Isopiestic studies of aqueous solutions at elevated temperatures III. {(1 − y)NaCl + yCaCl 2}

Abstract

Isopiestic studies have been made on aqueous solutions of the mixed electrolyte {(1 − y)NaCl + yCaCl2} over the temperature range 383.5 to 474.2 K. Sodium chloride was used as the reference electrolyte 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. Pitzer's mixture equation was fitted to each set of osmotic coefficients with a standard deviation of fit (in the osmotic coefficient) ranging from 0.0033 to 0.0039. One mixing parameter (in addition to the pure-electrolyte parameters) is adequate to satisfactorily describe the results. The mixing parameter is small (the maximum contribution to the osmotic coefficient is only about 0.01) and can be neglected in dilute solutions. Activity coefficients for NaCl and CaCl2 in the mixed-electrolyte solution were calculated from Pitzer's equations. Both the osmotic and activity coefficients showed a monotonic decrease with increasing temperature and became much less dependent on molality. The activity coefficient of CaCl2 is much more dependent on the composition of the mixture than that of NaCl. Excess Gibbs free energies for {(1 − y)NaCl + yCaCl2} are positive and become more positive as the temperature increases.