118 HYDROGEN ION CONCENTRATION 



ciprocal effect in an ionized solution of a completely dissociated 

 electrolyte, whose molecule jdelds n ions. Then the electric con- 

 ductance of this solutionis such as if only the fraction n X e"'^''"^^ of 

 the mol were freely motile, while the remainder of the mol did not 

 participate in the conduction of the current. This latter part of the 

 mol is the one which, according to the earlier conceptions, was con- 

 sidered as the undissociated molecule. But according to Ghosh the 

 electrolyte is completely ionized, just as according to the modern 

 theory of structure of crystals, a salt in its soHd cr>^stalUzed state is 

 totally ionized. Only at this juncture a deviation from the ordinary 

 conception of an ion slips in, as it were. From the very beginning 

 of the theory of electrolytic dissociation the term ion stood for a 

 charged molecule (or "half-molecule") which moves under the in- 

 fluence of the electric current, but which was not originally split off 

 from the uncharged molecule by the action of the current. Thus two 

 properties had to be interrelated in order to characterize the ion: 

 the free charge and the free motion of the particles. After it 

 had been recognized that even in the crystal only these charged 

 "half-molecules" and no salt-molecules proper were present, these 

 were likewise called ions, although they lack the second charac- 

 teristic, the free motility. Therefore, in accord with the definition 

 of an ion, we shall either say: a dissolved electrolyte is always 

 completely dissociated into ions, but only a fraction of it is freely 

 motile; or: a dissolved electrolyte is always only partially dissociated 

 into ions. It does appear, however, that the first mentioned, the 

 newer conception of Ghosh, will become the dominant one in the 

 future. It is even very possible that this conception is valid not only 

 for the strong electrolytes, but for all electrolytes in general. If this 

 is true, then a weak acid becomes under all conditions what we 

 designated earher as a "pseudo-acid" (see page 37), which exists in 

 two tautomeric forms, of which one is not dissociated at all and the 

 other completely dissociated. 



The value a = — , when it is taken not as denoting the degree of 



Aco 



dissociation but rather in the sense of Ghosh as expressing the 

 effect of the autopotential A, is calculated as e~^^"^^, or we can 

 state it as 



A = -nRT In a 



