DISSOCIATION OF STRONG ELECTROLYTES 



123 



iabove expression. All that is necessary is to choose the concentration 

 Ci SO small that the activity factor could be assumed to be = 1. 

 Such determinations can be made for all those ions for which rever- 

 sible electrodes are available, especially H+, Cl~, 0H~; and elec- 

 trotytes such as, for example, HCl, KOH, KCl, NaCl, etc. are 

 appropriate for this purpose. 



Thus Noyes and Mac Innes^" employing this method found the 

 values given in table 21 for the activity factor. 



TABLE 21 



34. The hydration of ions and its significance for the activity 



Bjerrum's approximation for the calculation of the activity factor 

 may be applied with sufficient accuracy to electrolyte solutions of 

 concentration up to about 0.2 molar, and, therefore, quite within the 

 limits of concentration most frequently encountered in physiological 

 investigations. But for higher concentrations it fails so completely 

 that, for example, for a 3 A^" solution of HCl the activity factor was 

 experimentally found to be > 1 (see table 21), while theoretically this 

 factor must decrease with increasing concentration. According to 

 Bjerrum the cause for this discrepancy lies in the fact that the activity 

 of the ions is influenced not only by the interionic electrostatic forces, 



" Journ. Amer. Chem. Soc. 42, 239 (1920). 



