Phase Equilibria in the Systems RbCl–TmCl3–HCl(22.12 %)–H2O and RbCl–LuCl3–HCl(21.37 %)–H2O at 298.15 K and Standard Molar Enthalpy of Formation of Rb2LuCl5·7H2O

Abstract

The solubility in the RbCl–TmCl3–HCl–H2O and RbCl–LuCl3–HCl–H2O quaternary water–salt systems was studied along the ∼22 % hydrochloric acid section by the isothermal solubility method at 298.15 K, and corresponding equilibrium diagrams were constructed. The compositions of solid phases were established by the Schkreinemakers’ wet residue method. The quaternary system RbCl–TmCl3–HCl–H2O is a simple eutonic type, but the quaternary system RbCl–LuCl3–HCl–H2O is complicated, and one incongruently saturating complex phase is formed in this system: Rb2LuCl5·7H2O. The compound Rb2LuCl5·7H2O was identified and characterized by the methods of powder X-ray diffraction, thermogravimetric, and differential thermal analysis. The crystal water was lost at (308 to 499) K. The standard molar enthalpy of solution of Rb2LuCl5·7H2O in water was measured to be (6.04 ± 0.17) kJ·mol–1 by heat conduction microcalorimetry, and its standard molar enthalpy of formation was calculated to be −(4009.9 ± 1.3) kJ·mol–1.