T H E AQUEOCS SOLUTION OF CUPRIC ASD FERROCS SCLPHhTES BY F. K . CAMERON A X D H. D. CROCKFORD
Introductzon. According to Etard' a brick-red salt crystallizes from an aqueous solution of ferrous sulphate and cupric sulphate, the composition being represented by the formula, S04Cu.S04Fe.zH20. ht a high enough temperature it loses all its water and becomes violet, though preserving its crystalline form. Keither salt is oxidized by fuming nitric acid, even a t ebullition. We have not been able to confirm any of these observations. Mixed salts of ferric sulphate and cupric sulphate containing also various proportions of sulphuric acid and water have been described often. Rut there seems to be little information regarding the solids stable in contact with solutions of ferrous sulphate and cupric sulphate. I t it commonly stated that they form mixed crystals, as do all the vitriols. T h e Materials used were C . P. grades of J. T. Baker Co. chemicals and the regular drug store Xujol. Methods of Analysts. Our first analyses of copper were by a volumetric method. Our results seemed to be running consistently high, so the method was abandoned and the copper precipitated electrolytically on platinum. The iron in solution was oxidized by heating to boiling with nitric acid, and removed by precipitating with ammonia, filtering, and weighed after heating the residue in a muffle t o red heat for several hours. Sulphates were weighed as the barium salt, and chlorine was weighed as the silver salt. It was found desirable to remove iron before determining copper; and it was, of course, necessary to remove all copper before incinerating the iron precipitates. The procedure followed when both bases were present was to heat the solution to boiling, add a little hydrochloric acid, and then somewhat more barium chloride than necessary to insure complete precipitation of sulphates; filter, and wash with a hot solution of hydrochloric acid, and then with hot water: to the filtrate and washings again brought to boiling, a slight excess of sulphuric acid was added, and the excess of barium precipitated was removed by filtration and discarded. The filtrate was then brought to a vigorous boiling, a little nitric acid added, and boiling continued for j-IO minutes. On cooling somewhat, concentrated ammonia water in excess was added with continuous stirring. The precipitated oxide of iron was washed on a filter alternately with concentrated ammonia solution and hot distilled water, until no trace of copper could be recognized in the washings. The filtrate and washings were evaporated to a small volume, thus getting rid of the excess of ammonia, acidified with sulphuric acid, and the copper precipitated electrolytically. Resolution and reprecipitation guarded against mechanical inclusions. Compt. rend, 87,
602 (1878).
BY F. K. CAMERON AXD H. D. CROCKFORD
7’0
Cupric Sulphate, Sulphuric Acid, and Water. This system was investigated by Bell and Taber’ a t 25”C, their results being quite satisfactorily confirmed later by Foote.* As our work was t o be a t 3ooC a part of the isotherm was determined for this temperature and covering the concentrations in which we were interested. The results are assembled in Table I. It appears that cupric sulphate is somewhat more soluble in aqueous solutions of sulphuric acid at 3ooC than a t 25OC.
TABLE I Solubility of Cupric Sulphate in Aqueous Solutions of Sulphuric Acid at 3ooC
cuso,
HzSO,
gms. per 100 gms. grns. per IOO gms.
Hz0 20.14 18.62 16.22 14.08
Hz0
7.38 17.13
20.65 27.76 49.61
7.82
* Calculated
cuso4
Hisod
gms. per 100 gms. gms. per
HzO 15.57
9.81 16.55
8.63
1 0 0 gms. HzO 16.12* 34.93* 14.61** 37.24**
from Bell and Taber: loc. cit.
**from Foote: loc. cit.
Sulphuric acid depresses the solubility of cupric sulphate. In contact with solutions of sulphuric acid up to about 49 per cent of sulphuric, the solid stable is the pentahydrate, CuS04.5H20. As concentration of sulphuric acid increases, the trihydrate, the monohydrate, and finally anhydrous cupric sulphate become in turn the solid phase. No evidence has appeared that solid complexes may appear containing sulphuric acid as well as water and cupric mlphate.
Ferrous Sulphate, Sulphuric Acid, and Water. Wirth’ has studied the solubility of ferrous sulphate in solutions of sulphuric acid a t 25OC finding that the solubility of the salt decreases with increase of acid. Up to solutions about 12th normal, the stable solid in contact with the liquids is ferrous sulphate heptahydrate, FeSO4.7H2O. As the concentration increases, the solid phase is the monohydrate, FeSOd.Hs0. Wirth’s results are given in terms of “normality” of the acid and grams ferrous sulphate (FeS04) per loo grams of solution, without the data necessary to compute the two solutes on a common basis. Our results obtained a t 3ooC are assembled in Table 11. Within the range of concentration involved the stable solid was ferrous sulphate heptahydrate. Kenrick4 has studied the solid phases stable in this system over a wide range of concentrations. He found that the constant solution, or concentration beyond which ferrous sulphate monohydrate is the stable solid, was J. Phys. Chem., 12, 175 (1908).
