104 LAURIE THE CONSTITUTION OF VI.-T%e Constitution of the Copper-Zinc and Copper-Tin Alloys. By A. P. LAuRrE B.A. B.Sc. IF instead of a simple metal a mixture of two metals is introduced into a voltaic cell the E.M.F. of the cell is that of the more positive metal. For instance a mixture of zinc and copper will give the E.M.F. of zinc. Several experiments in a cuprous iodide cell with plates of copper having pieces of zinc soldered into them of about 3$m of the whole area of the plate showed this. On first connecting with the electro-meter the deflection was about half that for zinc (the deflection for copper being of course zero) but it gradually crept up ultimately attaining a value some 4 or 5 hundredths of a volt below the valu THE COPPER-ZINC AND COPPER-TIN ALLOYS.105 for zinc. This gradual increase of E.M.F. waq due to some change i n the electrical condition of the compound plate and is probably to be explained by the gradual polarisation of the copper plate by the local action between i t and the zinc soldered on to it. But in what-ever way these points are explained the results are sufficient t o show that a mixture of two metals gives the E.M.F. of the more positive metal and that this holds good as long as at least 0.2 per cent. of the positive metal is present. If however in place of a mixture of zinc and copper we have an alloy of the two metals which has been formed with a development of heat and which therefore is of the nature of a chemical com-pound the result will probably be different for it is obvious that the dissolution of the zinc will in this case necessitate the decomposition of this zinc-copper compound whereby energy is absorbed and therefore the E.M.F.of the cell will probably be correspondingly lowered. Now there are three possible ways i n which zinc-copper alloys may be constituted. First they may be merely mixtures of zinc and copper; in that case they would give the E.N.F. of zinc in the voltaic cell. Second they may be of the nature of the solution of sulphuric acid in water; in that case a series of such alloys beginning with 100 per cent. of copper and ending with 100 per cent. of zinc would probably show a gradual rise of E.M.F. in the cell from the value for copper to that for zinc. Third one or more of the series of alloys may be a definite atomic compound the rest being solutions of this cornpound or compounds in an excess of zinc or copper; in that case the E.M.F.would probably rise by a jump when a series of alloys were tested a slight excess of zinc over that necessary for the compound causing a great alteration of E.M.F. in the cell. Further this jump would probably occur where the percentages of zinc and copper corresponded with some simple molecular formula. I n practice the difficulty which presents itself is that of obtaining homogeneous alloys. This difficulty can only be overcome by taking great care in their preparation and by preparing very large numbers of them. In this way any error from this cause may be expected to show’ itself. My zinc-copper alloys were prepared as follows -A certain quantity of copper and of zinc were weighed out, amounting together t o about 9 of an oz.and fused in a Fletcher’s furnace under borax ; when fused the alloy was vigorously stirred with an iron rod coated with borax and poured instantly into a little trough cut i n a block of charcoal. The ingot thus formed was cooled 106 LAURlE THE CONSTITUTION OF weighed and then broken across; if it seemed homogeneous in section the upper layer was filed off the ingot marked and laid aside for testing by the electrometer. Some zinc always burned off and the percentage of zinc and copper was calculated from the weight of copper taken and from the weight of the alloy obtained; this of course assumes that there is no loss of copper-in fact it is but very slight and also necessitates clean casting which is easily managed if the flux is sufficiently fluid.The stirring was always kept up till the last instant before emptying the crucible. The alloys corresponded in appearance to the descriptions already published the yellow tough “brass ” being replaced by white brittle alloys when the copper was diminished below 50 per cent. and these again giving place to hard grey alloys when it was below about 15 per cent. On testing a piece of brass wire in the cell it gave no deflection, showing a diminution of E.M.F. = 0.6 volt. It probably contained about 30 per cent. of zinc. One of my alloys containing about 25 per cent. of zinc gave the same result. As 40 per cent. of copper was approached a slight increase of E.M.F.showed