Phase equilibria in the RbX/CsX + CuX2 + H2O (X = Cl, Br) systems at T = 298.15 K and the standard enthalpies of formation of seven double salts

Abstract

The phase equilibrium of ternary systems RbX/CsX + CuX₂ + H₂O (X = Cl, Br) at T = 298.15 K were investigated by the isothermal dissolution equilibrium method, in which the composition of equilibrium solid phases was determined by the Schreinemaker’s wet residue method. In the system RbCl + CuCl₂ + H₂O, there were three crystallization regions corresponding to RbCl, Rb₂CuCl₄·2H₂O and CuCl₂·2H₂O, respectively. Five crystallization fields corresponding to CsCl, CsCuCl₃, Cs₃Cu₂Cl₇·2H₂O, Cs₂CuCl₄ and CuCl₂·2H₂O were formed in the system CsCl + CuCl₂ + H₂O. The ternary system RbBr + CuBr₂ + H₂O had three crystalline regions corresponding to RbBr, Rb₂CuBr₄·2H₂O and CuBr₂. In the system CsBr + CuBr₂ + H₂O, the solid-phases of CsCuBr₃, Cs₂CuBr₄, besides CsBr and CuBr₂, were discovered. The results were compared with available literature data for cesium bromide/copper bromide salt-water system. The seven new solid phase were characterized using the X-ray diffraction method and thermogravimetric/differential analysis. The dissolution enthalpies of Rb₂CuCl₄·2H₂O, Rb₂CuBr₄·2H₂O, CsCuCl₃, Cs₃Cu₂Cl₇·2H₂O, Cs₂CuCl₄, CsCuBr₃ and Cs₂CuBr₄ at T = 298.15 K were measured. Their standard enthalpies of formation were obtained, and the results were in order with −(1714.4 ± 3.2) kJ·mol⁻¹, −(1533.6 ± 3.3) kJ·mol⁻¹, −(697.8 ± 3.1) kJ·mol⁻¹, −(2418.9 ± 6.1) kJ·mol⁻¹, −(1134.8 ± 3.2) kJ·mol⁻¹, −(563.5 ± 3.2) kJ·mol⁻¹ and −(966.2 ± 3.3) kJ·mol⁻¹, respectively.