DISSOCIATION OF STRONG ELECTROLYTES 113 



trates this point for KCl, HCl, KOH and Na-acetate, calculated 

 from the conductivity measurements of Kohlrausch and his 

 coworkers. 



It has been attempted purely empirically to define the value of k 

 in different ways, and we have an empirical expression given by 

 van't Hoff- as well as one by Rudolphi.^ Van't Hoff's formula is 



Of' X c , 

 = k 



= k 



(1 - aV 



and Rudolph i's 



(1 -a) 



Both are interpolation formulae that are practically usable. 



Arrhenius* had already attempted an explanation of these findings. 

 He assumed that because of the contraction of the volume of 

 water ("electrostriction") accompanying the process of solution of 

 salts, alterations occurred in the dielectric constant, the osmotic 

 pressure and the motility of the ions. As a consequence of this the 

 dielectric constant and hence the dissociating capacity of the solvent 

 upon the electrolyte are increased with the increasing concentration 

 of the electrolyte which is being dissolved. In order to demonstrate 

 this idea Arrhenius showed that the dissociation constant of a weak 

 electrolyte (acetic acid) increases when a strong electrolyte (NaCl) is 

 added to its solution. Thus the inverting capacity of acetic acid for 

 <;ane sugar was also materially increased by the addition of salt. 

 After allowing for that portion of the effect which he ascribed to the 

 purely catalytic effect of the salt, he observed a still further accelera- 

 tion of the catalysis by the salt, which he ascribed to the increased 

 dissociation of the acetic acid effected by the presence of the salt. 



But even without such elaborate presentations, the foresight of 

 theoretical chemists has long recognized that this contradiction to the 

 mass law is only an apparent one. As Nernst puts it in his textbook: 

 "it is highly probable, although on still unknown grounds, that 

 neither the electric conductivity nor the osmotic pressure (lowering of 

 the freezing point) give us an accurate scale for measuring the degree 

 of dissociation." 



2 van't Hoff, Zeitschr. f. physikal. Chem. 18, 300 (1895). 

 » C. Rudolphi, Zeitschr. f. physikal. Chem. 17, 385 (1895). 

 * Sv. Arrhenius, Zeitschr. f. physikal. Chem. 31, 198 (1899). 



