222 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 

 vided that C/ and C/' are equal. Equation 51, therefore, is the analyti- 



V V 



cal expression of the isohydric principle. In the limit, as the second com- 

 ponent in the mixture disappears, the equation reduces to that for the 

 first salt alone. In the case of mixtures of electrolytes without a com- 

 mon ion the same expression applies. 



Equation 51 is not the only function which might be assumed to hold 

 for the mixture which reduces to the form of Equation 11 in the case 

 of a solution of a single electrolyte. We might assume for the mixtures 

 a function of the form: 



P t 

 (52) -^-=F(P.) 9 



^u 



where again P^ is the ion product. In the limit the concentrations of 



the positive and negative ions become equal for the solution of a single 

 salt, and consequently this equation reduces to the form of Equation 11. 

 The isohydric principle, or more generally, the iso-ionic principle, is a 

 consequence of the law of mass-action, but, when the law of mass-action 

 fails to hold, there is no reason for assuming that Equation 51 rather 

 than Equation 52 is correct, for both reduce to the same limiting form 

 in the case of a solution of a single electrolyte. We may, therefore, 

 inquire which of the two functions corresponds most nearly with the 

 experimental values. 



In order to test the functions in the case of mixtures, it is obviously 

 necessary to measure some property of these mixtures by independent 

 means, as, for example, the conductance of a mixture of electrolytes. 

 Assuming that the conductance of the ions in the mixture is the same as 

 that of the same ions in pure solutions, it is possible to calculate the 

 specific conductance of the mixture, if the form of the conductance func- 

 tion for the pure electrolytes is known, and if a function is assumed for 

 the mixture. If the assumed function is correct, then the calculated 

 specific conductance for the mixture should correspond to the measured 

 specific conductance of the mixture within the limits of experimental 

 error. If the calculated and observed values do not correspond, it fol- 

 lows that the function assumed for the mixture is not correct. That an 

 equilibrium actually exists in the mixture appears to be beyond question, 

 although the exact nature of the reaction may be somewhat in doubt. 



Bray and Hunt 2 have measured the specific conductance of mixtures 

 of sodium chloride and hydrochloric acid in water at 25. They have 

 likewise calculated the specific conductance of the mixtures, assuming 



a Bray and Hunt, J. Am. Chem. Soc. S3, 781 (1911). 



