A STUDY OF THE SODA-ALUM SYSTEM. 11 J. T. DOBBINS AND J. A. ADDLESTON Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina Received August 30, i93.4
The soda-alum system has been studied by Dobbins and Byrd (2), who determined the 25°C. isotherm and found soda-alum as a stable phase at that temperature. The demonstration of the existence of soda-alum at 25°C. made it seem advisable to continue the investigation further to determine the range of temperature over which it exists as the stable,phase. During the progress of the present investigation, Astruc and Mousseron (1) published the data for this system at 5", 18", and 42"C., and concluded that soda-alum does not exist below 11"C., nor above 39°C. I n the present paper are given the results of a study of the system at 0", 30°, and 42°C. EXPERIMENTAL PROCEDURE
Solutions were made, using C.P. sodium sulfate and aluminum sulfate. For each isotherm a series of solutions was prepared containing varying concentrations of aluminum sulfate in contact with solid sodium sulfate, and another series containing varying concentrations of sodium sulfate in contact with solid aluminum sulfate. For the 0" and 30°C. isotherms sodium sulfate decahydrate was used, and for the 42°C. isotherm, anhydrous sodium sulfate was used. The solutions were saturated with the respective salts at temperatures slightly higher than the temperature at which they were to be studied, thus producing equilibrium in a shorter time. Approximately 50 cc. of solution in contact with solid was prepared for each sample, and the bottles placed in the constant temperature bath. The bottles were shaken several times daily, and it was found that equilibrium was reached in less than two weeks. For the 0°C. isotherm, the bottles were kept immersed in a container of crushed ice and water placed in a large refrigerator.
Sampling After equilibrium was reached, the solutions were allowed to settle completely so that the supernatant liquid was perfectly clear. Samples of the liquid phase were withdrawn by means of pipets and immediately transferred to weighing bottles and weighed. These samples were then diluted to a known volume. 637
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The solid phase presented much difficulty in some cases, owing to the finely divided condition of the solid. In all cases, portions of the solid phase, with adhering liquid, were dipped from the bottles, as much of the liquid as possible was drained off, and the residue transferred to a weighing bottle, weighed, and made up to a known volume.
Methods of analysis On account of the difficulty of obtaining satisfactory analyses for sulfate in the presence of large amounts of aluminum, it was found advisable to make direct determinations of sodium sulfate and aluminum sulfate. Water was determined by difference. TABLE 1 0°C.isotherm RESIDGE:
LIQUID
BAMPLE NO,
NazSOi
Ala(S01)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
6.45 6.06 5.80 5.55 5.40 5.02 4.84 4.70 4.38 4.20 4.05 3.85 2.88 2.12 1.31 0.00
0.00 3.15 5.20 6.26 7.08 9.33 9.77 11.43 14.99 16.47 19.40 22.25 24.23 25.55 26.42 27.97
31.74
3.71
29.65 8.33 0.50
8.78 39.33 44.63
Sodium was determined volumetrically by the method of Dobbins and Byrd (3), and aluminum was determined gravimetrically by the method of Dobbins and Sanders (4). RESULTS
The data for the three isotherms are given in tables 1 to 3 and the results are plotted in figures 1 to 3. DISCUSSION OF RESULTS
An examination of figure 1 shows that the 0°C. isotherm for this system consists of two solubility curves, with no evidence of compound formation. The composition of the solid phase, determined by means of tie-lines
STUDY OF SODA-ALUM
SYSTEM.
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connecting the composition of the liquid phases with the composition of the respective wet residues, shows that the solids in equilibrium with the TABLE 2 30°C. isotherm BAYPLE NO.
1 2 3 4 5 6 7 8 9
.
