CALCIUM S U L P H A T E IN AQUEOUS SOLUTIONS OF POTASSIUM ,4ND SODIUM SULPHATES'
BY F. K. CAMERON AND J. F. BREAZEALE
This study of the solubility of calcium sulphate, or gypsum, is in continuation of the series on the solubility of this substance in solutions of various electrolytes reported in several earlier coniniunications from this laboratory. T h e solubility in solutions of sodium sulphate at 22' C,2 has already been given and is here again given for 2.5' C, with more points on the solubility curve. The solubility of calcium sulphate in potassium sulphate was made especially interesting by the formation of the so-called double salt, CaK2(S04)2.H20. This double sulphate of potassium and calcium was first prepared by Phillips, and later shown to be identical with the double sulphate, CaKQ(S04),.H20, found together with sylvine, KC1, in Kalusz, and formerly called Kaluszite.3 Later, on account of this relation to polyhalite, it was called syngenite. Ditte,4 and more recently van 't Hoff and \Nilson5 have prepared this compound. Van 't Hoff and Wilson determined the composition of the solution in equilibrium with both calcium sulphate and syngenite at 25' C, and applied these data to determining the conditions governing the formation of syngenite. Calcium Sulphate and Potassium Sulphate All the solubility determinations here described were made at 2 5 O C and the method of procedure was practically the same in all cases. A series of bottles was filled with solutions of Published by perniissioii of the Secretary of Agriculture. Jour. Phys. Chern. 5 , 643 (1901) ; and Bulletin No. 18, p. 51, Division of Soils, U . S. Ilept. of Agric.. 1901. See Jahresb. Fortsch. Chern. 1872, 1142. * Comptes rendus, 84i 86 (1877). 5 Sitzungsber. Akad. Wiss. Berlin, 1900,1142.
336
F. K. Camerbn andJ. F. Breazeale
varying concentrations with respect to the more soluble salt, and an excess of calcium sulphate added to each. They were then allowed to stand twelve to ,fourteen days with frequent shaking, and finally allowed to settle for several days in the thermostat kept constant at 25' C. Portions of the solution were withdrawn from time to time and analyzed to determine when final equilibrium had been established. In order to ensure accurate data in this respect, in one series the solutions were heated for several hours at 6 5 O C, and then kept in the constant temperature bath for fifteen days. Another series stood for some days at room temperature (about 2 2 ' C) and was then placed in the constant temperature bath for fifteen days. T h e agreement between the results from the two series was quite satisfactory and showed beyond reasonable doubt that final equilibrium conditions can be obtained in both cases. When equilibrium has been established as above described, fifty cubic centimeters were withdrawn, carefully weighed and the calcium estimated by precipitating as calcium oxalate and weighing as oxide in the usual manner. T h e effect of the presence of the relatively large quantity of potassium sulphate on the precipitate was tested in a separate set of experiments and was found to be so slight as to be safely negligible. I n the higher concentrations of potassium sulphate the precipitate had to be washed very thoroughly to remove the excessive amount of salt, but the calcium oxalate itself was influenced very little, quantitatively. T h e potassium sulphate was determined by evaporating another portion of the solution to dryness and subtracting the weight of calcium sulphate found by analysis from the weight of the residue. All results are expressed in grams per liter of solutions, although the weight of a liter of the solution is also given in each case, making it possible to compute the solubility on the basis of mass of solvent instead of volume of solution. T h e results of a preliminary series were as follows :
Calcium SzcFhate i n Aqueous Solutions Calcium Sulphate in Solutions of Potassium Sulphate Preliminary Series Grams K,SO, per liter
Weight of 1000cc. of solution
Grams CaSO, per liter I .60r
1032.18 1046.28 1060.35 1075.54
4.88 9.86 19.71 29.72 41-91 54.25 80.04 99.68
1 504 1.534 1.582 1.073 0.551 0.398 0.252
1003.80 1007.54 101g.15 1022.86 1023.64 1024.14 1026.92 1027.32
5.09 9.85 '9.57 28.35 30.66 31.15 35.19 35.79
1.563 1.446 1.485 1.553 1 * 587 1.549 1.257 1.213
I 003.6 I
1007.54 1015.54
,
1022.82
1 I
337
I? K. Cameron a n d J R Breazeale
333
liter. The first curve shows the conditions of concentration for equilibrium between potassium sulphate and calcium sulphate when the solution is in contact with calcium sulphate or gyp-
I
Grams K 2 S 0 4 or No,SO,
0
25
50
Fig.
per liter of solu. 75
8I
I
0
Preliminary calcium sulphate - potassium sulphate series
A
Calcium sulphate - potassium sulphate series.
x
Syngenite -potassium sulphate series. Calcium sulphate -sodium sulphate series.
s u m as solid phase and the second curve the conditions with syngenite as solid phase. The intersection of the two curves gives the stable triple point where the solution is in equilibrium with both gypsum and syngenite as solid phases, This triple point is lower in respect to calcium sulphate than is found by van’t Hoff and Wi1son.I To test further the position of the triple point the procedure followed by these investigators was used, and the results obtained show that the values for the triple point as found by extrapolatiotl on the curves here given are substantially correct. 1
loc. cit.
[Gramsper liter of solution Grarnsper liter of solutioii CaSO, K,SO,
1
__
-___
Weight of 1000cc. of solution.
Grams K,SO, per liter
I
Grams CaSO, per liter
I 1013.08
1015.78
16.31 19.87
1020.01
25.01
1024.54 1036.82 1058.I O 1085.91
30.83 46.99 75-45 112.87
I I ~
1.495 1.529 1.537 1.565 0.810 0.451 0.330
F.K.Cameron a n d / F.Breazeale
340
concentration of sodium sulphate at 22' C as formerly determined in this laboratory' is similar in many respects to the corresponding curve for solutions of potassium sulphate. I t was deemed advisable, however, to obtain the curve for solution of sodium sulphate at 25' C in order to establish this similarity with certainty. T h e figures obtained are giveti in the following table, and are illustrated in part in the figure. Calcium Sulphate in Sdlutions of Sodium Sulphate 1
Weight of 1000cc. of solution
1001.26 1007.59 1 0 1 1.45 1020.46 1031.48 1039. I 2 1079.47 1096.47 I 142.66 1176.47 1212.00
Grams Na,SO, per liter
2.390 9.535 14.132 24.369 36.979 46. I 50 94.220 I 15.084 146.612 205.105 257. I O 0
~
I
GramsCaSO, per liter
1.650 1.457 1.388 1.471 1.563 I . 650 I .g80 2.096 2.234 2.503 2.650
With increasing concentration of the more soluble salt, there is at first a decrease in the solubility of the calcium salt, such as would be indicated by the electrolytic dissociation hypothesis ; and then a regular increase which might be explained by assuming the formation of ionic complexes in the solution by chemical union between the solvent and one or more of the solutes, or, by a change in the density of the solvent. Criteria are unfortunately yet wanting to enable 11s to discriminate be tween these suggestions. The principal difference between the two curves is that the one representing solutions of sodium sulphate does not end with the appearance of a solid double salt, none being formed at 25' C until the solution is saturated with respect to sodium sulphate decahydrate." Bureau of Soils, U. S. Department of Agriculture, Washington,D.C. 1 2
loc. cit. Van 't Hoffand Chiaraviglio, Sitzungsber. Akad. Wiss. Berlin, 1 8 9 9 , S r ~ .
