Activity Coefficients in Aqueous Salt Solutions: 2, Hydration Theory Equations.

摘要

A new set of phenomenological equations for describing the activity coefficients of aqueous electrolytes has been derived, based on the hydration theory concept of Stokes and Robinson, but using the ''differentiate down'' approach in which an expression is first defined for the excess Gibbs energy. Separate equations are given for the activity of water and the activity coefficients of ionic solutes. The new equations incorporate an empirical but thermodynamically consistent scheme for using an average ion size parameter in the Debye-Hueckel part of the model. This permits the new equations to be applied to mixtures of aqueous electrolytes. The new equations, applied to the case of a pure aqueous electrolyte, do not reduce to the familiar Stokes-Robinson equation owing to a minor difference in how the Debye-Hueckel model is presumed to apply to formally hydrated solutes. As a first step in evaluating the usefulness of the equations, we have fit a ''two-parameter'' (ion size plus hydration number) model to data for a number of pure aqueous electrolytes. The quality of the fits is excellent in many cases, but there are indications that more satisfactory results would be obtained by fixing reasonable values for the ion sizes and compensating for the loss of a fitting parameter by including ion pairs in the models, by including virial coefficient terms in the equations, or both. 17 refs., 3 figs., 1 tab.