LAWS OF ELECTROLYTIC DISSOCIATION 



25 



a. From the conductivity oj pure water. F. Kohlrausch and R. 

 Heydeweiller^'^ determined the conductivity of absolutely pure 

 water. Since the conductivities of the H- and OH-ions were known, 

 the concentration of these ions could be calculated from the ob- 

 served conductivity of pure water. At 25° the product [H+] X 

 [0H-] = k„ = 1.1 X 10-1^ 



b. From the catalytic capacity oj pure water. At the suggestion of 

 van't Hoff, J. J. A. Wijs^^ determined the velocity of saponification 

 of methyl acetate by pure water. Tlie OH-ions function as catalyzers 

 and the velocity of the reaction is proportional to their concentra- 

 tion. This observed velocity was compared with that of the same 

 saponification carried out by the use of an alkali solution of known 

 [0H~] concentration and the [0H~] of pure water was easily calcu- 

 lated from these data, and furthermore in pure water [H+l = [0H~]. 

 Here it was found that at 25° [H+] X [0H-] = k^ = 1.44 X 10-'\ 



c. From the degree of hydrolysis oj salts. Salts of a weak acid and 

 a strong base in aqueous solution are in part split into free acid 



TABLE 6 



and free base, which in turn yield to the solution by electrolytic 

 dissociation H+ and 0H~ ions. But since the base is stronger, its 

 dissociation predominates, and as a result there is an excess of 

 0H~ ions over H+ ions. In this case the dissociation constant of 

 the water may be derived, if the dissociation constant of the weak 

 acid is known. This point will be taken up again later. Or the 

 [0H~] may be determined by a catalytic experiment as above in (b). 

 Arrhenius^^ obtained from the hydrolysis of sodium acetate at 

 25° 



kw = 1.21 X 10-1* 



From the hydrolysis of other salts several investigators obtained 

 similar figures. Lunden^4 found from a study of the hydrolysis of 

 salts of a weak acid and a weak base the figures shown in table 6. 



1^ F. Kohlrausch and A. Heydweiller, Zeitschr. f. physikal. Chem. 14, 317 

 (1894). 



1^ J. J. A. Wijs, Zeitschr. f. physikal. Chem. 11, 492 and 12, 253 (1893). 

 18 Sv. Arrhenius, Zeitschr. f. physikal. Chem. 1, 631 (1887). 

 ''' H. Lunden, Journ. de Chim. phys. 5, 574 (1907). 