itself amounting at 36 per cent. of copper t o about 0.1 volt ; so that from nothing up to 66 per cent. of zinc the E.M.F. had gradually risen about 0.1 volt. It gave a deflection of about 0.5 volt. All the alloys containing less than this amount of copper though varying a good deal continued on this higher level of E.M.F. The results are qiven in detail in Table A (p. 107) and Plate 4 (p. 116). The want of homogeneity of the alloys is brought out by this table but a t {,he same time is not sufficient in any way to conceal the striking shape of the curve of E.M.F. showing its sudden rise. It is to be remembered that although the alloys may have an average composition very close to that given in the table the electrometer will select t,he portions of the alloy containing an excess of zinc.These experiments indicate in the first place a considerable develop-ment of heat in the formation of brass lowering the E.M.F. of the cell about 0.6 volt and further that the greater part of this develop-ment of heat is due to the formation of a compound CuZn (32.8 per cent. Cu). After these experiments were completed I determined to test whether the zinc-copper alloys could be separated by differences in their specific gravity. Por this purpose imitating Matthiessen’s experiment I raised a porous pot about 3 in. high by 8 in. across to a bright red in a Fletcher’s furnace and poured into it an alloy containing a little over 34 per cent. of copper. The furnace being again closed and the The next alloy tested contained about 33 per cent.of copper THE COPPER-ZINC AND COPPER-TIN ALLOYS. 107 TABLE A.-Zinc copper Alloys. Percentage of copper. 100.0 75.0 54.4 50.5 49.3 47.2 46.1 4;3*9 41.7 37.0 36.5 35.9 33.8 S3.2 31.9 319 30.3 29.2 28.0 26.8 26-2 25.7 24-3 23.0 19.1 18.7 16.4 16.2 14.5 14-1 12.4 12.1 3.5 Zinc. E.M.P. in volts. - 0.020 -0.020 -+ 0.040 0.070 0.070 0.075 0-065 0.070 0.080 0.085 0.085 0.160 0.530 0.520 0.540 0.580 0.570 0-600 0.580 0-490 0.460 0.5 70 0.475 0.600 0.580 0-420 0.580 0.580 0.590 0.580 0.590 0.600 0.590 0.600 blast kept on the alloy was kept fused for at least 30 minutes. It was then allowed to cool the porous pot broken and the cylinder of alloy after it had been filed flat on one side was connected with the electrometer and gradually lowered into the cell.If there had been any regular settling out of alloys according to specific gravity one end should have had a higher E.M.F. than the other. If then the end of lowest E.M.F. was first plunged in the liquid its gradual immersion would cause an increase of the deflection on the scale. No such effect was produced however for no matter which end was first plunged in the E.M.F. continued at about 0.1 volt varying slightly. The same alloy fused up with fresh zin 108 LAURIE THE CONSTITUTION OF to reduce the percentage of copper to about 20 per cent. gave a11 along its length an E.N.F. of about 0.5 volt varying slightly. There is therefore no serious want of homogeneity produced by differences of specific gravity All these experiments tend to confirm the trust-worthiness of the results first obtained.As the slow cooling in an upright position seemed to have no injurious effect six alloys were made up of about 2 oz. each and cast in rnouldei~’~ earth in an upright position as sticks about 2 in. long and A portion about 1 in. long mas broken from each of these sticks by knocking off the two ends ; it was then filed flat on one side and plunged into the cell. The results with these six sticks are given in Table B. I t will be noticed that with one exception the deflections increase as the zinc increases with con-siderable regularity. They are evidently more homogeueous than the first set of alloys.in. in diameter. in it My next experiment was as follows. Taking alloy No. ti of the set Table B I ground it to powder and tested its E.11II.F. by packing into a minute glass vessel shaped like a tobacco pipe with a platinum wire down the stem and into the bowl this bowl being plunged beneath the liquid in the cell. It gave an E.3I.F. of about 0.5 volt but as the experiment failed unless the dust was wet and carefully packed no deflection being obtained I next filled the bowl with mercury mixed i n some of the dust and obtained again an E.M.F. of about 0.5 volt. Some of this dust was next treated with dilute sulphuric acid ; hydrogen was given off but after boiling f o r some time with the acid all effervescence ceased or almost ceased, although fresh acid was put on.The acid contained some zinc but no copper. On burnishing the dust it gave the same bright white lustre as before. The dust when introduced into the bowl of the glass pipe both with and without mercury gave the deflection of about 0.08 rolt so that apparently the excess of zinc had been removed by the acid. TABLE B. Percentage of copper. 