l 1 l 0.0363 1 0 l 1 l 0.018 1 0.11 :'1 § XVI. Variation of Results. The experiments given in the above table are either means of a large number or clse characteristic results selccted from several. It would be superfluous to give the results of more experiments than are here given since they all bear the same evidence as to the nature of the action and the circumstances which influence it. Often in comparing together several results of experiments per-formed as nearly as possible under similar circumstances considerable variation on either side of the mean was observed.From what has been said above it is not difficult to see that this would naturally be the case from the minute and unavoidable circumstances which influ-ence the action namely :-(1.) A very slight change in the density of the acid. (2.) A minute alteration in the amount or kind of impurity in the metal so small that different thcugh contiguous pieces of the same sample of metal may givc considerably different results as is shown by the greater concordance observed when several similar experiments are performed with the same piece of metal than when performed with different pieces of the same sample. (3.) The mode in which the sulphide is deposited; since a small alteration in temperature or in the purity of the metal makes a con-siderable difference in the compactness of the film.(4.) The mode in which the pieces of copper are placed in the liquid ; since it appears that between two pieces touching each other, the film of sulphide and sulphate is deposited so compactly as t o m:ake them adhere together. (The pieces of copper which hare been in contact with each other are invariably thicker after the action tha SULPHURIC ACID ON COPPER. 135 Time allowed. those which have not been in contact and also on the former the sul-phide leaves a darker and more indelible stain than on the latter.) (5.) The difficulty of keeping the temperature of the bath exactly constant for any length of time except the temperature employed be 100" c. (6.) And lastly the fact that the thickness of the copper varies sufficiently (especially when it is in foil as in the majority of the above cases) to cause the same weights of it to show considerably different amounts of surface.But in spite of the impossibility of getting very accurate and con-cordant results by performing a number of experiments in each case,% sufficient concordance is obtained to leave no doubt whatever as to the true reaction. The amount of variation may be seen in Table VII (and indeed in most of the previous tables) where the extreme results obtained in a few classes of experiments are given. Percentage composition Percent- of insoluble age of residue. Copper dissolved. -TABLE VII. Shouting Exteqit of Variation. of Results. 3. Pure 4. Pure ., 5. Pure ,. 6. Pure 8.Impure} " * ' ' 9. Impure . . . . . 7. Impure 100 100 135' 137 100 100 100 __I-I 1 tugs. 90 ) 97.0 90'6 90 , I 85.6 1 72.1 cus. Proportion of Co;Fper as I cu,s. cuso4 -- I - - I - 14.58 85 -42 18 *18 1 81.82 18.12 I 81 *88 16 so0 j 84 .oo $ XVII. Othei- Opiiiions as to the Action. The explanation of the action of sulphuric acid on copper given above is a t variance with t,hose already given by others who have studied the reaction. Berzelius (Tratite'de Chimie iv 324) mentions the formation of a black insoluble powder oxidisable by hydrogen nitrate and which * I have performed about 250 experiments on this subject. t Same piece of copper used in both experiments 136 PICKERING ON THE ACTION OF " appears to be " copper subsulphate though this body requires only 56-76 per cent.of copper whereas the copper in the residue after the extraction of free sulphur is never less than 66.316 per cent. in addi-tion to which recent experiments show that copper subsulphate is a soluble and not an insoluble body. B a r r u e l (Journ. de Pharnz. xx 13 1834) showed that the action between copper and sulphuric acid takes place at ordinary tempera-tures if sufficient time be allowed ; though he obtained a far smaller amount of action than that given in Table I. He states as has also heen noticed above that a t low temperatures the sulphur dioxide formed is dissolved in the liquid and he considers that this reacts on the copper forming protosulphide and protoxide of copper the latter afterwards dissolving in the acid.The only experiment which he adduced in support of this view was to enclose some copper in a her-metically sealed tube with some sulphurous acid. After six months a brownish body appeared in the liquid containing sulphur and copper, and whicli there is no reason to doubt was a sulphide of copper. But though sulphur dioxide in sohition may attack copper under certain circumstances still it is not difficult to prove that in the action of sul-phuric acid on copper it does not do so to any appreciable extent for if some copper be heated in an aqueous solution of hydrogen sul-phite and a constant current of snlphur dioxide be drawn through the liquid during the experiment it will be found that the copper remains perfectly untarnished and nnattacked whereas under similar circum-stances but using hydrogen sulphate instead of hydrogen sulphits a large amount of copper is found to have been dissolved and converted into sulphide.Rarruel like Berzelius does not appear to have made any quantitative analyses of the black residue. Maurnen8 (8.12~. Chim. Ph?ys. 1846 3rd series xviii 311) after performing several analyses of the insoluble residue arrived at the fol-lowing conclusions as to its formation and composition :-That besides the main reaction forming copper sulphate sulphur dioxide &c. there is another distinct reaction consisting of four dis-tinct steps and giving rise to four different bodies-(1.) Copper subsulphide Cn,S. (2.) An oxysulphide of copper Cu0,2Cu2S 01- Cu,S,O. (3.) Another oxpsulphide of copper CuO,2CuS or Cn,8,0.(4.) Another oxysulphide of copper CuO,CuS or Cu,SO. The first of these steps namely the formation of the subsulphide is the same as that which I consider takes place. As regards the other steps it is difficult to understand how Manmen6 failed to see that) the percentage of copper and sulphur together invariably made up 100 per cent. of the black residue. Th SULPHURIC ACID OX COPPER. 