Phase Equilibria on the Reciprocal Quaternary System K+, Rb+ // Cl–, and Borate–H2O at T = 323.2 K and p = 94.77 kPa

The isothermal dissolution method was applied for the phase equilibrium experiment of the quaternary system K+, Rb+ // Cl–, and borate–H2O at T = 323.2 K and p = 94.77 kPa. The solubilities and refractive indices (nD) of the system were determined experimentally. The coexisting salts of the quaternary invariant points were identified using the X-ray diffraction (XRD) method. Based on the measured data, the stable phase diagram and the figures of water content and refractive index (nD) vs composition were plotted for this system at 323.2 K. The results show that the system belongs to a complex type with the formation of a solid solution [(K, Rb)Cl]. The stable phase diagram consists of three quaternary invariant points, seven isothermal dissolution curves, and five crystallization regions corresponding to KCl, RbCl, RbB5O6(OH)4·2H2O, K2B4O5(OH)4·2H2O, and [(K, Rb)Cl]. The size of the RbB5O6(OH)4·2H2O crystallization zone is the largest among the coexisting salts in the quaternary system, which means it is the most easily crystallized from the mixed solution containing potassium, rubidium, chloride, and borate.