An Equation of State for Electrolyte Solutions Covering Wide Ranges of Temperature, Pressure, and Composition

An equation of state has been developed for electrolyte solutions over wide ranges of temperature, pressure, and composition. The equation is in terms of the Helmholtz free energy and contains three terms accounting for the various interactions in solution: a Peng−Robinson term for the interactions between uncharged species, a Born term for the charging free energy of ions, and a mean spherical approximation term for the electrostatic interactions between ions. The equation of state was fitted to experimental data for 138 aqueous electrolyte solutions at 25 °C and 1 bar and to data for aqueous NaCl, NaBr, CaCl2, Li2SO4, Na2SO4, K2SO4, and Cs2SO4 solutions from 0 to 300 °C and from 1 to 120 bar. The equation does well in correlating activity coefficients, osmotic coefficients, densities, and free energies of hydration over the entire range of conditions. The simplicity and wide range of applicability of the new equation of state make it particularly well suited for use in engineering applications.