PROPERTIES OF AQUEOUS SALT SOLUTIONS 319 



concentrations of both ions produced by dilution. And in reality this 

 conclusion, if regarded as a general expression of the facts, is entirely 

 unwarranted. It is true that for certain typical combinations of salts 

 those for which from one molecule of each salt results by ionization 

 not more than one ion of the kind not common to the salts the 

 principle here stated does coincide with the requirement of the Mass- 

 Action Law. But for combinations not so characterized the Mass-Ac- 

 tion Law predicts, as is readily seen upon formulating the equations, 

 a conductivity of the mixture widely divergent from that actually 

 found, and, therefore, from that expressed by the principle under 

 consideration. This last statement applies, for example, to the mix- 

 tures before referred to of potassium sulphate with sodium sulphate, 

 and of potassium sulphate with copper or magnesium sulphate, the 

 first of which have been studied both with respect to their conductiv- 

 ity and freezing-point. The Law of Chemical Mass- Action here again 

 shows itself entirely inapplicable to the phenomena connected with 

 the ionization of salts. The opinion of some investigators that the 

 deviations from this law indicated by the conductivity were only 

 apparent, and that they were attributable to variations in the migra- 

 tion-velocity, has arisen, no doubt, from the fact that they have con- 

 fined their attention to di-ionic salts, and have failed to recognize, on 

 the one hand, the striking divergences from it exhibited by tri-ionic 

 salts, and, on the other, the substantial correspondence of the con- 

 ductivity and freezing-point results. 



Combining this principle in regard to the ionization of mixed salts 

 in solution with the empirical concentration law of Storch for single 

 salts, we are led to the conclusion that the ratio of the concentration 

 of the un-ionized part to the product of the concentrations of the two 

 ions (but in the case of tri-ionic salts not raised to a power correspond- 

 ing to the requirements of the Mass-Action Law) is a function of the 

 sum of the equivalent concentrations of all the ions in the solution and 

 of that alone. 1 This ratio is, moreover, roughly inversely proportional 

 to the square root of the total ion-concentration. 



The correctness of this principle is further demonstrated by the 

 fact that with its aid the conductivity of a mixture of two salts with- 

 out a common ion can be computed from their separate conductivities. 

 This is shown by the conductivity measurements, made by Archibald 

 and more recently by Sherrill, upon mixtures of potassium chloride 

 and sodium sulphate, or of sodium chloride and potassium sulphate. 

 Up to at least 0.2 normal concentration, the agreement between the 

 observed and calculated values is within 0.5 per cent. On the other 



1 This is expressed mathematically by the following equation in which c, and c a 

 represent the equivalent concentrations of the two salts, and yi and y 2 their de- 

 grees of ionization in the presence of each other: 