187 explanation perhaps lies hid in an unintelligible passage which occurs in his paper (p. 316) in which he mentions that granulating the copper was an expedient whicli he used to render easg the separation of the oxysulphide from the unattacked copper and at the same time to prevent any copper remaining unattacked. In the same passage he refers to a “ matter insoluble in acids,” which existed i n large quanti-ties in the copper turnings which he employed ; this matter concen-trated itself in the copper sulphides increasing their weight by as much a,s one-twentieth and in many of his analyses he made allowance for this though in what way he does not state.The first oxpsulphide contains nearly the same percentage of copper as the subsulphide ; the others contain percentages intermediate between that of the sulphide and protosulphide. Thus-Cu2S contains . . Cu 79.814 S 20.186 0 0.0 CuO.ZCu,S contains Cu . . 7!>-82l S . . 16.150 0 . . 4.029 cuo.2cus ?> Cu 70.356 S 23.726 0 5.918 c u 0 . c u s )) Cu . ‘72.511 S 18.339 0 9.150 cus ? ? Cu 66.408 S 33.592 0 0.0 The last three steps which he considers to take place suppose the formation of three bodies the existence of which has never been proved and is not supported even by the majority of his own analyses, as he says that the percentage of copper or sulphur required by the formuls Cu4Sz0 Cn,S,O Cu2SO were never exactly obtained by him and the strongest proof which he adduces for the formation of these bodies is the fact that Pelouze has shown the existence of a body 5CuS,CuO.If Maumen6 had let the action continue longer, he would have ascertained that the last state of the residue is by no means one in which the copper- amounts to as much as 72.414 per cent. but only to 66.316 and taking into consideration the free sul-phur which he does not appear to have recognised indefinitely less.In addition to the improbability of copper oxide being formed and remaining unattacked in presence of excess of strong hydrogen sulphate the following experiment disproves its formation. A piece of copper bent at right angles was placed in a large watch-glass containing hydrogen sulphate so that half the metal should be im-mersed in the acid and half should project into the air the whole heing placed under a bell-jar t o prevent diminution in the density of the acid by absorpt,ion of moisture from the atmosphere ; the portion of the metal in the air being unacted on or acted on only to a very small degree performed the part of an electronegative metal towards the copper in the acid which becoming thus more electropositive was acted 011 to a greatly increased extent (on an average about five times as much as if it was entirely immersed) ; and instead of the relative VOL.XXXIII. i 138 PICKERIKL'G ON THE ACTION OF SULPHURIC ACID ETC. proportion of sulpliide formed being increased as when rendered electropositive under other circumstances it was greatly decreased, so that only a trace of it could be discerned. Now this could not be due to the fact of the copper being made electro-positive since doing so incyeases the amount of sulphide ; it must therefore have been due t o the copper sulphide being oxidised a t the time of its appearance by the oxygen which would be liberated at the surface of that portion of the copper which is immersed in the acid since the whole arrangement would form a galvanic cell consisting of a metal a liquid and a gas.If therefore this black residue consists of or contains copper oxide it i 3 wholly imonceivable that the action of more oxygen can diminisli the amount of oxide formed especially when this oxide according t o Maumen6 is the peroxide. The fact that the presence of free oxygen will diminish the proportion of the insoluble residue was also shown by acting on some copper first with acid carefully freed from oxygen by boiling and by passing a current of carbon dioxide through it and then with acid through which a current of air was passed throughout the experiment a larger proportion of sulphide being formed in the former than in the latter case. The determinations of the sulphur dioxide made also give conclusive proof that the residue can contain no oxygen.The fact already noticed that the sulphide is a t first of a brownish colour does not argue at all that any body besides the sulphide is formed nor does Maurnen6 consider it to do so since he agrees with me that a t the time when the brown colour is niost conspicuous the body consists undoubtedly of copper subsulphide only ; the brown colour at first visible appears to be due merely t o the smallness of the amount of the subsulphide present which moreover is exactly similar to that produced by a small quantity of the protosiilphide when suspended in a liquid. Unfortunately Naumen6 gives no particulars as to the circumstances under which his cxperimeuts were performed so that they cannot be repeated. Calvert and Johnson (Jourm. Cham. SOC. xix 435 1866) per-f'ormed a few experiments on the relative effect of strong and dilute sulphuric acid 011 copper at temperatures of 130" amti 150" C. the re-snlts of which as mentioned above agree moderately well with those given on p. 134. T h y mention the formation of copper subsulphide, and consider it to be due to the liberation of free sulphur which latter afterwards combines directly with the metal. This view they sup-ported by no experinleiits on the subject or analyses of the sulphide, but held it for the following reasons :-(1.) No hydrogen sulphide is evolved. (2.) Sulphur is volatilised GLADSTONE AND TRIBE ON THE ACTION ETC. 139 (8.) From the similarity between this action and that of sulphuric acid on tin, The first two of these reasons have already been discussed and much weight can scarcely be given to the last reason when in the account of their experiments on the action of hydrogen sulphate on tin given in the same paper they state “that hhe action of various strengths of sulphuric acid upon tin differs entirely from that which they exert upon copper.” They failed to obtain any action below 130” C. and considered that none took place in spite of the fact that Barruel as early as 1834 had proved that it took place even at ordinary atmo-spheric temperakures. Experiments detailed above show that tthe difference in action obt,ained at low temperatures (except indeed where the action 5Cu + 4H,SO = Cu,S + 3CnS0 + 48,O alone takes place) up to 270” C. is one of degree only and not of kind