256 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



At present we have in sea-water in 1,000 parts 1.239 grams or 0.009 

 gram molecule of calcium sulphate, 1.617 grams or 0.0135 gram molecule 

 of magnesium sulphate, and 0.860 gram or 0.005 gram molecule of potassium 

 sulphate.' Such a solution would contain 0.0275 mole or 0.055 gram equiv- 

 alent of total sulphates. The degrees of ionization of calcium and magnesium 

 sulphates are practically the same,^ and while potassium sulphate ionizes 

 far more readily, it forms a relatively small component of the system, and 

 we can, with sufficient accuracy for our purpose, consider the degree of 

 ionization for the sulphates to be that of a 0.05 equivalent magnesium sul- 

 phate solution or 48 per cent. Then 



Cso4 = 0.0275x0.48 = 0.0132 



Cca = 0.0091 X 0.48 = 0.0044 

 and 



CcaXCso, = 0.000058 



Since for a saturated solution the solubility product constant is 0.000059 

 (KcaS04)>' we see that this sea- water would be almost saturated with gyp- 

 sum were it not for the modifying influence of the presence of a large pro- 

 portion of sodium chloride and other salts in it.* 



A purely aqueous solution of the above composition should be nearly 

 saturated with gypsum and a large part of the latter should crystallize out 

 during its concentration say to one-quarter its original volume. If at the 

 beginning, when the solution is practically saturated with gypsum, it also 

 at the same time be considered to be saturated with calcium carbonate and 

 bicarbonate in equilibrium with a partial pressure of 0.0003 atmosphere 

 carbon dioxide, we would have (see p. 250) a maximum concentration of 

 carbonate ions rt nmoo 



Then, according to equation 42 (p. 252) we must have sufficient calcium 

 bicarbonate in solution to give a concentration of acid carbonate ions 



VQ 04 V 10—'' 

 7 0X10-" XQ0003XQQ41S3X 2.8X10 



= 0.00039 



and the concentration of the ionized calcium carbonate would be 0.000195 

 gram molecule. Its degree of ionization is 74 per cent, calculated by the 

 method used before, so the total calcium bicarbonate in solution is 0.000264 

 gram molecule per liter. 



• Chamberlin and Salisbury, Geology, p. 309. 



' Kohlrausch and Holborn, loc. cit., p. 200. For 0.01 eqmvalent the degrees of ioniza- 

 tion are 63 and 65 per cent respectively. 

 » Page 250. 



* In this calculation we are not including a study of the effect of sodium chloride, 

 but are limiting ourselves to an examination of the effect of sulphates alone in an aqueous 

 solution. It is imderstood that the calculations are only for a rough orientation, the sim- 

 plicity of the laws for dilute solutions being lost as solutions become more concentrated and 

 more complex. Vide also E. C. Sullivan, Journal of the American Chemical Society, 27, 

 .529 (1905). 



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