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.