60 CUNDALL :VITL-The Action of Xuiphuric Acid on Copper.By (the late) JAMES TUDOR CUNDALL (with appendix byMUNGO MCCALLUM FAIRGRIEVE).SOME time ago, in order to demonstrate the formation of coppersulphide in the preparation of sulphur dioxide by the action ofconcentrated sulphuric acid on copper turnings, I poured the darkliquid obtained into a large quantity of water, when I was surpriseTHE ACTION OF SULPHURIC ACID ON COPPER. 61to find that a bright red precipitate was produced instead of ablack one.A number of experiments were then made, in which differentsamples of copper were heated under varying conditions withsulphuric acid in order to investigate the nature of this precipitate.It was found a t first that the formation of the red precipitate wasapparently capricious, variations in the sample of copper, and ofsulphuric acid, in the time of action, and in the temperature havingno constant effect; but in general it was noticed that the brightestred precipitates were usually obtained when the copper andaulphuric acid were heated until a brisk action occurred, the flamethen removed, and the liquid allowed to remain a little beforebeing poured off.The necessary delay was shorter the more finelydivided the copper. I n order to avoid contaminating the precipitatewith copper sulphide or particles of copper carried over mechani-cally in the liquid, the latter was filtered through asbestos in aGooch crucible into the water.It was then found that not only was the red precipitate formedwhen a sample of copper was acted on by successive quantities ofacid, but also if portions of the liquid were drawn off a t all stagesof the action until the copper was nearly all dissolved.The red precipitate was formed most strikingly when the filtrate,although clear, was of a reddish-brown colour, and only appearedas the l i p i d diffused into the water.It was extremely finelydivided, only settling after some hours. When collected and wmhedon a Gooch filter it assumed a metallic appearance, forming brightfilms of a coppery lustre t.hat floated in the crucible. The weightsof the precipitate were small, and could not be materially increasedby increasing the amount of the acting substances or the time ofaction. They varied from 0.03 to 0.22 gram in weight, and werequite similar whether commercial copper turnings, “ pure precipi-tated copper,” finely divided copper obtained by reducing purecopper oxide in hydrogen, or very pure electrolytic foil free fromarsenic were used.The precipitates were then analysed, 0.135 gram yielding 0.135gram of copper, and 0.223 gram yielding 0-220 gram of copper.They were therefore practically pure copper.Other samples wereexamined for sulphur by conversion into barium sulphate, yieldingonly 0.24, 0.16, and 0.03 per cent.The red precipitate being therefore metallic copper, is formed inall probability by the decomposition of cuprous sulphate by wafer,as shown by Recoura (Compt. rend., 1909, 148, 1105):and 8 sample of cuprous sulphate prepared by Recoura’s methodc ~ 8 o , = c u s o , + c u .. . . . . (1gave with water a red precipitate identical in appearance with thatobtained by the process described above.Suspicion having been aroused that the concentration of thesulphuric acid was the condition deciding the formation of the redprecipitate, a series of experiments was made, in which sulphuricacid of various strengths was made to act on pure copper foil. If" pure distilled sulphuric acid " (D15 1.8399) containing approxi-mately 95.4 per cent. of H,SO, was used, the red precipitate couldnot be obtained, and i t was only on diluting it until the percentageof HzSO, was reduced to about 94 that it definitely appeared,becoming well marked a t 91 per cent., and perhaps at its best atabout 87 per cent.A t lower percentages red precipitates wereproduced, but the action of the acid on the copper only continuedof itself if the mixture was heated repeatedly.On examining the course of the action in the various cases itwas seen that the reddish-brown liquid mentioned above as givingthe red precipitate best when poured on to water was always t obe noticed as pouring off the copper, but whereas in the weakeracids it persisted, in the more concentrated it rapidly became darkgreenish as it passed away from the neighbourhood of the copper.If the reddish-brown liquid is allowed to remain or is gentlyheated it darkens and deposits a greyish-black precipitate. Thisprecipitate when collected and washed resembles cuprous sulphide,and a portion weighing 0.0210 gram gave 0.0158 gram or 75 percent.of copper (Cu,S=:79.9 per cent. Cu and crUS=66*5 per cent.Cu).When more strongly heated the reddish-brown liquid becomesgreenish-black, and deposits a greenish-black precipitate, of which0.318 gram gave 0.2114 gram (=66*5 per cent.) of copper, and wastherefore pure cupric sulphide, although if the heating is continuedthe precipitab dissolves with evolution of sulphur dioxide :It thus approximates t o cuprous sulphide.CUS + 4H2S04= CuSO, + 450, + 4H,O . . . (2)The final result of the reaction, if carried as far as possible,results in the maximum production of sulphur dioxide, as indicatedby the equation:showing a ratio of copper dissolved to sulphur dioxide set free of1 to 1 (which may be observed by w i n g that no solid residue isobtained).I f the cupric sulphide is left unacted on, the ratio of copper tosulphur dioxide rises to 3 : 1, and the ratio of copper as coppersulphate t o copper as sulphide becomes 5.7 : 1, a relation whichmay be expressed by the formula:C?u+2R2S04=CuS04+SOz+2H,0 .. . . (3)6Cu + 8H2S04 = 5CuS0, + CUS + 250, + 8H90 . . (4THE ACTIQN OF SULPHUHIC ACID ON COPPER.. 63If plenty of copper is lsft unacted on, cuprous sulphide isobtained, and the ratio of copper used to sulphur dioxide producedrises to 6 : I, the equation for this being:GCU + 6H2S0, = CUSO, + CU,S + SO, + 6H2O . . (5)Note on Abozie.The late Mr. Cundall left the draft of his research thus inc0.m-plete, but from an earlier manuscript and from conversation withhim I gather that he might hawe completed it and summarisedhis results somewhat as follows:The primary reactions of copper and sulphuric acid may berepresented by the following equations :(0) 8Cu + 4H2S04 = 3c’u2s04 + C 9 S + 480,( h ) 2Cu + 2H,SO, = C!u,S04 + 2H20 + SO,,(c) 5Cu2S0, + 4H2S0, = Cu,S + 8C%SO, + 4H20,( d ) crU,8 t 2H2S0,= CuS + CuSO, + 2H20 + SO,,(e) CuS + 4H280, = Cuso, + 450, + 4H20.Ckprous sulphate is the primary product, both when the sulphuricacid is concentrated, and when it is diluted with water [“Thereddish-brown liquid was always to be noticed as pouring off thecopper,” see above], although a maximum value is given when thediluted acid approximates to one containing molecular proportionsof sulphuric acid and water. In hot concentrated acid, however,the cuprous sulphate is even more unstable than in the slightlydiluted acid, and reacts almost at once, as indicated by equation ( c ) .Equation (a) is apparently specially a lcw-temperature reaction,and it is interesting t o note that if combined with (c) in theproportion of 5 (a) + 3 ( c ) we get:5cU + 4H&30, = cu2s + 3cuso4 + 4H20,one of Pickering’s fundamental reactions (T., 1878, 33, 112).whilst the secondary reactions of the products formed are :andEquation (5) is made up of a+ 2b + c,To get the ordinary equation for the completed action (3) it is,of course, possible t o use either I ‘ a ” or ‘‘ b ” in combination withMr.Cundall’s results, therefore, on the whole confirm the finalresults of Pickering, although the mechanism of the changesinvolved is different, inasmuch as cuprous sulphate is regarded asthe primary substance formed.SCIEXCE DEPARTMENT,THE EDINBUXGH ACADEMY.and (4) ), ,, U.+2b+c+2d.( I c,’) KC d,” and (( e.>