Thermodynamics of electrolyte solutions without mean ionic concentrations

Abstract

A model is developed to bridge the gap between non-electrolyte and electrolyte solution thermodynamics. Solutions containing non-charged particles A and B are mixed at the fixed ratio ν _A /ν _B and the model entity E = ν _A A + ν _B B is defined. Expressions for the chemical potential of E in reference molal solutions are derived. A generalised Henry's law in terms of the total solute particles is employed. Standard states for E are regarded as formed from standard solutions of A and B by a two-step process: the ideal mixing of fixed amounts of these solutions followed by a concentrating step. The stoichiometric standard state for E corresponds to identical standard concentrations for A, B and E. Another convenient standard state corresponds to a concentration for E which, for ν _A + ν _B > 2, is lower than for A and B by a factor equal to the ratio concentration/mean concentration of E. This indicates that the use of mean concentrations arose from a bias for standard states of unit concentration. The activity coefficient of E is introduced in terms of the excess chemical potential and its value on a given concentration scale does not depend on the choice of standard state. It is identical to the conventional mean activity coefficient. The charging process leading to strong electrolyte solutions can only affect activity coefficient values. Comparison with other approaches is made.