Application of Free Volume Theory to Ion Pair Dissociation Constants

Using Kirkwood's zeroth‐order partition function for a particle in solution, an equation has been derived for the dissociation constant of an ion pair. The equation contains three adjustable parameters: the distance of closest approach, a, of the ions; E_s, the difference in solvation energies of the ions and the ion pair; and ($\mathrm{gv\sigma ̄}$ ) which is a function of the free volumes available to the ions and ion pair. The latter two parameters account for specific solvent effects other than that due to the dielectric constant of the solvent. Comparison of the theory with the experimental data in several forms has been made.