DISSOCIATION OF STRONG ELECTROLYTES 125 



it may be assumed that there is a molecular attraction between 

 every individual ion and all water molecules, which rapidly decreases 

 with the distance from the water molecule. In this way we arrive 

 at the idea that every ion is surrounded by a layer of closely adhering 

 water molecules, and this in turn is surrounded by other layers of 

 progressively less adhering water molecules. At the same time it is 

 to be assumed that the reactivity of the ions (or their "activity") 

 is diminished by being bound by water, and that this curtailment 

 decreases in the presence of but little water. 



Attempts to determine the hydration capacity of different ions^^ 

 by various methods based on the first conception outlined above led 

 to contradicting results. Thus Nernst gives the following figures for 

 infinitely dilute solutions (taken from Riesenfeld) : 



H(0) K(20) Ag(3o) iCd, Cu(55) Na(70) Li (150) 



OH(IO) ISO4, I, Br, Cl(20) N03(25) ClOsCSS) 



The figures in parentheses represent the numbers of bound H2O- 

 molecules. On the other hand Bjerrum found: 



H(8) CI (2) K(0) 



Thus the uncertainty concerning such figures is very great, and 

 each of the many methods tried yielded different values. But at all 

 events this uncertainty would seem to speak against the idea of 

 stoichiometrically defined hydrates and it should be interpreted as 

 being in favor of the theory of a general attraction existing between 

 the ions and the surrounding water-molecules. ^^ 



On the other hand, almost all investigators agree on the serial 

 sequence of the degree of hydration of the ions. Thus it is always 

 observed that the hydration of the H-ion is the least of all. With the 

 alkali metal ions the degree of hydration decreases with increasing 

 atomic weight in the manner. 



Li > K > Na > Rb > Cs 



while with the halogen ions, reversely, it increases with increasing 

 atomic weight, but not to so great an extent. 



The obvious interpretation of this phenomenon is the following: 

 The attraction for the water is evidently exercised by the free charge 



12 Nernst, Theoret. Chem. 7. Aufl. 1913, P. 413. 

 " K. Fajans, Naturwissenschaften 9, 729 (1921). 



