Thermodynamic model for aqueous electrolyte solutions with partial ionization

An equation of state has been developed to describe the thermodynamic properties of single electrolytes in water over a wide range of temperatures from 25 C to near the critical point of the solvent. The new equation of state was obtained from an analytical expression of the Helmholtz free energy containing three major contributions: (1) a discrete solvent term to account for the short-range interactions between uncharged particles based on the Peng-Robinson equation of state, (2) an ion-charging term described by the continuum-solvent model of Born, and (3) a charge-charge interaction term given by the explicit mean-spherical-approximation (MSA) expression. The thermodynamic model proposed here incorporates the chemical equilibrium of the dissolved electrolyte, allowing for the calculation of the corresponding degree of dissociation of the salt at different temperatures. The present equation of state was applied to the representation of the mean ionic activity coefficients, osmotic coefficients, standard free energies of ion hydration, and densities for NaCl, CaCl₂, K₂SO₄, and MgSO₄ salts in water over a wide range of temperatures and salt molalities. The results indicate good agreement between the experimental data and the calculations generated from the present equation of state.