Ionic interactions in solutions. II. The theoretical basis of the equilibrium between free and pairwise associated ions

It is shown from recent results based on the statistical mechanics treatment of ionic solutions that the notion of an equilibrium between free ions and paired ions which wasa priori introduced in the chemical model of Bjerrum is essentially correct. The mass-action law which is deduced from such a model is indeed derivable from the more exact treatments of Mayer, Rasaiah, and Friedman starting directly from Hamiltonian models and without any assumptions as to the chemical structure of the ionic solute in the solution. The transformation relation from a primitive-model result to any short-range Hamiltonian model equation is given. It allows a simple mathematical treatment of experimental data for dilute solutions, the derivation of useful information on the Gurney solvation effects, and an interpretation of the multistep association concept.