124 HYDROGEN ION CONCENTRATION 



but also by the degree of hydration of the ions^^ which changes with 

 the concentration. Only the essential features of this conception 

 will be touched upon at this juncture. 



There are many grounds for beheving that there exists an affinity 

 between the molecules of a dissolved substance and the solvent, and 

 that the solvent is not merely the space in which the molecules are free 

 to move. This affinity for the solvent becomes even more probable 

 in the case of ions. At least two theories can be presented concerning 

 the nature of this affi.nity. 



In the first place, it may be assumed that each ion forms a hydrate 

 stoichiometrically with a definite number of water molecules analo- 

 gously to the formation of salts with water of crystallization. It 

 can be conceived that in concentrated solutions with an insufficient 

 excess of water the hydration of ions does not reach its maximum 

 extent. But ions of different degrees of hydration cannot be con- 

 sidered equivalent from the standpoint of the mass law. It can 

 further be conceived that only the few unhydrated ions that are 

 present take part in the electromotive activity or in chemical reac- 

 tions, or stated in other words, that the ions are first dehydrated 

 when thej^ become active. It is also conceivable that a very small 

 number of water-free ions are in a state of equilibrium with the 

 hydrated ions. Then it would follow that the active ionic mass is 

 proportional to the concentration of the water-free ions. As long as 

 the water is present in a large excess the ratio of water-free to 

 hydrated ions will remain constant, and the active masses of the 

 ions will be proportional to their respective concentrations. On 

 the other hand, in concentrated solutions, this ratio is disturbed in 

 the direction of the water-free ions, and the activity of the ions 

 will be apparently too great. Bjerrum had even attempted to 

 calculate the degree of hydration of ions from this deviation of 

 the observed from the calculated activity values. In reality it 

 can be easily observed from the preceding table how at the higher 

 concentrations the factor fa exceeds the value 1, which without these 

 conceptions would be quite inexplicable. 



The second possible theory would be based on the supposition that 

 there are no hydrated ions formed in a stoichiometric way. Instead 



" O. Sackur (Zeitschr. f. physiol. Chem. 70, 493 (1910)) had already made 

 use of the theory of hydration in an attempt to explain the above discrepancy 

 in the case of strong electrolytes. 



