A STUDY O F T H E SODA-ALCJI SYSTEM BY J. T. DOBBINS A S D R . M. BYRD
Soda-alum was discovered by Gehlen' in 181j. This discovery was confirmed by the work of Zellner* and other early investigators. The existence, however, of soda-alum has been questioned by several experimenters, one of whom was Ostwald, who claims that the potassium in ordinary alum may be replaced by rubidium or caesium but not by sodium or lithium. The work of more recent investigators, A ~ g e Smith,4 ,~ W a d m ~ r e Leffman ,~ and Strock,6 and others has proved conclusively the existance of this compound. The general method employed by these invest,igators for the preparation of sodaalum was to put together solutions of sodium sulphate and aluminum sulphate in equivalent quantities and concentrate it by heating. The literature does not afford any information as to the limits of concentration between which soda-alum may be prepared, and it appears that little attention was paid to the temperature other than to say that the alum may not be prepared above 3oOC. There is also some dispute as to the behavior of soda-alum at ordinary temperatures. Some investigators claim that it does not, effloresce at ordinary temperatures, while others contend that it does. Therefore, this investigation mas undertaken to see if soda-alum could be prepared at ordinary temperatures, z j°C. being selected, and between what limits of concentration this takes place. Experimental Preparation of System: Sodium Sulphate-Aluminum Sulphate-W ater A series of solutions was prepared of varying concentrations with respect to aluminum sulphate, and with sodium sulphate present in excess. Another series of solutions of varying concentrations of sodium sulphate, and with aluminum sulphate present in excess was prepared. These solutions were a t 2 j°C. placed in a constant temperature bath, which was correct to o.oIT., and allowed to come to equilibrium. After equilibrium had been reached, about z 5 grams of the liquid phase, and about z j grams of the wet residue were weighed out and diluted to a definite volume, from which aliquots were taken for analysis. Aluminum was determined by a method developed in this laboratory by Dobbins and Sanders,' the sodium determined volumetrically by a method developed by Dobbins and Byrd,sthe sulphates determined gravimetrically by precipitating them as BaS04, and the water determined by difference. -
* Personal communication to Joseph S . Fuchs; Schweiggers Journal, 18, 377. Schweigger's Journal, 18, 344 (1816). 3Compt. rend., 110, 1139-1140 (1890). J. Am. Chem. S O C . ,31, 2 4 j (1909). s P r o c . Chem. Soc., 21, 150 (1905) K A m .J. Phar., 100, 474 (1928). Excerpt from Doctoral Dissertation University of N. C. (1931). 8Excerpt from Doctoral Dissertation University of N.C. (1931).
J. T. DOBBINS AND R. M . BYRD
3674
In order to ascertain if any appreciable hydrolysis had taken place, in which case it would have been necessary to treat the system as a four component instead of a three component system, a complete analysis was made on several of the samples. By arbitrarily assuming the experimental values of sodium to be correct and calculating their equivalent of sulphate, then calculating the a aluminum sulphate equivalent to the remaining sulphate, the extent of variation of the calculated values of aluminum sulphate from the experimentally determined values could be found. If hydrolysis had taken place, the calculated values for aluminum sulphate would be higher than the experimental values. These results are listed in Table I.