* J. Am. Chem.
SOC., 37, 2 9 0 (1915).
‘2.anorg. Chem., 79, 364 (1913). J. Phys. Chem., 12, 693 (1908).
CUPRIC AND FERROUS SULPHATES
7x1
about 78 grams of sulphuric acid per hundred grams of water. From Kenrick's data it appears that, as concentration of sulphuric acid increases, a t about a concentration of 459 grams of sulphuric acid per I O O grams of water, the compound with the formula 2 FeS04.H2S04becomes the solid phase, replacing the monohydrate. Above concentration 85 5 grams sulphuric acid per I O O grams of water, the stable solid has the formula FeS04. H2S04, and above concentration 1592 grams sulphuric acid per I O O grams of water the stable solid has the formula FeS04.3H2S0,. We have dealt with no such concentrations in the present investigation. But we consider it possible that
FIG.I Solubility isotherms for aqueous solutions of cupric sulphate and ferrous sulphate.
similar solids have appeared in our work and that it is desirable t o call attention a t this point to the facts that ferrous sulphate unites with sulphuric acid in the solid phase, under favorable conditions, whereas cupric sulphate appears not to do so under any conditions.
TABLE I1 Solubility of Ferrous Sulphate in Aqueous Solutions of Sulphuric Acid at 3ooC FeS04
Gms per 100 gms Hz0
16.89 17.53 18.08 18.56
H ~ O I Grns per IM) gms Hz0
FeSO, Grns per 100 gms
Hz0
2j.20
19.30 '7.43 18.65
32.60
'7-79
14.99 '9.96
HlSO. Gms per IOO gms HB 41 -85
'9.50
31-29 25-57
BY F. K. CAMERON AND H. D. CROCKFORD
712
TABLE I11 Mutual Solubilities of Cupric Sulphate and Ferrous Sulphate in Water. Stated in grams dissolved salt per I O O grams water. 25"C* 30°C 4ooC* cuso4 FeS04 cuso4 FeSOl cuso4 fe504 21.59 2 2 . I2
22.37 22.00
22.61 18.37 17.61 '5.79 15.76 11.18 9.61 8.48 4.33 3.03
3.44 4.94 8.93 I O . 96 13.08 13.55 14.64 16.67 16.04 19.82 21.33
23.53 22.49 20.54
18.95 16.02 21.98 21.j6 20.67
3.13 11.51
17.16 19,61 22.7j
3.29 6.86 11.82
27,70 27
39
27.7;
31.68 23.41 21.92 19,73 19.75 17.0:
16.89 12.83 I O . 30 9.12 4.81 3.71 28. j 7 28.78 25.16
22.28
25.78 26,61
* Calculated from data by
28.07
3.47 5.11
8,43 16.14 22.14 20.06 20.94 23.51 23.89 25,83 25.07
28.68 31.46 32.48 36.19 37.09 10.11
12.47 19.6j
Agde and Barkholt: loc. cit.
Cupric Sulphate, Ferrous Sulphate, and Tt'utey. Agde and Barkholt' by studying the change in composition on the cooling of numerous mixtures of cupric sulphate, ferrous sulphate, and water obtained data from which to chart several isotherms for this system. R e have recalculated their data for 25°C and 4ooC and assembled them in Table I11 and Fig. I . At lower concentrations with respect to ferrous sulphate, the stable solid phase is copper sulphate pentahydraie, CuS04.gH20, while the liquid solutions of higher concentration are in equilibrium with members of a series of solid solutions. The limiting member of this series of solutions, at the transition point, probHydrolysis of the ably has the composition, 3CuS04.~H20.zFeS04.7H20. salts had little effect. As long as cupric sulphate pentahydrate alone is in contact with t,he solutions, increase in content of ferrous sulphate has little effect. It is noteworthy that they attained equilibrium conditions involving the formation of a solid solution so very quickly. Our experience has not been so happy in attempting to realize this system as one of three components. Stock solutions were prepared by warming an excess of cupric sulphate crystals and cupric oxide in distilled water; and ferrous sulphate crystals with cleaned wire nails and water. From these a series of solutions were prepared, solid cupric sulphate in excess being added
' Z. angew. Chem., 39, 851 (1926)
CUPRIC AND FERROUS SULPKATES
7 I3
to the solutions of ferrous sulphate, and an excess of solid ferrous sulphate to the solutions of cupric sulphate. Each member of the series was “seeded” with a few crystals of a solid solution. Prevention of oxidation by air was attempted by superimposing a heavy layer of Xujol. This device was but moderately successful, as, in time, some solid ferric hydrate always appeared. Analysis of five of these solutions in which there showed not more than appreciable amounts of ferric iron are here given. The solutions had been kept in a thermostat at 3ooC for 11 days, meanwhile being shaken vigorously at frequent intervals. Examination of the solid phases, carefully dried by pressing betwen filter papers, was disconcerting. No distinctions in crystal form were observable, even with a hand magnifying glass. But there was a marked gradation in color from blue through the intermediate shades to a strong green. Generally crystals of a quite different color and composition could be picked from the mass corresponding to one and the same solution. The analyses of these samples always showed a discrepancy between the sulphuric acid content, as calculated from the metals found, and that actually found. A few typical cases are given in Table IF’.