34.1 31.5 31.0 30.6 30.1 29.9 E.M.F. in volts. 0.160 0.420 0.560 0.480 0.5 30 0.570 These results certainly indicate the existence of a compound, CuZn, the sudden rise of E.M.F. being difficult to interpret in any other way and seemed to me sufficiently encouraging. At the sam TKE COPPER-ZINC AND COPPER-TIN ALLOYS. 109 time the zinc-copper alloys and especially those containing a high percentage of zinc are difficult of preparation and very little is known about their physical properties.It seemed better therefore to select for further experiments some well-known series of alloys in order to check the results of this new method against the conclusions of other experimentalists. For this purpose the tin-copper alloys seemed especially suitable. In the first place Riche (Compt. rend. 55 1862) states that the curve of densities for these alloys has two maxima corresponding respectively with the alloys Cu4Sn and Cu3Sn. The carve of den-sities given in Thurston’s lZeport* on the copper-tin alloys though not showing two maxima rises to a maximum over the region of these two alloys falling off towards copper and towards tin. Riche further states that these two alloys alone are not subject to liquation.Matthiessen (PhiZ. T y a n s . 1860) failed to find any indication of the existence of these compounds on testing the electric conductivity of these alloys but this has been shown by Lodge to be due t o his not having investigated that portion of the curve. Lodge (Phil. Afag., 1879) finds a minimum conductivity at Cu4Sn and a maximum con-ductivity at Cu,Sn. Again the curve for heat conductivity given by Calvert and Johnson (Phil. T r a n s . 1858) shows a minimum at Cu4Sn and maximum at Cu,Sn. Finally the induction balance curve as determined by Roberts-Austen (Phil. X a g . 1879) shows the same maximum and minimum. There is consequently a con-siderable amount of evidence pointing to the existence of the com-pounds Cn3Sn and CulSn in this series of alloys a mass of evidence first I believe collected and pointed out by Roberts-Austen.These alloys therefore seemed to be peculiarly suitable for testing the new method. The compound CuaSn I did not expect t o find but the com-pound Cu3Sn would in all probability be indicated. In the first place, however I thought it best to again test the behaviour of compound plates adopting a different arrangement. A Daniel1 cell was constructed containing a large cylindrical copper plate coated with india-rubber on the back and with a front surface of 500 sq. cm. This plate was placed in n solution of sul-phate of zinc. A glass tube plugged with plaster of Paris and con-taining a copper wire in a solution of sulphate of copper formed the other pole of the cell.This wire was coiinected permanently to the electrometer. A thin strip of zinc of breadth 5 mm. wa,s arranged so that it could be covered to any depth in the solution. Either the zinc plate or the copper plate or both could be connected t o the electrometer by means of mercury cups. The copper plate was first * Report on Copper-Tin Alloys United States Board to test iron steel &c.: Washington 18’79 110 LAURIE THE CONSTITUTION OF connected. It gave n small deflection of about 0.03 volt. The zinc plate was then immersed in the solution to the depth of 5 mm. and then connected by a wire to the copper plate ; immediately the spot of light on the scale began to move and time observations were taken. The results are plotted on Plate 1 (p.116). The deflection for the zinc alone is also marked on the curve. On disconnecting the zinc plate, the spot of light moved steadily down to the old deflection for copper alone an apprecialde time elapsing before the polarisation of the copper plate ceased. The zinc on removal was found to be corroded, a considerable amount of zinc having been consumed in polarising the copper plate. Similar experiments were made with a cell con-taining an acid solution of stannous chloride and a copper wire coated with cuprous chloride for the other pole. In this case also a similai-curve was obtained on connecting a tin plate to the copper plate in the same way. In both cases the surface of the positive metal amounted to about -&5 part of the surface of the copper plate.No experiments were made to test the effect of distributing the tin over the copper surface though I am disposed to think that this would pmbably increase the rate a t which the