The Thermodynamics of Ion Association

Two important factors which determine whether ionic association is present in an electrolyte solution are the charge on the ions and the dielectric permittivity of the solvent. Their roles are clearly seen on the basis of Coulomb s law, which shows 

Derivation of the thermodynamic equations for an electrolyte system with ion pairing follows the same procedure given for a weak electrolyte. However, in the following the ion pairing equilibrium is defined in terms of an association process. For a 1-1 electrolyte, ion pairing is described as 

Here and px e the standard chemical potentials for the two ions on the molarity scale, and that for the ion pair. 

As a result of ion pairing, a fraction a, of the dissolved electrolyte remains in solution as ions and a fraction (1 — a,) exists as ion pairs. Thus, the chemical potential of the electrolyte can be expressed as 

This result shows that the chemical potential of the electrolyte can be expressed in terms of the activities of the two ions without considering that of the ion pair. It was obtained earlier for weak electrolytes as equations (3.6.38) and (3.6.40). 

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