TABLE I Determination of Hydrolysis of A l ~ ( P 0 ~ ) 3 NalSO, assumed
Al?(SO,)r by
A i z ( S 0 1 3) Experimental
0 .ooyc
28.987,
2.27
27.27
3.92 6.29
26.45 23.04 18.26 14.2j 12.84
29.29% 27.64 26.68 23.07 18.66 14 59
to be correct
difference
10.74
16.55 18.89
T.4BLE
12.80
11
Percentage composition of liquid and of residue in system : Sodium sulphate-aluminum sulphate-water at 25OC. Liquid Sa834
AL(SO,),
HzO
0.00
29.29
70.71
0.00
2.27
27.64
j0.09
I
3.92 6.29
26.68 23.07 18.66 16.24
69.40
1.64
$0.64 70.60 i o . 06 Cn 46 69.02
11.50
10.74
t 3 . 70 15.44 16.30 16.55 r8.89
15.10
14.63 11-59 12.80
18.90 19.06 19.62 '9.85 20.13 20.84
12.28
21.55
0.03
9.18 7.28
6.83 4.93 2.11
68.86 68.31 68.82 j1.76 73. I O 73.32 74.04 77.05 78.45
78
47.22
38.54
14.18
41.71 31.10 32.98
60.05 56.65 57,40 52.84
15.30
23.47
61.23
19.37 40.94 39.91 40.56
24.75
55.84 57.43
41
44.10
52
1.63 1.54
58.55
1.08
58.36
0.00
55.90
3675
THE SODA-ALUM SYSTEM
The data listed in Table I show that the extent of hydrolysis was insignificant and we were able to treat this as a three component system. The results of the analysis of the liquid phase and the iTet residue are recorded in Table 11. The percentage by weight of anhydrous sodium sulphate and aluminum sulphate as tabulated in Table I1 are plotted in the conventional way on a triangular diagram. The curve for the liquid phase has two transition points, one giving a very short segment AB on the aluminum sulphate side, a much longer segment BC in the middle, and a fairly long segment CD on the sodium
,51
FIG.I Solubility of l;a2SOeIoH20 and Alz(S04)3.18HzO in H 2 0at 2j" C
sulphate side. The breaks in the curve at B and a t C indicate, of course, that a compound has been formed between the sodium sulphate and the aluminum sulphate. In order to determine what compounds exist in contact with the solutions represented by these three curves, ssmples of the net residue were analyzed and the results plotted on the triangular diagram. The tie lines drawn from points on segment AB through the corresponding points of the solid phase, intersect a t a common point, which represents the composition of hydrated aluminum sulphate of composition . ~ ~ ~ ( S O ~ ) ~ . I SLikewise H Z O . the tie lines drawn from points on the liquid phase segment CD through the corresponding solid phase points, intersect at a common point, which represents the cornposition of the solid in that field, the compound being hydrated sodium sulphate of composition NazS04.IOHZO. The compound of primary interest in this system is the one in equilibrium with the solution represented by segment BC. In order to establish the identity of this compound, several samples of the wet residue corresponding to vital points on the liquid phase curve were analyzed and the tie lines drawn. The point of intersection of these lines corresponds exactly to the percentage composition calculated for soda-alum or ~ ~ z S O ~ - A ~ Z ( S O ~ )The ~ . comZ~H~O. position of this compound was the principal point of contention, and the analysis of it shows conclusively that it is soda-alum.
3676
J. T. DOBBINS AND R. M. BYRD
The point B on the triangular diagram is evidently very near the transition point for the field of soda-alum and hydrated aluminum sulphate, as the tie lines on either side of it pass through the point of composition of hydrated aluminum sulphate on one side and soda-alum on the other. On the other hand, point C is the transition point for the field of soda-alum and hydrated sodium sulphate. This is shown by the analysis of the solid phase which is represented by the point F. At the composition represented by point C, sodaalum and hydrated sodium sulphate exist together in contact with the same solution. The apparent limits of formation of soda-alum at 25°C. lie between points B and C on the liquid phase curve. Consequently, soda-alum may be prepared at this temperature by taking concentrations which lie between these limits. It is obvious from Fig. I that a straight line joining the point representing solid soda-alum and the apex representing pure water, would cut the curve representing solutions in equilibrium with the solid alum, z.e. the components other than water would have the same ratio in both liquid and solid phase. Hence, evaporation at z 5 O C . of a solution of soda-alum will yield soda-alum without decomposition. It was found, in the course of this study, that a white, apparently amorphous mass was first formed which later was transformed into small crystals of soda-alum. These crystals grew into larger ones, the growth taking place from the top of the solid layer downward. This peculiar behavior was observed by Spence,g and others. Also it was observed that a rather appreciable efflorescence took place in these crystals. This was one of the outstanding points of dispute by some of the other investigators. This point is in agreeagreement with results observed by Leffman and Strock.'O
Summary I. A study of the system: Sodium sulphate-aluminum sulphate-water has been made. 2. Soda-alum may be prepared at z5OC. between limits of concentration represented by segment BC on the triangular diagram. 3 . Soda-alur., effloresces quite readily at room temperature when exposed to the air. 4. Soda-alum can be obtained by evaporating a t 2 j°C. an aqueous solution containing from 5 percent Ka2S04 and 26 percent A12(S04)3to 19 percent NasS04 and 8 percent Alz(SOa)3.
linzverszty of .Vorth Carolzna, Chapel " d l , CQtdZnQ.
9Chem. News, 22, 181 (1870). Am. J. Phar., 100, 474-478 (1928)
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