Solid–Liquid Phase Equilibria in the Ternary Systems Rb⁺,Mg²⁺//Cl–-H₂O and Cs⁺,Mg²⁺//Cl–-H₂O at T = 288.2 K

The phase equilibria of the ternary water–salt systems Rb+,Mg2+//Cl–-H2O and Cs+,Mg2+//Cl–-H2O at 288.2 K were studied by the isothermal dissolution equilibrium method, the solubilities, densities, and refractive indices of these two systems were measured experimentally, and the corresponding phase diagram, density diagram, and refractive index diagram were plotted. At 288.2 K, the ternary systems Rb+,Mg2+//Cl–-H2O and Cs+,Mg2+//Cl–-H2O are complex systems with similar structures of the phase diagram. Both phase diagrams consist of two invariant points, three isothermal dissolution curves, and three crystallization regions. The multitemperature diagram illustrates the complexity of the ternary systems, with double salt formation occurring at different temperatures while maintaining a constant content of crystal water. As temperature increases, the crystallization region of salts decreases, indicating an increase in solubility of salts within the two systems. According to the experimental data, the interaction parameters of the ternary system and the Ksp of the double salt were fitted. The Pitzer model was applied to predict the phase diagrams, and a comparison of the experiment and theoretical calculation revealed a high degree of agreement between the predicted and experimental phase diagrams.