264 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



dall, 25 discussed in the preceding section, that even polar molecules, such 

 as we have in the weak acids, are affected only to a slight extent by the 

 presence of strong acids. The depression of the concentration of the 

 un-ionized fraction of a strong electrolyte in equilibrium with its solid 

 phase, due to the addition of other electrolytes, has been ascribed to 

 interaction between the ions and the un-ionized fraction of the first salt, 

 and the salting out effect in the case of non-electrolytes has been cited in 

 support of this hypothesis. From the foregoing analysis, however, it 

 would appear that the behavior of non-electrolytes, as well as that of 

 weak electrolytes, in the presence of strong electrolytes, lends little sup- 

 port to this hypothesis. On the whole, it appears much more likely that 

 the concentration of the un-ionized fraction varies as a function of the 

 nature of the added electrolyte, and that, in general, it varies less than 

 indicated by the calculated values given above. 



According to the above calculation, the ion product varies consider- 

 ably with the concentration of the added electrolyte and depends, to a 

 considerable extent, upon the nature of this electrolyte. Observations 

 on the solubility of salts in the presence of other salts indicate that, even 

 in the case of strong electrolytes, the ion product remains approximately 

 constant on the addition of other electrolytes. 26 It is at once evident 

 that if the concentration of the un-ionized fraction is only slightly de- 

 creased on the addition of a second electrolyte, the concentration of the 

 ions is appreciably smaller than that derived from calculations on the 

 basis of the iso-ionic principle. The result is to render the value of the 

 ion product approximately constant and independent of the concentration 

 of the added electrolyte. 



As we have seen, the conductance of mixtures of hydrochloric acid 

 and sodium chloride, calculated on the assumption that the equilibrium 

 in the mixture is governed by the isohydric principle, is not in accord 

 with the experimentally determined values. On the other hand, we saw 

 that, in the more dilute solutions, the observed values agree very nearly 

 with the values calculated on the assumption that in the mixture the equi- 

 librium conforms to Equation 52. It is evident that if C remains con- 

 stant in the mixture, P^ will likewise remain constant. If, therefore, 



the concentration of the un-ionized fraction of a salt remains constant, 

 the ion product should also remain constant according to this principle. 

 Assuming this principle to hold, we may calculate values for the con- 

 centration of the un-ionized fraction and for the ion product in the case 

 of a salt in equilibrium with its solid phase in the presence of a second 



** Kendall, Zoc. oit. 



"Stteglitz, J. Am. Chem. Soc. SO, 946 (1908). 



