FERRIC SULFATE I N AQUEOUS SOLUTIONS OF OTHER SULFATES F. K. CAMERON Department of Chemistry, University of North Carolina, Chapel Hill, N . C . Receivsd December 9, 1996 INTRODUCTION
Ferric sulfate, Fez(S04)8, with aqueous hydrogen sulfate forms a threecomponent system, but it is only in high concentrations of the acid that ferric sulfate is truly a component. At lower concentrations it is hydrolyzed, and the stable solid in contact with the liquid is either a basic sulfate or a solid solution of basic character. Furthermore, at ordinary temperatures no ferric sulfate of definite composition can exist as stable solid in contact with an aqueous solution unless the ratio of SO3 to Fez03 be greater in the liquid phase than in the solid (1). Consequently, it would be expected that the addition of ferric sulfate to the aqueous solution of another sulfate must necessarily produce a four-component system. However, alums, pseudo alums, and other double salts have been reported as crystallizing from clear solutions. Similar observations have been made on analogous aluminum compounds. The crystallization of ordinary potassium or ammonium alum is commonplace. Dobbins and his coworkers have shown in this laboratory that sodium alum is stable over wide ranges of concentration and a fairly wide range of temperature (2). Occleshaw has reported the pseudo alum FeSOa.Alz(S04)a. 24&0 related to the mineral halotrichite (3); the manganese compound MnS04 .Alz(SO&-24H~0and others are known. The possibility exists, therefore, under some conditions of temperature and concentration, of considering an aqueous solution of ferric sulfate and another sulfate as a three-component system. From preliminary experiments it appeared difficult to obtain solutions from which some indefinite ferruginous material would not precipitate. It was attempted to add sufficient excess of sulfuric acid to prevent the indefinite solid from precipitating and to maintain a constant ratio between the excess acid and water. This latter condition was impracticable of complete realization. But the indefinite ferruginous material being apparently but slightly soluble, it was decided to ignore it with the slight 689
690
F. K, CAMERON
TABLE 1 Ferric sul.fate in aaueous solutions of other sulfates at 86°C. LIQUID PHASE
-8m' g-
E P R CE
1.4 1.5 1.7
c
I _
IR
cet
43.1 39.2 32.8 2.3.7 20.6 18.0 16.3
26.6
9.0 1.4 0.5
-
Fj 2.83 2.94 3.34' 3.38 4.56 5.43 5.32 6.21 8.0
11.7 11.9 11.8 11.5 11.3 12.2t 12.ot 12.5t 13.1t 13.7t
,. 8
g
I wI -
-
w -
per cent
er ce,
it7
c
cei
7 ?
(NH4)zS04.Fe~(S01)3.24Hz0 and inde-
0.98
41.9
0.:
0.9
40.9 21.1
O.!
2.5
0.8
-
I '
I I ceni
17.8 14.3 31.8 B.8 L3.1 11.2 9.02 6.04 2.35 0.85 0.69 0.31 0.10 0.02 1.25 1. B 1.53 1.@ I .a!
- -
-
s2
-
R cent
r eefi
0.4
87.3 85.6 n6.6 86.0
4.4 4.3 3.8 4.2
3.3 2.3
3.9 3.6
1.7 1.4 1.9 2.1
30.5 29.4
1.58
0.59 0.68 0.55
0.26 0.11 1.27 1.41 1.86 2.24 2.7
SOLID P E A 8 1
I
v
8
per cen
-
8
44.5 t 0 . 7 44.2 t l . l 44.4 t 2 . 3
1.9 2.2 3.3 4.8 11.7 14.8 17.3 40.8 44.2
REBIDUE
-
?r
cen
finite mixture (NH&SO4. Fe~(S04)8.24H~O (NH4)&304.Fez(SO&.24H~0 (NH4)zS04.F e ~ ( S 0 4 ) ~ . 2 4 H ~ 0 (NH4)zSO~. Fe~(S04)3.24H~O ("4)zSO4' Fez(S0&.24H~0 (NH4)&04.Fez (SOr) 24Hz0 (NH4)zS04~Fez(SO&~24H~0 and (."4)80, (NHI)zSO~ (NHI)zSO~ (NH4)zSO4
FERRIC SULFATE IN SOLUTIONS OF OTHER SULFATES
691
TABLE I-Coneluded
I
LIQUID PHASE
RESIDUE
per cmt per cent pm cen
~mcent per cent per cent
0.38 0.82 1.36 2.01 2.53 2.61 3.071: 3.261 4.90$ 5.301: 8.16 10.47 11.29 15.23 18.39 19.04 19.23
20.24 21.30 22.06 22.50
