10 HYDROGEN ION CONCENTRATION 



best for aqueous solutions in which water is the solvent medium. 

 Whenever any electrolyte is dissolved in water, the resulting solu- 

 tion contains not only the ions of this electrolyte, but also the ions 

 yielded by the water, namely, H+ and 0H~ ions. While the degree 

 of the dissociation of water is indeed very slight, still, the purest 

 water shows a definite capacity to conduct an electric current. 

 This fact can only be explained on the basis of its electrolytic disso- 

 ciation, be it ever so slight, if Arrhenius's theory is to be accepted 

 as being correct. F. Kohlrausch and A. Heydeweyller* obtained 

 after repeated distillations of water under rigorous precautions a 

 final value for the conductivity of water, which value could not be 

 diminished or altered by further purifications. In this way they 

 were the first to establish the numerical value for the dissociation 

 of water. Subsequently many other methods were developed for 

 the determination of the same value, which will be described later, 

 and which corroborated Kohlrausch's findings. 



The degree of dissociation of water is, as was already stated, very 

 slight, i.e., the H+ and 0H~ ions have a very great tendency to 

 unite and to form H2O. On the other hand, most electrolytes 

 when dissolved in water dissociate into ions to a much greater 

 extent, and some are even completely dissociated. This unique 

 behavior of water has two noteworthy consequences : 



1. If a strong electrolyte, such as KCl for example, which yields 

 neither H+ nor 0H~ ions, is dissolved in water, then the number 

 of H+ and OH" ions yielded by the water itself is so insignificant 

 in comparison with that of the other ions present, that many of the 

 properties of the given solution may be completely accounted for by 

 the ionization of the dissolved electrolyte, as, for example, the con- 

 ductivity and the depression of the freezing point. The dissocia- 

 tion of the water itself is not, however, influenced by a strong 

 electrolyte dissolved in it. 



2. But, if an electrolyte which itself yields either H+ or OH" 

 ions be present in an aqueous solution, then the dissociation of the 

 solvent, water, is fundamentally affected. These special electro- 

 lytes have such remarkable properties that they had been placed 

 in a separate class long before the theoretical basis of their peculiar 

 behavior was clearly understood. The electrolytes which give off 



* F. Kohlrausch und A. Heydweyller, Zeitschr. f. physikal. Chem. 14, 317 



(1894). 