plate becomes completely polarised. These experimenh seem to me to open out an interesting method of investigating polarisation. By weighing the zinc before and after some light might be thrown on the thickness of the molecular layer necessary to polarise the copper plate but I did not wait to make any such experiments. I f then one of our tin-copper alloys contained only 0.1 per cent. of its surface of tin merely mixed with it we might hope to detect i t on the electrometer. Preparation of the Alloys. The alloys were prepared from the best grain tin and the best copper 99.9 per cent.obtained from Johnson and Matthey’s. A weighed quantity of copper was in each case melted in a plumbago crucible under charcoal hydrogen bubbled through it to be sure of the absence of suboxide remored from the furnace a weighed quantity of tin rapidly added and thoroughly mixed and the alloy poured into an open mould. The casting formed a cylinder about 13 cm. long by 0.8 em. diameter. The aggregate weight of tin and copper was 100 grams. Prof. Roberts-Austen kindly allowed these alloys to be prepared with the assistance of Mr. Groves a t the Royal Mint. They had thus the advantnge of being made by one skilled in the melting and mixing of metals. They had all a fine appearance on fracture. The alloys prepared were as follows : THE COPPER-ZINC AND COPPER-TIN ALLOYS.111 (1.) (2.) (3.) (4.) (5.) (6.) (7.) (8.1 Cu percent 100 95 90 85 80 75 70 68.16 (9.) (10.) (11.) (12.) (13.) (14.) Cupercent . 67 64 61.64 60 40 0 8 and 11 correspond to the two supposed compounds Cu,Sn and Cu4Sn. Subsequently a fresh set of alloys were prepared of the following percentage composition :-(1.) ( 2 . ) (3.) (4.) (5.) (6.) T’inper cent . 37 38 39 40 41 42 I t was thought a,dvisable to analyse certain of those alloys that were shown to be of especial importance in the ultimate results. These analyses agreed pretty closely with their supposed composition. Alloy 11 was found to contain 61.7 of copper. Alloy 12 analysed at both ends was found t o contain 60.4-60.6 Alloy of 39 per cent.in the second series was found to cont:iin per cent. of copper. 61.2 of copper. T h e Cells used. The cells used consisted either of a solution of stannous chloride and a copper wire coated with cuprous chloride or of stannous sulphate and a copper wire within a porous partition containing copper sulphate. Such tin cells have been carefully tested and found t o yield a definite E.M.F. (Phil. Mug. 1886 [5] 21 13). Experiments with the Electrometer. I first determined to test all the alloys in succession in a solution of stannous chloride acidified with hydrochloric acid. The composi-tion of the solution was as follows SnC1 33 grams water 1750 c.c., strong pure hydrochloric acid 17.5 C.C. The cell consisted of a shallow glass dish with a wooden cover drilled full of holes through each of which one of the sticks could be dropped and its E.31.P.tested by the electrometer; in this way all the alloys were tested under exactly similar conditions. The othei- pole of the cell con-sisted of a copper wire coated with cuprous chloride. The results of the first set of obsemations in which the rods were polished with emery paper and then immersed are shown in Curve ( a ) , Plate 2 which gives the mean of this and the next two methods of testing the alloys. The numbers thus given are the mean of two or three independent sets of observations. In the first place the sudde 112 LAURIE THE CONSTITUTION OF rise in E.M.F. on passing Cu,Sn. will be noticed. No corresponding effect is observable on passing Cn4Sn but the alloys give slightly irregular deflections in the cell as shown by the zigzag of the line joining the observations.The irregular numbers are probably clue to ephemeral causes the metals not entering into the action of the cell and consequently coatings of gas &c. producing variable deflec-tions as has been noticed in the case of platinum plates. Certain precautions are necessary in order t o get trustworthy and uniform results in experiments of this kind. I n the first place the cell used should be so designed as to give a definite E.M.F. free from polarisation effects such as the Daniell cuprous chloride or iodine cell. I n the second place the solution in which the alloys are placed should not direct,ly attack the metals in the alloy. I n the third place, the salt used should be a salt of the more positive metal.For instance I have failed to obtain constant results from a cell contain-ing common salt in place of stannous chloride no doubt due to gas, polarisation of the alloy complex secondary actions &c. In