TABLE I\’ Composition of Precipitates in contact with Aqueous Solutions of Cupric Sulphate, Ferrous Sulphate, and Hydrogen Sulphate Color
CuSOa per cent
FeSOa per cent
Blue
20.64
35.50 37.04 21.33
12 j
Green ,
22.42
20.42
2j,64 25.81
HI SO^
per cent
- 1’33 - .56
HzO
per cent 45.21
16. j o
20.62
41,I O 33.60 35.03
31.49
9.67
33.02
24.60
Three dilute solutions of ferrous sulphate were prepared, and cupric eulphate added in excess. S o Sujol was added, but the air was removed by displacement qith carbon dioxide. At the expiration of six days the solutions werc analyzed and the accompanying solid phases, dried between filter papers, proved to be cupric sulphate pentahydrate containing no detectable amounts of iron. These flasks were then seeded, each with a few crystals of a solid solution. At’ the end of another period of five days, each flask contained more or less ferric hydroxide and some green crystals as well as blue crystals of cupric sulphate pentahydrate. The green crystals contained both iron and copper. Their color was decidedly different from the bluish color of the seed crystals. Apparently, with extraordinary care or by good luck, a three-component system containing cupric sulphate, ferrous sulphate, and water can be realized. Ordinarily, however, one deals with a four-component system, and more commonly where hydrogen sulphate may be considered most conveniently as the fourth component. C u p r i c Sulphate, Ferrous Sulphate, S u l p h u r i c Acid, and Ti-ater. A series of solutions was prepared, a part by adding an excess of solid ferrous sulphate
BY F. K. CAMERON A S D H. D. CROCKFORD
714
to solutions of cupric sulphate, and a part by adding solid cupric sulphate to solutions of ferrous sulphate. To each solution in the series was added the same excess of sulphuric acid. The containers were quite filled and tightly stoppered. Under these conditions there was practically no oxidation of the iron. After being in the thermostat for about three weeks, a t a temperature of 3ooC, with intermittent but frequent shaking, samples of the solutions approximately of 4.5 grams each, were analyzed, the results being assembled in Table V. The solids in contact with five of these solutions were carefully separated by filtration on a Buchner funnel, quickly dried between heavy filter papers, pulverized and quartered for a subsample for analysis. I n each case the whole solid phase was carefully inspected with a hand glass before pulverizing. But while it appeared that characteristic crystals of cupric sulphate pentahydrate were present in each case, it also proved hopeless to separate them mechanically from the remaining crystals. The analysis of the solids are included in Table V.
TABLE V Composition of Liquid and Solid Phases, a t 3ooC, in the System of Cupric Sulphate, Ferrous Sulphate, Sulphuric Acid, and Water Sample
No. I
2
3 4 5 6 7 8 9 I
2
3 7 9
Solution CuSO, FeSOI HlSO, HzO per cent per cent per cent per cent
18.81 16.21 13.99 13.89 13.82 13.67 13.38 13.22 11.09
3.63 7.17 11.32
6 . 2 1 71.36 4 . 5 1 72.11 4.58 7 0 . 0 1 10.85 5 . 4 9 69.77 11.24 4 . 6 9 7 0 . 2 5 1 1 . 0 9 5 . 3 8 69.86 11.62 5 . 0 6 6 9 . 9 4 12.09 4 . 5 6 70.13 13.55 5 . 5 0 6 9 . 8 6
Solid CuSO4 FeS04 H&OI HzO per cent per cent per cent per cent
34.98 39.38 43.59
4.63 5.56 7.74
24.86 18.60
12.58
35.52 36.56 36.09
26.92
15.24
20.32
37.52
12.50
18.16
27.04
42.31
Probable Solid Phases : CuSO4.jHzO and FeS04:8.31HzS04:~8.7 jHzO CuS04.5Hz0 and FeS04:j.I 7 H ~ S 0 4 : 2I. 7 5Hz0 CuS04.5Hz0and FeSO4:z.5~H2SO4:~2.48Hz0 CuSOI. 5HZO and FeS04:2.o6HzSO4:I 2.3 I Hz0 CuSO4.5HZOand F ~ S O ~ : Z . ~ ~ H Z S O ~ : I ~ . ~ ~ H Z O
Inspection of the figures in the table shows that the solubility of either salt is depressed by the other and by the sulphuric acid. The solution figures when plotted on the equilateral tetrahedron and projected on the CuSO4FeSO4-HZSO4triangle fall on or very close t o a smooth curve. In this system of a t least four components there must have been two solid phases present, since there was but one liquid and vapor, in each of the containers. It seems most reasonable to assume, although not definitely proven, that one of the solid phases in each case is cupric sulphate pentahydrate,
