ISOPIESTIC STUDIES OF AQUEOUS-SOLUTIONS AT ELEVATED-TEMPERATURES .2. NACL + KCL MIXTURES

Isopiestic studies have been made on aqueous solutions of the mixed electrolyte NaCl + KCl over the temperature range 383 to 474 K. Sodium chloride served as the isopiestic standard for the calculation of osmotic coefficients. The molality range covered in this study corresponded to about 0.6 to 6 mol kg-1 for NaCl. The system of equations developed by Pitzer was fitted to each set of osmotic coefficients with a standard deviation of fit (in the osmotic coefficient) ranging from 0.0025 to 0.0044. Only one mixing parameter (in addition to the pure-electrolyte parameters) was necessary to describe the results adequately. Activity coefficients for NaCl and KCl in the mixed electrolyte system were calculated from the equations. The activity coefficients showed a monotonic decrease with increasing temperature and became much less dependent on molality at the higher molalities. The activity coefficient of NaCl is much more dependent on composition than that of KCl. The trace activity coefficients of NaCl and KCl are very nearly equal over this temperature range. The ability to describe the NaCl + KCl system with only one mixing parameter in Pitzer's equation is equivalent to Harned's rule. Excess Gibbs free energies calculated from the mixing parameter were, to a very good approximation, independent of temperature. © 1979.