11 13
29.61 28.10 27.75 27.37 27.04 27.00 26.93 26.67 24.20 15.95 12.58 9.20
14 15 16 17 18 19
5.43 5.21 3.18 2.30 1.55 0.00
10
RESIDUE
LIQUID
NanSOc
Alz(S0r)a
0.00 3.35 5.14 6.55 8.77 10.44 11.42 11.73 13.55 17.73 19.49 22.68 25.51 26.54 27.20 29.56 30.87 31.21 31.99
NanSOa
Alz(SO4)r
39.20
2.17
39.52 30.70 21.48 16.00
' 3.40 11.90 19.85 24.21
7.22 8.24 1.10 0.92
29.55 32.40 44.05 42.26
TABLE 3
42°C.isotherm I
I
RESIDUE
BAYPLE NO.
Nan504
1 2 3 4 5 6 7 8 9 10 11 12
32.94 29.10 27.13 25.70 21.12 18.74 10.31 8.03 5.49 2.22 1.89 0.00
0.00 6.83 10.11 12.43 13.94 15.19 21.35 22.65 26.94 31.32 32.21 34.24
75.92 76.63 20.69 18.48 12.92
3.86 4.07 23.77 26.31 31.45
8.03
31.79
3.26
46.40
saturated solutions are sodium sulfate decahydrate and hydrated aluminum sulfate. It is evident that soda-alum does not exist at 0°C. Figure 2 shows the presence of soda-alum in addition to the two com-
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pounds present at 0°C. The limits of concentration between which sodaalum may be prepared are 26.9 per cent sodium sulfate, 11.4 per cent aluminum sulfate, and 4 per cent sodium sulfate, 30.5 per cent aluminum sulfate.
IOH,O
FIQ.2. THESYSTEM Na2SOrAla(S04)~H20AT 30°C.
Examination of figure 3 reveals that the 42°C. isotherm is similar to that of the 30°C. isotherm. The solid phases are anhydrous sodium sulfate, hydrated aluminum sulfate, and soda-alum. The determination of the
STUDY OF SODA-ALUM SYSTEM.
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composition of the compound in contact with the middle section of the isotherm presented much difficulty. The solid was very finely divided, and it was impossible t o obtain samples comparatively free of the mother liquor. Analysis of the wet residue and extension of the tie-lines to their intersection gave no exact information, since it was impossible to obtain three tie-lines to intersect at one point. Hydrolysis had taken place to some extent, and the excess aluminum present made the intersection of any tie-lines vary greatly from the composition of the alum. The inability to remove the liquid from the residues caused the composition of the residues to be so close to the liquid curve that a small amount of hydrolysis produced a very great effect on the direction of the line.
FIQ.3. THESYSTEM Na2SO~-Al~(S04)s-H~0 AT 42°C.
To ascertain the composition of this solid phase, the tell-tale method was used. A small amount of copper nitrate was added to the saturated solutions. Samples of the liquid and the wet residue were taken, and the copper in each was determined electrolytically. From the amounts of copper present, the amount of mother liquor adhering to the solid phase was calculated. Calculations indicated that the solid phase in contact with the middle segment of the 42OC. isotherm consisted of soda-alum with varying excesses of aluminum oxide ranging from 4 per cent to 6 per cent on the extreme left end. On the right portion of this segment of the isotherm, the region of high sodium sulfate concentration, an excess of sodium sulfate was found in the solid. This was most probably due to adsorption. In order to verify these conclusions as to the composition of the compound in question, a microscopic examination was made of the solid.
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This showed conclusively that the substance was composed of small isometric crystals showing the flattened octahedra of the alums. Since hydrolysis had taken place to some extent, the system should theoretically be treated as a four-component system. However, since the liquid curve is fairly smooth, it is evident that hydrolysis is not a large factor and may be disregarded. . SUMMARY
The 0", 30" and 42°C. isotherms for the soda-alum system have been determined. There is no alum formation at 0°C. Evidence of the existence of soda-alum as a solid phase at 30°C. and 42°C. was established. Hydrolysis takes place to some extent above 30°C. REFERENCES (1) ASTRUCAND MOUSSERON: Compt. rend. 196, 423 (1932). (2) DOBBINS AND BYRD:J. Phys. Chem. 36, 3673 (1931). (3) DOBBINS AND BYRD:J. Am. Chem. SOC.63, 3285 (1931). (4) DOBBINSAND SANDERS:J. Am. Chem. SOC.64, 178 (1932).
