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 (n_D) 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 (n_D) 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.

中文翻译：

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在 T = 323.2 K 和 p = 94.77 kPa 时倒四元系统 K+、Rb+ // Cl– 和硼酸盐–H2O 的相平衡

等温溶解法应用于四元体系 K ⁺、Rb ⁺ // Cl ^-和硼酸盐-H ₂ O 在T = 323.2 K 和p = 94.77 kPa 的相平衡实验。该系统的溶解度和折射率 ( n _D ) 是通过实验确定的。使用X射线衍射（XRD）方法鉴定了四元不变点的共存盐。根据实测数据，得到稳定相图和含水量和折光指数（n _D) 与该系统在 323.2 K 下的组成作图。结果表明该系统属于复杂类型，形成固溶体 [(K, Rb)Cl]。稳定相图由三个四元不变点、七个等温溶解曲线和五个结晶区组成，分别对应于 KCl、RbCl、RbB ₅ O ₆ (OH) ₄ ·2H ₂ O、K ₂ B ₄ O ₅ (OH) ₄ · 2H ₂ O 和 [(K, Rb)Cl]。RbB ₅ O ₆ (OH) ₄ ·2H _{2 的大小}O结晶带是四元系共存盐类中最大的，这意味着它最容易从含有钾、铷、氯化物和硼酸盐的混合溶液中结晶出来。