238 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



we find as a matter of experience that the proportion 



CNaXCci xp-v 



— 7^ w; 



v^NaCl 



is not a constant for different concentrations, as the corresponding expres- 

 sion is for all other kinds of reversible reactions, including the ionization of 

 weak bases and weak acids. The above proportion for a strong electrolyte 

 grows larger with increasing concentration — the most promising explana- 

 tion of this apparently abnormal behavior being perhaps that the ionizing 

 power of the solution is changed by the presence of considerable numbers 

 of ions (the "salt-effect" of Arrhenius). 



This result, applied to the ionization of calcium carbonate (equation 2) , 

 would mean that the proportion 



Cca X CcOa (6) 



, , CcaCOa 



IS not a constant. 



It is ordinarily assumed that for the saturated solutions of difficultly 

 soluble salts the molecular concentration of the salt (here CcacOa) is a 

 constant at a given temperature in all aqueous solutions: we would expect 

 then, from what has just been said, that the product CcaXCcacOa could 

 not have a constant value, since it is the numerator in the variable propor- 

 tion (6). As a matter of experiment, however, A. A. Noyes, Findlay, and 

 others found in a number of carefully studied cases of similar salts that 

 the solubility products are constants, whether the given salt is present alone 

 or in the presence of another salt modifying its ionization, such as a salt 

 with an ion in common with it. 



The peculiar discrepancy between the empirical results expressed in (5) 

 and (6) on the one hand, and the empirical result expressed in the constancy 

 of the ion product on the other hand, as just explained, seems to find a 

 satisfactory explanation in the work of Arrhenius on the solubility of 

 salts in salt solutions.* 



Arrhenius shows that the old view must be abandoned that in the pres- 

 ence of excess of the solid salt, the solubility of the molecular or non-ionized 

 salt is a constant in different saturated solutions {i.e., in solutions saturated 

 in the presence of other salts). His experiments on the solubility of the 

 silver salts of various organic acids (silver acetate, valerate, etc.) in the 

 presence of varying amounts of the sodium salts of the same acids show 

 that the solubility of the non-ionized silver salt grows smaller with increased 

 concentration of the sodium salt present. In other words, there is no con- 

 stant molecular solubiUty of a precipitate, as has so long been assumed. 

 The case is analogous to the decreased solubility of gases, such as oxygen or 

 carbon dioxide, in salt solutions as compared with pure water— a fact which 

 is used below in the discussion of the equilibrium conditions. 



Arrhenius uses his data as an argument against the correctness of some 

 of Noyes's conclusions. No attempt, however, seems to have been made by 

 Arrhenius in this paper to study the ion-products for the silver salts in his 

 own experiments, and calculations were therefore made with his material to 



' Zeitschrift fur Physikalische Chemie, 31, 221 (1899). 