44.75 1.48 44.15 1.93 43.79 3.19 43.32 2.98 42.25 2.47 43.25 1.12 42.60 1.56 41.71 1.99 39.80 2.86 38.42 3.61 36.60 1.90 28.36 0.25 23.38 1.16 16.64 0.04 9.59 0.21 8.70 0.18 7.52 1.73 5.73 -0.21 3.02 -0.53 2.50 0.31 1.36 0.63
0.04 48.14 -4.43 Basic ferric sulfate 0.05 48.51 -4.09 Basic ferric sulfate 0.08 47.40 -4.08 Basic ferric sulfate Basic ferric sulfate 1.63 44.74 -4.44 Basic ferric sulfate Basic ferric sulfate 4.10 44.25 -4.05 FeSO4.7Hz0 and a basic ferric sulfate 7.90 40.10 -2.99 FeS04.7HzO and a basic ferric sulfate 10.36 41.54 -3.95 FeS04.7HzO and a basic ferric sulfate 6.91 43.41 -4.47 FeSO4.7HzO and a basic ferric sulfate FeS04.7H10 and ferric sulfate 49.89 6.26 -0.55 FeSO4.7HzO FeS04. ~ H z O 53.15 2.86 0.45 FeS04.7H~0 FeS04.7H~0 FeS04.7H~0 FeS04.7HzO 22.96 1.07 0.20 FeSO4.7HaO FeSO4. ~ H z O FeSO4 ~ H z O 49.70 0.50 0.45 FeSO4.7HaO
* Solution obtained by dissolving the alum K Z S O ~ . F ~ ~ ( S O ~ )in~water , ~ ~ Halone. ZO
t Probably, points on a boundary curve of a four-component system.
1: Probably, points on a boundary curve of a four-component system.
excess of acid and proceed as if dealing with a three-component system, meanwhile acquiring the data necessary to treat the several systems as of four components. Series were made of aqueous solutions of ferric sulfate and either ammonium sulfate, potassium sulfate, or ferrous sulfate, and agitated continuously in thermostats for upwards of four months. From time to time the liquid phases were analyzed, until they had reached a steady state. The final results for 25°C. are given in table 1. Recalculated to the basis of moles per 100 grams of solution they are plotted on the right-angled isosceles triangle, in figure 1, as offering the best comparisons of them. The data are included for ferric sulfate in aqueous solutions of sulfuric acid, recalculated from analyses by Constable (3). ,
THE JOURNAL OB PHYSICAL CBBMISTRY, VOL.
40, NO. 5
692
F. R. CAMERON AMMONIUM SULFATE-FERRIC
SULFATE-WATER
The system was studied at 25°C. by J. E. Hunter (5). Much difficulty was encountered in preparing solutions from which basic ferruginous solids would not precipitate on standing. In contact with solutions containing from 1.7 per cent ammonium sulfate and about 44.5 per cent ferric sulfate to 17.3 per cent ammonium sulfate and 16.3 per cent ferric sulfate, the stable solid is the alum (NH4)2S04.Fez(SO&. 24H20. A t lower concentrations of ammonium sulfate the solid is a more basic precipitate of iron sulfates, while at higher concentrations the solid is ammonium sulfate. The tie lines for solutions and corresponding residues do not give a perfectly unique crossing for the alum, but ones reasonably close to one another, Analysis of a dry residue gave 41.5 per cent ferric sulfate and 13.6 per cent
\\\
P ' /
FIG.
1
ammonium sulfate against 41.5 per cent ferric sulfate and 13.9 per cent ammonium sulfate as required by the alum formula. The ammonium sulfate precipitates were all more or less contaminated by yellow ferruginous solids. A few only in which the contamination was slight are recorded here. To prevent this contamination would require the addition of much sulfuric acid and would markedly affect the solubility of the ammonium sulfate; the system would then become, inevitably, a four-component one. To prove the solid phase was ammonium sulfate, the carefully filtered, clear, mother liquors were placed in contact with solid ammonium sulfate for several days. Analysis of the liquid phase then showing no change in composition, it was concluded they were already saturated with respect to ammonium sulfate when filtered, and the residues were essentially that salt.