the fourth place the alloys must always be carefully cleaned with fresh emery paper before being plunged in the solution. With the elec-trometer internal resistance can be ignored and many arrangements of cells are consequently available. For instance I sometimes place the inner solution in a glass tube drawn out to a fine c;zpill,sry o r sometimes use as my cell a U tube constricted at the bottom and plugged with plastey of Paris. With reference to alloy 12 a t which the rise occurs two points are worthy of notice.First the rise is instantaneous there is no creeping up of E.M.F. as in the case of the compound plates described before. This is probably due to the distribution of the free tin all over the surface of the alloy instead of being located at one point. The next point is that in about 10 or 15 minutes the deflec-tion disappeared. This can easily be accounted for by supposing the free tin t o exist in pockets and not in veins ; these pockets wouId soon be consumed loy the local action and the E.M.F. would Lhen fall. One confirmation of this is the fact that it is only necessary to reburnish the surface with emery to restore the origiiial E.M.F. Some additional facts favouring this view will be mentioned presently. It is of some interest to note the actual surface of free tin to which the deflection is probably due.If we suppose 1 C.C. of the alloy to be immersed in the solution then the whole area of the rod present, leaving o u t the end amounts to about 2.5 sq. cm. I f we suppose the alloy t o contain 1 per cent. of tin evenly distributed over the surface, this gives 0.025 sq. cm. as the actual surface of tin in action. The E.M.F. of the alloys were next tested in a solution of stannous chloride containing no free acid. The results are given in Curve ( a ) THE COPPER-ZIKC ASD COPPER-TIN ALLOTS. 113 Plate 2. The only difference noticed here was the very gradual disappearance of the deflection of 12 the X.M.F. remaining practically the same for an hour. The polished surface of the alloys having thus been tested t8hey were next broken across the centre and the broken ends plunged in the solution.The yesults are given in Curve (a) Plate 2. The next experiment -as to test the effects of amalgamation. Alloy 11 and alloy 12 were both amalgamated. The E.M.F. were respectively 0.008 volt and 0.26 volt. The only difference caused by amalgamation was that the deflection given by alloy 12 was now permanent. This is of interest as confirming the view taken of the condition of the free tin on the surface. The mercury doubtless eats into the a.lloy and dissolving the tin brings it to the surface. It will also tend to check the violence of the local action. It is also curious to note that the alloy 11 is not affected by amalgamation m i l that consequently the compound is seemingly unaffected by solution in mercury.The alloys were next tested in a cell consisting of stannous sulphate in acid solution and copper sulphate round a copper wire, divided by a porous partition. The results are given in Curve ( b ) , Plate 2. It will be noted that a distinct deflection is given by all the alloys. Alloys of the second series were now prepared and tested. The results obtained were the same and have not therefore been plotted. The alloys were all amalgamated. One alloy however behaved differently namely 3. This alloy gave uncertain values sometimes high sometimes low. If the results of the two groups of alloys are compared together we find that-1st series alloy containing 38.3 per cent. tin gives the low deflectioii 0.008 volt. 2nd Beries alloy containing 38.8 per cent.gives an irregular deflection. 1st series alloy containing 39.5 per cent. gives a definite but vanishing deflection 0.26 volt. 2nd series alloy containing 40 per cent. (not analysed) does the same. 2nd series alloy containing 41 per cent. (not analysed) gives a permanent high deflection. 2nd series alloy containing 42 per cent. (not analysed) gives a, permanent deflection. It will be noticed how closely their percentage compositions and their behaviour on the electrometer agree. Having completed these measurements I next searched carefully for CuaSn by two methods. I n the first place I used a cell in which They are the mean of two distinct sets of observations. VOL. LIII. 114 LAURIE THE CONSTITUTION OF copper gave a definite E.M.F.as well as tin so as to get rid of the small irregular deflections noticed in the former cells. For this purpose silver and sulphate of silver were substituted for the copper and sulphate of copper. In this cell copper gave a deflection of 0.482 volt and tin of 