Isopiestic Determination of the Osmotic and Activity Coefficients of NaCl + SrCl2 + H2O at 298.15 K and Representation with an Extended Ion-Interaction Model

Isopiestic vapor-pressure measurements were made at 298.15 K for aqueous NaCl + SrCl 2 solutions using NaCl(aq) as the reference standard. The measurements for these ternary solutions were made at NaCl ionic strength fractions of y 1 = 0.17066, 0.47366, and 0.82682 for the water activity range of 0.9835 ≥ a w ≥ 0.8710. Our results, and those from two previous isopiestic studies, were combined and used with previously determined parameters for NaCl(aq) and those determined here for SrCl 2 (aq) to evaluate mixing parameters S θ Na,Sr = (0.0562 ± 0.0007) kg·mol -1 and ψ Na,Sr,Cl = −(0.00705 ± 0.00017) kg 2 ·mol -2 for an extended form of Pitzer's ion-interaction model. These model parameters are valid for ionic strengths of I ≤ 7.0 mol·kg -1, where higher-order electrostatic effects have been included in the mixture model. If the fitting range is extended to the saturated solution molalities, then S θ Na,Sr = (0.07885 ± 0.00195) kg·mol -1, and ψ Na,Sr,Cl = −(0.01230 ± 0.00033) kg 2 ·mol -2 . The extended ion-interaction model parameters obtained from available isopiestic data for SrCl 2 (aq) at 298.15 K yield recommended values of the water activities and osmotic and activity coefficients.