FERRIC SULFATE I N SOLUTIONS O F OTHER SULFATES
693
I t was shown by dilatometer measurements that ammcnium ferric alum is stable below 36"C., above which temperature it decomposes. POTASSIUM SULFATE-FERRIC
SULFATE-WATER
This system has been studied by D. A. Pickler (9). According to the International Critical Tables, potassium ferric alum, KzS04.Fe2(S0&. 24H20,is stable to 33°C. (7). In contact with its aqueous solution, Pickler has found that it decomposes above 17°C. to form another double sulfate of potassium and iron with the simultaneous formation of a very basic iron complex of low solubility. Satisfactory crystals of the alum were obtained by crystallizing from solutions at 0°C. and removing adhering mother liquor and basic ferruginous material with filter paper. A series of solutions was prepared, from alum, ferric sulfate, potassium sulfate, and appropriate amounts of sulfuric acid and water, and agitated for four months a t 25"C., when a steady state had been realized. The data in table 1 are results of the final analyses. In contact with solutions from 2.83 per cent potassium sulfate and 27.8 per cent ferric sulfate to 11.7 per cent potassium sulfate and 0.8 per cent ferric sulfate the solid phase is a double sulfate. At lower concentration of potassium sulfate the ferric sulfate hydrolyzes with precipitation of a more basic complex, and a t higher concentrations the solid phase is potassium sulfate, KzSOa. A plot of the tie lines between solution and corresponding residue data indicates with reasonable certainty that the double salt contains two reacting weights of potassium sulfate to one of ferric sulfate. On the line through the origin and corresponding to all mixtures in this proportion, points were selected corresponding to different proportions of water of crystallization. Tie lines from the point corresponding to 14 molecules of water of crystallization to the solution points passed more closely to the points for corresponding residues than tie lines from any other point. Consequently, and beyond reasonable doubt, the formula for the double salt is 2KzS04. Fez(SO&. 14HzO. The solution and residue designated by an asterisk in table 1 were made from potassium ferric alum and water alone. The plotted tie line passes very close to the point representing the double salt. The solutions designated by a dagger (t) in table 1 were obtained in an effort to realize a condition where pure potassium sulfate would alone exist in the solid phase. The solubility of this salt evidently increases much more rapidly with addition of sulfuric acid than do the ferric complexes present. A boundary curve, in part, was realized where the solid phases are potassium sulfate and the double salt just described. In the threecomponent system obtained by keeping excess sulfuric acid a t a minimum, the range of concentrations is very short over which potassium sulfate alone is the solid.
694
F. IC. CAMERON FERROUS SULFATE-FERRIC
SULFATE-WATER
The literature covering the effects of other sulfates on the solubility of ferrous sulfate has been summarized recently (8). In general, the solubility of ferrous sulfate is depressed in the presence of another sulfate. The formation of double salts is common. At high temperatures only is the hydrolysis of serious moment. The effects produced by the addition of ferric sulfate to solutions of ferrous sulfate have been studied by C. C. Hudson (5). The data for solutions agitated at 25OC. for seven months are included in table 1. There is a transition point or “constant solution” containing 8.16 per cent ferrous sulfate and 36.6 per cent ferric sulfate. With higher concentrations of the ferrous salt, the stable solid phase in contact with the solutions is FeS04.7H20. With concentrations less than 8.16 per cent ferrous sulfate, the nature of the solid phases is uncertain. Examination of the data shows, a t very low concentrations of ferrous sulfate and hydrogen sulfate in the liquid phase, the solid phase approximates in composition the basic sulfate Fe20s. 2.5SOa.7H20 recently described or a more basic ferric complex (solid solution) (1). As the concentration of ferrous iron increases in the liquid phase with more hydrogen sulfate the former increases in the solid phase, i.e., ferrous sulfate is a component of the solid phase. One is dealing with a four-component system and two phases are present in a mixture of solids. Calculated as oxides, for treatment as a four-component system, we find the solutions varying from one containing FeO, 0.18 per cent, Fez03, 17.9 per cent, SO3, 28.26 per cent, and H20, 53.64 per cent, to one containing FeO, 2.51 per cent, Fe20a,15.37 per cent, SOa, 28.94 per cent, and H20, 53.18 per cent. The range of concentrations covered is so narrow that graphical treatment i s