THE SOLUBILITY O F CALCIUM S U L P H A T E I N SOLUTIONS O F N I T R A T E S BY ATHERTON SEIDELL AND JOSEPH G. S M I T H
'
T h e solubility of gypsum in aqueous salt solutions has been studied by a number of investigators and has from time to time been given attention in this laboratory." A careful examination of the literature upon solubility determinations of mixtures of salts, shows that of all substances the solubility of calcium sulphate has been investigated in probably a larger number of different salt solutions than that of any other compound.3 It, therefore, appears to offer the best opportunity for a comparative study of the increasing or diminishing solubility effects produced by more or less concentrated aqueous solutions of different salts. A difficulty, however, presents itself at once, in that the data are to a considerable extent only qualitative, and in the earlier papers, where quantitative results are given, these are usually in such form as to make a strict comparison with more modern work almost impossible. T h e work described in this paper was undertaken for the purpose of extending the systematic observations which have so far been made in this laboratory and elsewhere upon the solubility of calcium sulphate in aqueous salt solutions. T h e solubility of calcium sulphate in ammonium nitrate solutions has been observed by Fassbender,4 but his experiments were more or less qualitative only. Droeze5 determined the solubility of gypsum in saturated solutions and in some cases other concentrations of the nitrates of ammonium, sodium, potassium and magnesium, but his figures are rather indefinite, Published by permission of the Secretary of Agriculture. Jour. Phys. Chem. 5, 643 (1901); Ibid. 7,571 (1903); Ibid. 8,335 ( 1904). Bibliography of Solubilities, by Atherton Seidell, sooii to be published. Ber. chem. Ges. Berlin, g, 1360 ( 1 8 7 6 ) . Ibid. IO, 330 (1877).
Atherton SeideZZ and_jose#h G. Smith
494
since one cannot determine from the description the precise concentrations employed, and an accurate comparison of his results with those observed later cannot be made. Making the comparisons, however, as best one may, quite large discrepancies are found, and it is evident that his results as well as those already mentioned are iiiainly useful in showing the directions of the solubility changes in thesystems studied. Cohn' gives determinations of the solubility of calcium sulphate in amnioniuin nitrate and in several other ammonium salts. T h e results of the calcium sulphate determinations are presented in terms of grams dissolved per IOO grams of solution, while the concentrations of the ammonium salt employed are stated in percentages of a standard solution, contained in IOO grams of water. By a tedious and indirect method it has been possible to recalculate the results to grams of salts contained in IOO cc of solution, and in this form to compare them with similar determinations made by another investigator. T h e results of this comparison for one of the salts employed, viz. : ammonium chloride, which was ' studied by Ditte,' shows a difference which is too great to be accounted for by ordinary analytical error. It appears that Cohn's results are subject to the criticisnis applied above to other investigations. From the preceding it is evident that satisfactory determinations of the solubility of calcium sulphate in solutions of nitrates have so far not been made. In order, therefore, to extend the observations to this class of compounds, the following experiments were undertaken : Experimental Part Saturated solutions of the nitrates of sodium, potassium, calcium and magnesium were prepared, and after being filtered and slightly diluted, the grams of salt per liter of solution were determined by evaporating to dryness and weighing in the case of the sodium nitrate and potassium nitrate, and by determining the calcium and magnesium gravimetrically and calculating to 2
I Jour, prakt. Chem. ( 2 ) 34, 43 (1887). Coniptes rendus, 126,674 ( ~ 8 9 8;) Ann. Chim. Phys. ( 7 ) 14,294 (1898).
Weight of 1000 cc. of solution
998.1 1016.3 1034.0 1068.4 1133.6 1191.6 1363.9 1390.4
Grams NaNO, per liter
___ Grams CaSO, per liter
25
2.084 4.252
50
5.500
0
IO0 200
300
600 655
7. I O 0 8.790 9.282 7.886 7.238
Atherton Seidell. andJosejh G. Smith
496
T h e results show that the solubility of the calcium sulphate increases to a marked degree in concentrations of sodium nitrate up to 300 grams per liter, but beyond this concentration the amount dissolved becomes less as the concentration of the nitrate increases. In the second table the results obtained in solutions of potassium nitrate are given.
TABLE 11. Calcium Sulphate in Solutions of Potassium Nitrate ____. ._
--
-
Weight of 1000CC. of solution
~~
Grams KNOs per liter ~
998.1 1008.I
0.0
12.5
1015.4 1032.I 1062.j 1092.4 1122.4
200.0
1153.9
260.0
2 5.0
50.0 100.0 I 50.0
I
Grams CaSO, per liter
2.084 3.284 4.080 5.255 6.855 7.907 8.688 6.278'
I n these solutions both the calcium and the sulphate were determined as in the preceding case. Up to a concentration of 2 0 0 grams of potassinm nitrate per liter, the amount of calcium sulphate in solution steadily increases, and the sulphate determinations agree with the calcium determinations, that is, the amount of calcium sulphate is the same whether calculated from the sulphate or calcium determinations. Beyond this concentration the sulphate and calcium determinations no longer agree, and differ very widely in fact, as is indicated in the table. This phenomenon was noticed whenever a potassium salt solution is used, but the concentration where this disagreement occurs varies with the different salts. This is probably due to the formation of a new solid phase, the double salt of calcium and potassium sulphates, CaK2(SOJ2.H20,or syngenite, and beyond this ___
Calculated from SO4 determination. Calculated from Ca determination.
Solubility
of Calcium SuZjhaie
Weight of 1000 cc. of solution
Grams Mg(NO,), per liter
998.1 1020.5 1039.8 1078.6
25 50
0
497
Grams CaSO, per liter ~
IO0 I
2.084 5.772 7.884 9.920
498
Atherlon Seide11 andyosejh G. Smith
T h e results show that the calcium nitrate decreases the solubility of the sulphate to a marked degree, with increasing concentration of the nitrate.
TABLE IV. Calcium Sulphate in Solutions of Calcium Nitrate ~~_____
I
Weight of 1000 cc. of solution
998.1
1013.8 1031.7
1067.3 I 136.9 1203.5
~
Grams Ca(NO,), per liter
2.084 1.238
0
25
1.196
50
1.134 0,929 0.759
I00 200
300
1265.6 1328.I
400
1352.0
544
0.569 0.403 0.346
500
Fig.
Grams CaSO, per liter
I
T h e above results with the four nitrate solutions are perhaps better seen from the accompanying graphic representation. T h e curves show very markedly the varying effects of the different salt solutions on the solubility of the calcium sulphate. Magnesium nitrate increases the solubility greatly. Sodium nitrate and potassium nitrate have about the same increasing effect ; the sodium nitrate shows the customary maximum values in the higher concentrations of salts containing no com-
s
499
mon ion ; the potassiuni nitrate is not plotted beyond the point where syngenite separated. T h e calcium nitrate depressed the solubility. In general, when no common ion is present the solubility is increased, while a common ion decreases the solubility. T h e decrease in the higher concentration of the sodium nitrate remains, however, unexplained by this general statement and is probably connected with the formation of molecular complexes with the solvent itself in these higher concentrations. It may be of interest i n this connection to state that a.comparison of the solubility of calcium sulphate i n the solutions of the nitrates here employed with the results obtained with the corresponding chlorides shows that in general a greater increase of solubility is noticed in the case of th,e nitrates. In conclusion the authors wish to thank Dr. F. K. Cameron for his kindly interest and advice. Bweau of Soils,
U.S. Department of Agriculture, Washington,D.C.
