The Dissociating Poivers of Free and of Combined Water. 121 



SOLVENTS. 



Water. The water was purified by the method of Jones and Mackay 1 

 as modified by Schmidt, 2 and had a mean specific conductivity of 1.8 X 

 10- 6 at 25 C. 



Isohydric Solutions. If two solutions of electrolytes are mixed the 

 conductivity of the mixture is, in general, less than the mean of the 

 conductivities of the constituents. If the two solutions contain a 

 common ion, however, there are concentrations at which they can be 

 mixed without affecting each other's conductivity. This fact was first 

 explained by Arrhenius. 3 He showed that if equal volumes of two 

 solutions of acids of certain concentrations be mixed, the conductivity 

 of the mixture is the mean of the conductivities of the solutions, pro- 

 vided there be no appreciable change in volume. Such solutions are 

 said to be isohydric. Arrhenius 4 defines them as follows : 



"Two solutions of acids are isohydric whose conductivity, or in other words, 

 whose electrolytic dissociation, is not changed if they are mixed." 



Arrhenius worked out the condition for two solutions containing a 

 common ion to be isohydric and has expressed it thus: 



-=^- m 



Vl t>2 



In this equation, a = percentage dissociation of the salt in solution, 

 Vi = number of liters of solution containing a gram-molecular weight of 

 the salt, m = number of common ions in each molecule of the salt. 

 /3, v z , and n are the respective symbols for the second solution. 



It was further shown by Arrhenius that two acids are isohydric if in a 

 unit volume they contain the same number of hydrogen ions. With 

 this principle in mind the investigator, in the course of his work, 

 determined the concentrations of five different pairs of salt solutions 

 when they fulfill the condition of being isohydric. This method 

 follows. 



In calculating the percentage dissociation by the conductivity 

 method 



Here, M' and /i" = molecular conductivities of the two solutions; 

 and //'oo = the conductivities at infinite dilution. 



From the method of Kohlrausch for calculating conductivity 



/0 s 

 (3) 



'Amer. Chem. Journ., 19, 90 (1897). 3 Wied. Ann., 30, 51 (1887). 



2 Carnegie Inst. Wash. Pub. No. 180, 135 (1913). 4 Zeit. physik. Chem., 2, 284 (1888). 