0.965 volt consequently a considerahle deflection for copper was combined with a marked difference of E.M.F. between copper and tin. There was no indication here of a CuiSn compound, the E.M.F. not rising in two jumps but only one. The second method was to use a cell in which copper gave a large deflection but in which the difference between copper and tin was small. Here I hoped that in the region between CutSn and Cn,Sn the curve would dip owing to the necessity of decomposing the mixture of the two compounds.No indications of this have been obtained as the following figures Copper 0.56 volt; alloy 5 0.55 volt; alloy 10 0.60 volt; tin, 0.73 volt. These experiments complete the work with the electrometer. I next experimented on the possibility of eating out the free tin from an alloy by placing it in the cuprous chloride cell and short circuiting. If the electrometer results were trustworthy an alloy containing an excess of tin over Cu3Sn would be attacked when used in place of the zinc plate in such a cell until the excess of tin had been removed. The E.M.P. would then fall practically to zero and all action would stop Cu3Sn Pemaining unaffected. For this purpose I selected alloy 13 (60 per cent. tin) and cast it into a thin plate about 1 in.x 2 in. X in. thick. This was placed in acid stannous chloride and connected with a copper plate in a paste of cuprous chloride inside a porous pot. After two or three days all action seemed to stop and the plate under the microscope in place of having a rough but homogeneous surface seemed made of bundles of needle-shaped crystals with cavities between. It had not however, been eaten through only a thin outer layer having been affected. This outer layer on being scraped off and analpsed was found to contain 45 per cent. of tin. These results being indefinite a cell was next constructed in which the finely-powdered alloy could be attacked. Another point attended to was making the cell air-tight, so as to prevent oxidation of the cuprous chloride and its partial solution diffusion and deposition as metallic copper on the alloy.The following diagram (p. 115) shows the cell used. This consists of a wide-mouthed 1 oz. bottle containing acid solution of stannous chloride ; in this solution is plunged a copper cup supported by two copper strips. A weighed quantity of the finely-powdered alloy is placed in this cup and covered with a disc OF linen to keep out possible impurities. The bottle is then corked show :-The cell used was an iodine cell THE COPPER-ZIXC AND COPPER-TIN ALLOYS. 115 by a glass tube covered at the end with parchment paper and con-taining a paste of cuprous chloride and a copper plate the whole being filled up with paraffin ; after pushing in the glass tube paraffin is poured round the neck of the bottle.The powdered sample of alloy taken was found t o contain on analysis 36.4 per cent. of tin. The current fyom the cell was allowed to flow through a little copper voltameter the plates of which were occasionally weighed ; after 36 hours the plates no longer increasing in weight the alloy was removed and analysed ; it still contained 44 per cent. of tin so that this experiment was not very successful. A second supply of alloy was again placed in the cup and the cell connected to the voltameter. The time curve of the Voltameter is given on Plate 3. When the curve had reached the point ( a ) the alloy was removed from the copper cup re-ground in an agate mortar and replaced in the cup ; immediately the current took a fresh start as is shown by the curve. When the curve had reached ( b ) the alloy was again removed, ground. and put back; this time very little additional tin was removed and the alloy was therefore considered fit for analysis. On weighing the residual alloy and calculating the tin lost the alloy apparently contained 38 per cent. of tin. On analysis it was found to contain 41.2 per cent. of tin. Evidently then this residual alloy agrees pretty closely in composition with the compound Cu3Sn. The difficulty evidently is to remove the last traces of free tin inclosed in portions of the alloy the re-grinding enabling this to be more com-I 116 CROMPTON AN EXTf!”ION OF pletely done. On putting alloy 11 in the copper cup for 35 hours, no change or indication of electrio current could be detected. These experiments therefore afford strong evidence that Cu3Sn is a compound of definite formula and considerable heat of formation, thus confirming the evidence obtained by other me5hods. It remains to apply the method to less known alloys with the view of throwing light on their oonatitution