BASIC COPPER SULPHATE
I
BY F. S. WILLIAMSON
Three Ihndred and twenty grams of crystallized copper sulphate, CuS04.5H20 were dissolved in distilled water and diluted to four liters. The solution was divided into four portions of one liter each. To these portions were added definite amounts of standard sodium hydroxide corresponding to 0.5, 1.0, 1.5,and 2.0 mols per mol of copper sulphate. Since all the copper was precipitated on adding two mols of sodium hydroxide, there was no point in adding more. The treatment of the precipitate and the method of analysis for sulphate was the same as in the preceding two papers. The copper was determined electrolytically. Approximately one gram of the sample was dissolved in 200 cc of distilled water containing 3 cc of concentrated nitric acid. The copper was then precipitated on a rotating copper cathode, using a platinum anode. Before the electrolysis the copper cathode was cleaned by dipping it momentarily into 1: 1 nitric acid and rinsing it with distilled water. It was then dipped in 95% alcohol and ‘flamed’ by burning of the adhering alcohol. The cathode was ‘flamed’ again after the electrolysis. The results of the analyses are given in Table I.
TABLE I added
0.5 1.0 1.5 2.0
Average
Average
Average
54.28 54.30 57.45 76.08
67.9 67.9 71.9 95.2
17.67 17.85 14.89 0,oo
The first two precipitates agree in composition within the limit of experimental error and indicate the existence of a 1
Williamson: Jour. Phys. Chem., 27, 380, 480 (1923).
790
F . S. Williamson.
Calculated for (CuO)r. SO3 .4Hz0 Calculated for ( C U O ) ~( S~0. 3 ) ~18Hz0 . Found by analysis
Jour. Am. Chem. SOC.,38, 1947 (1916).
CUO
SO8
HzO
67.7 67. G 67.9
17.0 17.9
15.3 14.5 14.3
li.S
Basic Copper Sulphate
791
larger amounts of oxide were darker in color and showed a tendency toward stratified settlement. This was taken as an indication of an excess of uncombined coppej oxide, a conclusion which received confirmation upon a microscopic examination which showed the light green crystals of basic salt and black particles of oxide side by side. The examinations failed to yield any evidence of crystals of basic salt of more than one sort. The crystals were very minute and the evidence of the microscopic examination is of itself perhaps of little importance; but offers some support to conclusions to be drawn later from the results of analysis.” When discussing the results Stearn and Young say that “the first fact of importance to be noted is the negligible effect of temperature on the composition of the salts formed under otherwise like conditions. So closely do the analyses of the salts produced a t the three temperatures check with one another, that it is safe to conclude that, within the limits investigated, the composition of the salts is independent of the temperature. “In the second place, it is to be noticed that the amount of copper oxide in the salt increases with the amount added, as is to be expected. The amount of copper oxide which can be assimilated by one molecule of copper sulphate seems to be limited to about two molecules. The evidence furnished by the experiments is not sufficient t o fix this limit beyond doubt, since the next higher amount of copper oxide used was four molecules to one of copper sulphate. Since, however, the salt produced in these cases contained large admixtures of unattacked copper oxide, and since the copper oxide in the solution was reduced practically to zero in all cases where two molecules of copper oxide to one of copper sulphate were used, and not where less than this amount was used, the presumption is very strong that this amount represents very closely the limit assimilable under the conditions of these experiments. “Another conspicuous fact shown by the tabulated re-
.
