AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 259 



CALCIUM SULPHATE, CARBONATE, AND BICARBONATE IN THE 

 PRESENCE OF SODIUM CHLORIDE. 



In both the previous cases discussed it was found that the absence of 

 calcium carbonate in gypsum crystalHzing from solutions containing cal- 

 cium carbonate in equilibrium with the carbon dioxide of the air would be 

 considered an indication of a very low partial pressure of the carbon dioxide. 

 Even if the great mass of the excess of calcium carbonate in solution were 

 deposited first, in the same locality or elsewhere, before the point of satura- 

 tion for gypsum were reached, the requirements for equilibrium would be 

 such as to keep so much of the carbonate in solution as to form an easily 

 discernible contamination of the gypsum formed by further concentration. 



In nature the crystalhzation of gypsum is supposed to occur usually by 

 the concentration of waters containing a large excess of other salts, notably 

 of sodium chloride, and the last question we shall try to consider now is the 

 effect of sodium chloride on the conditions discussed in the preceding parts. 



A salt like sodium chloride which has no ion in common with gypsum 

 should, according to the law of mass-action, increase the solubiUty of the 

 latter up to a certain point; the chloride and sulphate must react to a con- 

 siderable extent to form calcium chloride and sodium sulphate: 



CaSO^ + 2NaCl ^ CaCl^ + Na^SO, 



As the ionization of the new salts in moderately concentrated solutions is 

 by no means complete, considerable amounts of calcium and sulphate ions 

 must be suppressed to form these salts in non-ionized form, and this would 

 lead to an increased solubility of calcium sulphate according to equation 

 (35), page 250. In fact, a rough calculation of the result of Cameron's ^ 

 determination of the effect of sodium chloride in 0.017 molar solution on 

 the solubility of gypsum, made with the aid of Arrhenius's principle of 

 isohydric solutions, led to a value for the ion or solubihty product for 



calcium sulphate ^ ^ .^ « 



CcaXCso4 = 9.5X10-' 



in the presence of the salt, as compared with 6.5 X lO"', the value of the solu- 

 bility product at 23° in the absence of salt. 



For concentrated salt solutions the conditions, as is always the case, 

 become more and more complex; we are more Ukely to have complex ions, 

 such as NaSO^, and double salts formed in large quantities and leading 

 not so much to abnormal changes in solubility as to changes which we have 

 no means of estimating at present.' Cameron, however, has given us a 

 large amount of empirical data which will be useful for the consideration of 

 our subject. In regard to gypsum and sodium chloride^ he finds the solu- 

 bility of the former is rapidly increased, rising from 2.37 grams calcium 

 sulphate (0.0174 mole) per liter to 7.50 (0.0555 mole) in the presence of 

 130 grams of sodium chloride, after which there is a gradual decrease in 



^ » Loc. cit., 5, 560. . , , . 



' As to the lowering of solubility of the non-ionized calcium sulphate in salt solutions, 

 comparable with the decreased solubility of carbon dioxide, see pp. 237, etc. 



17 