CCPRIC AND FERROUS SULPHATES
715
CuS04.5H20. This assumption made, the composition of the other was computed for each case and the results assembled under Table V. Since there is apparently a continuous and progressive variation in the composition of the solid phase, with the progressive variation in the composition of the liquid phase, we may conclude that the solid phase containing ferrous sulphate also contains sulphuric acid and water in solid solution. Apparently we are dealing with a series of solid solutions, although the data for the solid in contact with solution KO.9 does not accord too well with the others. Another series of solutions was prepared, but designed to cover a wider variation in the concentrations of the components. I n this series, the solid phases were not separated from the supernatant liquid by filtration. Hydrochloric acid, about one per cent or a little more, was added to each solution as a “tell tale.” After standing a t room temperature for about three weeks, being shaken vigorously several times a day, the samples were then immersed in the constant temperature bath for 2 2 days. Samples of the solutions of about approximately 6.5 grams weight each were withdrawn for analysis. Residues of solid and mother liquor were also taken for analysis at the same time, these residues varying in mass from slightly less than two grams, to over five grams. The samples were each dissolved and made up to 500 cc volume, and analysis made on aliquots. Considerable difficulty was met in making the chlorine determinations. The “chlorine ratio” for liquid and residue was considered sufficiently reliable to be used in but five out of the nine cases. The final computations are assembled in Table VI.
TABLE 1‘1 Composition of Liquid and Solid Phases, a t 3ooC,in Systems of Cupric Sulphate, Ferrous Sulphate, Sulphuric Acid, and Water Solutions Sample CuSO, FeSO, H2S0, HzO No. per cent per cent per cent per cent I 2
3 4 5 6 7 8 9 I
4 6 7 9
16.05 15.40 13.93 11.80
11.63 9.17 5.86 3.44 1.91
2.09 5 . 7 1 3.58 6.16 5.39 7.62 1 1 . 2 2 9.81 11.01 8.56 9.68 9.46 10.40 1 1 . 3 5 11.33 11.13 12.41 12.49
76.14 74.86 73.05 67.16 68.79 71.68 72.40 74.10 73’17
Solids HzS04 FeSO, per cent per cent per cent
CuSO,
25.18
3.68
48.53
7.79
32.51
9.60
Hz0
-
per cent
38.62
34.08
24.06 18.56 19.36 22.41
21.53 22.23
35.84 36.00
4.01 33.73
21.80
40.45
Probable Solid Phases : CuSo4.5Hz0 and FeS04:13.6gH~SO~:55.98H~O CuS04.5HZ0and F ~ S O ~ : I . ~ O H ~ S O ~ : ~ . Z ~ H ~ O CuS04.gHzOand FeS04:I .80H2S04:IO.I 2H20 CuSOr.gH20 and FeS04:1.j4HzS04:9.44H20 CuS04.gH20 and FeS04:o.ggHsS04:g.gH20
BY F. K. CAMERON AND H. D. CROCKFORD
716
These figures suggest the same conclusions as do those in Table V. They are charted in Fig. z . It will be observed that the data for solution KO.4 falls far from the smooth curve indicated by the data for the other solutions. Whether equilibrium had not been reached in this solution or the analytical work was faulty in determining the excess of acid t o be added, it gives an adequate reason for suspecting the correctness of the composition found for the solid solution in contact with this solution. There seems to be no reasonable escape from the conclusion that the solid phases in this system are the pentahydrate of cupric sulphate and one of the members of a series of solid solutions containing ferrous sulphate, sulphuric acid, and water.
K S04
A
s
cu -O /J
FIG.z Solubility a t 30" of cu ric sulphate and ferrous sulphate in aqueous soyutions of sulphuric acid.
Summary. It has been shown that, at 3 0 T : I . The solubility of cupric sulphate in water is depressed by sulphuric acid. 2 . The solubility of ferrous sulphate in water is depressed by sulphuric acid. 3 . Cupric sulphate and ferrous sulphate mutually depress one another's solubility in water. The presence of sulphuric acid augments the depression. Two isotherms have been charted. 4. A satisfactory explanation of the two solid phases in contact with an aqueous solution of cupric sulphate, ferrous sulphate, and sulphuric acid, is that one phase is cupric sulphate pentahydrate, CuS04.5H20, and the other a solid solution of ferrous sulphate, sulphuric acid, and water. Department of Chemistry, L'ni1,ersity of S o r t h Carolina