792
F. S. Willianzson
sults is that in all cases the amount of water decreases as the amount of copper oxide increases, and that as a limit the molecules of oxide and water to one of sulphate approach equality in the most basic salt a t the approximate value of two. I n so far as is deducible from the evidence obtained in this investigation, the most basic salt which could be prepared by the method used would have approximately the formula CuS04.2Cu0.2H20. Whether there is a lower limit to the amount of copper oxide which may be contained in a basic salt is not determinable from the evidence in hand, and remains a subject for further investigation.” In their summary Stearn and Young say that “the evidence obtained by a study of these salts [prepared by the direct action of copper oxide on solutions of copper sulphate] makes it seem probable that they are not definite compounds, but rather are to be regarded as a three-component system in which all three components, CuS04, CuO, and H 2 0 (or if one prefers CuO, SOa, HzO) are continuously variable within limits. The evidence indicates that the maximum number of molecules that are capable of being taken up by one of copper sulphate is two, and that the formula for the most basic salt is probably quite close to CuS04.2CuO. 2H20.” Such a salt would have the composition: CuO, 67.3; SOa, 22.6; and H20, 10.1. There seems to be some question whether Stearn and Young washed their precipitates sufficiently in case there was any adsorption. There is also nothing in their experiments to disprove the possibility of a salt of a formula approximating CuS04.3CuO.4Hz0. It must be remembered, however, that the conditions are quite different. The salt obtained by Stearn and Young presumably represents equilibrium conditions and is apparently crystalline. The salt obtained by precipitation may be merely a case sf adsorption. The constancy of composition in sulphates of aluminum, chromium, and copper is no proof of a definite compound, because the precipitation. takes plaqe under fairly comparab conditions. All
Basic Copper Sulphate
793
that we are not dealing with two phases in the ordinary sense of the term. It would be interesting to heat the precipitates with caustic soda a t 50-25” for 15-88 days before washing them and then to analyze them. Unfortunately, that was not possible at the present time. There is a good deal of evidence that the product obtained by the action of alkalies on copper sulphate solution has the composition CuS04.3CuO. 4H20. By washing copper sulphate containing ammonia of crystallization with water, Kanel obtained the data given in Table 111, using his atomic weights.
TABLE I11
I Calculated for CuSO4.3CuO. 4Hz0 Found by analysis Found by analysis
Cuoby difference
I
67.5 67.3 67.5
1
SO8
I
HzO
17.1 16.4 17.3
1
15.4 16.3 15.2
This same substance was obtained by adding an insufficient amount of caustic potash to a copper sulphate solution, On adding just enough to precipitate all the copper Kane obtained a precipitate which analyzed for CuS04.7CuO. 12H20;but of course there is no evidence that this was not a mixture. Denham Smith2 was unable to make this latter salt. Srqith confirms Kane, however, as to the existence of CuS04.3CuO. 4Hz0, p. 498. “This salt is obtainable in a great variety of ways. It is precipitated when a cold solution of sulphate of copper is mixed with an insufficient quantity of carbonate or of caustic soda, or potash, to decompose it completely. It may be prepared by digesting together cold,-equivalents of sulphate of copper, and of well-washed precipitated osride of copper; by adding a solution of potash to a warm solution of sulphate of copper until a greenish blue precipitate falls, and no copper remains in solution; by treating a cold solqtion Ann. Chim. Phys., [ 2 ] 72, 270 (1B9). Phil. Mag., [31 23, 501 (1845).
794
F . S. Williawson
Calculated for CuSO4.3CuO.4H20 Found by analysis
CUO
so3
Hz0
67.6 67.5
17.0 17.0
15.4 15.5
The results obtained by Pickering2 agree very closely with those obtained by myself. I n the analyses given in Table V, the data refer to the anhydrous salt. Up to 1.4 mols KOH the composition of the precipitate is practically constant. The percentage of copper oxide in Table I is 79.2 which is higher than all except one figure obtained by Pickering. This is quite a little below the 79.9 calculated for CuS04.3CuO; but Pickering thinks that this is due to insufficient washing. In another experiment in which the precipitate was washed a good deal more, the percentage of copper oxide rose to 79.7. If we assume that the sulphate determinations are about half a percent high through inadequate washing, occluded barium chloride, or both, then Phil. Mag., [4]24, 124 (1862). Chem. News, 47, 181 (1883).
Basic Copper Sulphate
Mol KOH added
0.16 0.24 0.36 0.47 0.47 0.67 0.67 0.71 0.71 0.93 0.93 0.93 0.93 1.40 1.72 1.97 4.00
795
% Tptal Cu precipitated
% CuO in anhydrous salt
lo.s 16.3 24.3 33.9
Light blue
60.8 68. 1 98.1
78.1 77.9 77.9 78.5 75.5 77.0 79.2 78.2 78.6 79.2 79.0 78.7 78.3 78.6
100.0 100.0
....
Dark blue
99.6 100.0
.... ....
46.6
.... .... ....
*...
....
,
Remarks “
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