256 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



where /S is the solubility of the first electrolyte in pure water and M 01 + 

 is the ion concentration in this solution. As the reciprocal of the total 

 ion concentration, 1/2C-, or the common ion dilution 2F^, decreases, the 



solubility decreases as a linear function 1 of this variable, approaching the 

 value S = & . at 1/2C . = 0. 



U> (/ 



If S, is a constant, as it is if the laws of ideal solutions hold, and if 



tv 



K! is a constant, then it follows from Equation 66 that 

 (69) M l *XX- = K 1 S u = K, 



where X~ is the concentration of the common ion in the solution, and K 

 is a constant. For an electrolyte in solution in equilibrium with its 

 solid phase, the product of the concentrations of the ions remains con- 

 stant, provided that the laws of dilute solutions hold. According to these 

 considerations, the solubility of a given electrolyte may be depressed to a 

 value which corresponds to the concentration of the un-ionized fraction 

 in a solution of the pure electrolyte in equilibrium with its solid phase. 



The foregoing relations are based on the assumption that the laws 

 of dilute solutions are applicable. As we have seen, this condition is not 

 fulfilled in solutions of strong electrolytes. The effect of the presence 

 of strong electrolytes upon the solubility of other strong or weak elec- 

 trolytes can, therefore, be determined 'by experiment only. 2 ** The con- 

 centration of the various molecular species in the mixture cannot be deter- 

 mined, even though the solubility of the first electrolyte is known, unless 

 a law is assumed governing the equilibrium of the various electrolytes 

 present in the mixture; and the results obtained for the concentration of 

 the ionized and the un-ionized fraction of the first salt in the mixture, 

 as calculated, will depend upon the laws assumed as governing the equi- 

 librium in the mixture. 



We shall first examine the effect of strong and weak electrolytes upon 

 the solubility of weak electrolytes; that is, electrolytes which conform 

 to the simple mass-action law. Such determinations have been made by 

 Kendall. 22 



In Table CII is given values for the solubility of a number of weak 

 acids in the presence of other acids, both weak and strong. 



The results are shown graphically in Figures 49 and 50. Considering 

 first the .solubility of orthonitrobenzoic acid and salicylic acid in th< 



m It is evident from Equation 69 that KI and 8 U might vary in such a manner tha 



their product would remain constant, in which case the ion product would remain con 

 stant. It is very improbable, however, that such a compensation actually occurs. 

 "Kendall, Proc, Roy. Soc. 85 A, 218 (1911). 



