2i 4 SCIENCE PROGRESS 



Development of the Chemical Theory of Ionisation 



The first suggestion that ionisation depends, not on the 

 physical properties of the solvent, but on a chemical equilibrium 

 between solvent and solute, was made by Arrhenius in his 1883 

 memoir as quoted above. This suggestion possesses obvious 

 merits, but it is only in the most recent years that its advantages 

 have been generally recognised. These may be broadly stated 

 as follows : 



(a) If, as Arrhenius suggested might be the case, " the active 

 part [of the electrolyte] is a compound of the [solute] and 

 solvent," the proportion of solute present in the active form will 

 depend on the relative concentrations of solvent and solute, and 

 will reach a maximum when the solvent is present in large 

 excess. The equilibrium will necessarily be " such that dilution 

 increases the active and diminishes the inactive part," and the 

 effect of dilution in increasing ionisation will therefore receive 

 at least a qualitative explanation if the chemical theory of ionisa- 

 tion is adopted. 



(b) There is abundant evidence to show that the tendency to 

 form hydrates rapidly decreases when the temperature is raised. 

 If ionisation depends on the formation of similar compounds, it 

 is clear that the ionising properties of a solvent must decrease 

 when the temperature is raised, and may be expected to vanish 

 at the critical temperature of the solution when the solute is no 

 longer able to retain the solvent in the liquid form. The con- 

 ductivity of a composite electrolyte usually increases with the 

 temperature, but this is due to the increased mobility of the 

 ions, and in every case is accompanied by a decrease in the 

 coefficient of ionisation, which becomes more and more pro- 

 nounced until at the critical temperature conductivity and 

 ionisation disappear together. 1 The effect of temperature on 

 ionisation is therefore entirely in accord with the requirements 

 of the chemical or " hydrate " theory of ionisation. 



(c) The part played by residual affinity in promoting ionisa- 

 tion also receives a satisfactory explanation if it is recognised 

 that the process involves the formation of additive compounds 

 of solvent and solute. Such compounds are not produced in 

 the case of solvents which are fully saturated, but water and 



1 For a discussion of the influence of temperature on conductivity see Bousfield 

 and Lowry, Proc. Roy. Soc. 1902, 71, 42. 



