The solubilities and physicochemical properties such as density and refractive index of the solution in the ternary systems KCl + MgCl 2 + H 2 O and KCl + RbCl + H 2 O were investigated at 298.15 K using an isothermal evaporation method. The crystalloid forms of the solid phase were determined using chemical analysis and an X-ray diffraction method. On the basis of experimental data, the metastable equilibrium phase diagrams and the physicochemical properties vs composition in the ternary systems at 298.15 K were constructed. In the metastable phase diagram of the ternary system KCl + MgCl 2 + H 2 O at 298.15 K, there are two invariant points, three univariant curves, and three crystallization fields corresponding to potassium chloride (KCl), magnesium chloride hexahydrate (MgCl 2 3 6H 2 O), and a double salt carnallite (KCl 3 MgCl 2 3 6H 2 O). In the metastable phase diagram of the ternary system KCl + RbCl + H 2 O, there are two invariant points and three crystallization fields corresponding to the single salts potassium chloride (KCl), rubidium chloride (RbCl), and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. Comparisons between the metastable phase diagrams at 298.15 K and at 323.15 K show that the crystallization forms have not changed, while the crystallization zones have slight changes. The calculated densities and refractive indices using an empirical equation have the maximum relative error < 0.016.

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The metastable phase equilibria in the aqueous quaternary system LiCl + KCl + RbCl + H2O were investigated at 298.15 K using an isothermal evaporation method. The solubilities and physicochemical properties such as the density and refractive index of the equilibrated solution were determined. On the basis of the experimental results, the stereo phase diagram, the projected phase diagram, the water content diagram, and the physicochemical properties versus composition diagrams were constructed. The projected phase diagram consists of two invariant points, five isothermal univariant curves, and four crystallization fields corresponding to single salts potassium chloride (KCl), rubidium chloride (RbCl), lithium chloride monohydrate (LiCl·H2O), and a solid solution of potassium and rubidium chloride [(K, Rb)Cl]. The two invariant points are cosaturated with three salts, which are LiCl·H2O + RbCl + (K, Rb)Cl and LiCl·H2O + KCl + (K, Rb)Cl, respectively. No double salt was formed. The crystallization region of the solid solution [(K, Rb)Cl] almost occupies the entire phase region, which shows that it is difficult to separate potassium from rubidium in chloride solution by only using evaporation and crystallization methods at 298.15 K. The calculated densities and refractive indices using empirical equations have a maximum relative error of <0.016.

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