difficult, and the data are insufficient to justify definite statements regarding the composition of the solid ferric complex encountered. I n table 2 are the results for a series kept at 50°C. for seven months. Again, at high concentrations of ferrous sulfate the heptahydrate, FeSO4.7H20, is the stable solid. The constant solution at the transition point contains 14.74 per cent ferrous sulfate and 32.94 per cent ferric sulfate. With very low concentration of ferrous sulfate in the liquid, the composition of the solid approaches the basic sulfate Fe2O3.2.5s03.7H20, but as the concentration of ferrous iron increases in the liquid it becomes an essential constituent of the solid phase or phases in contact with the liquid. Ferric hydroxide is very slightly soluble in solutions of ferrous sulfate a t ordinary temperatures. In table 3 are the results of agitating the hydroxide in contact with solutions of varying composition at 5OoC.for seven months. Though small, the solubility is appreciable and increases relatively more as the concentraton of the ferrous sulfate increases. The analysis of the residues shows a relatively greater absorption of SO4 than of ferrous iron
695
FERRIC SULFATE IN SOLUTION8 OF OTHER SULFATES
in the solid. Consequently it would seem necessary to assume that basic ferrous sulfates are present in the solids as well as basic ferric compounds. The data for liquids appear to fall on a boundary curve for a four-component system, but the nature of the two coexisting solid phases is indefinite. TABLE 2 Ferric sulfate in aqueous solutions of ferrous sulfate at 6OOC. LIQUID PHASI
I
RISIDUI
$ 1
8k
c 8 - -Per
wr cent per cen
C I
0.19 0.67 3.67 6.18 6.28 8.02 10.55 14.74 19.40 21.76
48.73 46.54 44.50 41.03 40.53 38.87 36.61 32.94 25.35 19.85 27.88 9.67 30.35 5.40 31.99 2.79
-
0.78 2.59 1.33 3.16 3.00 5.20 2.50 1.09 0.95 1.01 0.22 0.30 0.67
er cent per cent per Odnl
0.49 58.06 -5.82 11.21 48.33 -6.21 11.20 47.69 -6.17 22.17 46.43 -6.10 54.95 10.74 66.20 4.61
0.30 0.20
2.80
0.51
53.97
Basic ferric sulfate Basic ferric sulfate Basic ferric sulfate Basic ferric sulfate and Basic ferric sulfate and Basic ferric sulfate and Basic ferric sulfate and FeS04.7H~0 FeS04.7H~0 FeS0,.7H~0 FeSO4.7HzO FeSO4 7Hz0 FeS04.7Hn0
FeSO,.’IHzO FeS0,.7H~0 FeSO4.7HzO FeS04.7HsO
+
TABLE 3 Composition of liquids and residues obtained by treating solutions of ferrous sulfate with ferric hydrate REBIDUB
F0
Fe
804
F0
Fe
804
per cent
per cent
per cent
per cent
per cent
per cent
3.50 4.27 5.15 6.72
0.15 0.19 0.22 0.25 0.36
6.34 7.61 9.55 12.00 15.47
0.96 0.14 0.14 2.10
40.03 38.18 38.18 45.00
5.95 6.80 6.80 8.43
8.38
I n dilute solutions of ferrous sulfate, the system is always a four-component one. No double salt, nor pseudo alum analogous to that described by Occleshaw, exists between 25°C. and 50°C. at any concentration of ferrous sulfate.
696
F. K. CAMERON
SUMMARY
It is shown that, generally, the addition of ferric sulfate to an aqueous solution lowers the solubility of another sulfate. At extreme dilutions when hydrolysis is nearly complete, sulfates which form disulfates with sulfuric acid, such as ammonium sulfate and potassium sulfate, may become more soluble, but over a very small range of concentration. With ferrous sulfate, no double salt is formed between 25°C. and 50°C. at any concentration of ferric sulfate. With ammonium sulfate, an alum is formed below 36"C., and the range of concentrations over which it exists at 25°C. is shown, with approximate accuracy. With potassium sulfate an alum exists below 16OC. over a wide range of concentration. It is not stable above 16"C., but a double salt, 2KPS04. Fez(SOJs. 14&0 is stable over all but very high or very low concentrations of ferric sulfate. The concentration limits are shown with approximate accuracy. At very low concentrations and a t rather high concentrations of ferric sulfate in the presence of another sulfate, the system must be treated as composed of four components. But over wide ranges of concentration and temperature the system may be considered as composed of three components, and so treated, practically, with the addition of small excesses of sulfuric acid. REFERENCES (1) BASKERVILLE AND CAMERON: J. Phys. Chem. 39,769 (1935). J. Phys. Chem. 34, 692 (1930). (2) CAMERON: E. W.: Doctor's dissertation, University of North Carolina, 1934. (3) CONSTABLE, (4) DOBBINSAND BYRD:J. Phys. Chem. 35, 3673 (1931); J. Am. Chem. SOC.63, 3285 (1931). (5) HUDSON, C. C.: Master's thesis, University of North Carolina, 1935. J. E.: Master's thesis, University of North Carolina, 1935. (6) HUNTER, (7) International Critical Tables: Vol. VII, p. 157. McGraw-Hill Book Co., New York (1926). (8) OCCLESHAW: J. Chem. Soc. 127, 2598 (1925). D . A , : Master's thesis, University of North Carolina, 1935. (9) PICKLER,
