In this work, the water activities of the NaCl–CaCl2–H2O and KCl–CaCl2–H2O ternary systems and of their sub-binary systems NaCl–H2O and KCl–H2O were measured using an isopiestic method at 323.15 K. The isopiestic composition line for the NaCl–CaCl2–H2O system was found to obey the Zdanovskii rule very well, whereas the KCl–CaCl2–H2O system deviated slightly. The addition of NaCl to an aqueous solution of CaCl2 decreased its water activity for all CaCl2 molalities. However, KCl was found to decrease and increase the water activity at low and high CaCl2 molality, respectively. The turning point appears at 8.0 mol·kg–1 CaCl2 solution. The Pitzer–Simonson–Clegg (PSC) model was applied to represent the water activity of the two ternary systems, and the calculated results are discussed.
Solubility isotherms of the ternary NaCl + RbCl + H2O system were elaborately determined at T = (273.15 and 298.15) K by an isothermal equilibrium method, and the results showed that there are only two solubility branches for the solid phase NaCl(cr) and RbCl(cr) at both temperatures. The liquid composition of invariant points at T = (263.15, 323.15, 338.15, and 348.15) K were also determined. The experimental data at T = 298.15 K were reproduced using a Pitzer–Simonson–Clegg activity model by adding a temperature independent ternary mixing parameter. Then, the solid–liquid equilibria in the ternary NaCl + RbCl + H2O system were predicted from the available binary parameters obtained in our previous study and the mixing parameter regressed at 298.15 K in this study. The model was further validated by comparing the predictions with the present data at other temperatures that were not used in parametrization. Finally, the phase diagram of the ternary NaCl + RbCl + H2O system in the temperature range from the lowest eutectic point to 373.15 K was constructed to lay the foundation for equilibria property predictions for more complex Rb-containing systems.
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