260 



EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



solubility again, which Cameron ascribes to the condensation of the solvent 

 in aqueous solutions of electrolytes.^ 



Entirely analogous effects must be anticipated for the action of sodium 

 chloride on calcium carbonate and bicarbonate, their ions also being sup- 

 pressed, for instance, according to 



CaCHCOa)^ + 2NaCl±? CaCl^ + 2Na(HC0,) 



calcium chloride and sodium chloride being formed in considerable quanti- 

 ties according to the principle of isohydric solutions.^ 



Again, the molecular solubility of calcium carbonate is liable to be 

 decreased, as was discussed for the sulphate, and we have in this case also 

 the fact that the solubility of carbon dioxide is considerably less in salt 

 solutions than in pure water, and this decreased solubility will reduce the 

 amount of bicarbonate dissolved approximately proportionately to the cube 

 roots of the change in the coefficients of absorption.* So we have forces 

 tending to increase the solubility as well as such as tend to decrease the 

 solubility of calcium carbonate. As a matter of fact, Cameron found that 

 at 25° for calcium carbonate and bicarbonate in equilibrium with air,* the 

 solubility was increased from 0.1046 gram bicarbonate per liter to 0.2252 

 gram by 51 grams (0.87 mole) sodium chloride, and then it was decreased by 

 additional sodium chloride. There is, therefore, at first an increase in solu- 

 bility and then a decrease, exactly as for gypsum. The characteristic bend in 

 the curve occurs earlier than in the case of gypsum, which was to be expected, 

 as the molecular solubility of carbon dioxide is also affected in this case. 



Cameron ^ also determined the effect of sodium chloride on the solu- 

 bilities of calcium sulphate, carbonate, and bicarbonate simultaneously, 

 i.e., in mixtures in equilibrium with solid gypsum, solid calcium carbonate, 

 and the air. This is the work that is of most interest and importance for us, 

 and as we shall use the data the table is reproduced here (table 7) . 



' Loc. cit., 5, 576. 



^ This has been shown to be reliable up to 0.4 normal solutions. (Summary by A. A. 

 Noyes, loc. cit.) 



' When no other calcium salt is present, the solubility changes according to the cube 

 root of the concentration of the carbon dioxide (equation (30), p. 247). When another 

 calcium salt, e.g., gypsum, is present in large excess, so that Cca may be considered constant, 

 the solubility changes approximately as the square root of the concentration of carbon 

 dioxide (equation (25), p. 246, and (42), p. 252). 



* No mention is made as to the partial pressure of the carbon dioxide, whether " labora- 

 tory air" or pure country air was used. 



* Loc. cit., 5, 653. 



