Ion-Association Models and Mean Activity Coefficients of Various Salts

Abstract

Calculations using the aqueous model from WATEQ and an aqueous model modified from WATEQ were compared to experimental mean activity coefficients for various salts to determine the range of applicability and the sources of errors in the models. An ion-association aqueous model was derived by least-squares fitting of ion-association stability constants and individual-ion, activity-coefficient parameters to experimental mean activity coefficients for various salts at 25°C. Salts of the following cations and anions were considered: Al+3, Ba+2, Ca+2, Cd+2, Co+2, Cs+, Cu+2, Fe+2, H+, K+, Li+, Mg+2, Mn+2, Na+, Ni+2, Pb+2, Sr+2, Zn+2, Cl-, ClO4-, F-, OH-, and SO4-2. The stability constants of the the derived model and the WATEQ model were in agreement for most two-ion complexes but were not in agreement for most complexes containing three or more ions. The largest discrepancies in stability constants were for complexes of Cu+2, Mn+2, Ni+2, and Zn+2 with Cl-. The derived-model calculations matched the experimental data for all salts to a concentration of about 2 molal, but the parameters of the model could not be defined uniquely by the fitting process. Alternative choices for the complexes included in the model and for the individual-ion, activity-coefficient parameters could fit the experimental data equally well.