LAWS OF ELECTROLYTIC DISSOCIATION 



19 



adsorption, the above multiplicity of ways of writing reactions is 

 not a futile exercise but quite a useful procedure. 



5. The influence of dissolved substances upon the dissociation 



constant of water 



It was stated above that every substance dissolved in water has 

 an influence upon its dissociation constant as a result of changes in 

 the pressure of the water vapor and in the mass of the undissociated 

 water. This influence is however very slight. The vapor pressure 

 of a unimolar solution of a nonelectrolyte (sugar, urea, etc.) is only 

 two per cent less than that of pure water, in solutions of binary 

 electrolytes this effect is approximately doubled. But molar solu- 



TABLE 2 



tions are of such great concentration that they may be left out of 

 consideration in this discussion. And even with such high concen- 

 trations the effect upon the value of kw is within the present limits 

 of error involved in the determination of k^. 



Let us next consider the conditions under which a dissolved sub- 

 stance markedly affects the dielectric constant of water. The 

 dielectric constant and kw diminish in value in aqueous solutions of 

 alcohol with increasing concentrations of alcohol. But in this case 

 also only the higher concentrations have a marked effect. The 

 figures in table 2 were obtained by Lowenharz^- for aqueous alco- 

 holic solutions at 25°C. 



" Lowenherz, Zeitschr. f. physikal. Chem. 20, 283 (1896). 



