An Equilibrium Model for the Calculation of Activity and Osmotic Coefficients in Aqueous Solutions

A procedure is described for the calculation of activity and osmotic coefficients which is based upon a knowledge of the equilibria in solution and assumed single-ion activity coefficients. The procedure permits one to introduce chemical equilibria of various types (ion-pairing, complexation, hydration, and hydrolysis) into a model which can be used to calculate values of the excess Gibbs energy and the activity and osmotic coefficients. Both the Debye-Hückel theory and Pitzer's expression are used to calculate the electrostatic contribution to the single-ion activity coefficients. Calculations have been performed on aqueous sulfuric acid, acetic acid, hydrofluoric acid, cadmium chloride, copper sulfate, and sodium carbonate. Properties which have been calculated are the excess Gibbs energy, the osmotic coefficient, the mean ionic activity coefficient, and Frank's single-ion activity coefficient function. Agreement between calculated and measured properties has been obtained up to molalities of ≈ 1.0 mol kg^-1.