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

The phase equilibrium of ternary systems RbX/CsX + CuX 2  + H 2 O (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 + CuCl 2  + H 2 O, there were three crystallization regions corresponding to RbCl, Rb 2 CuCl 4 ·2H 2 O and CuCl 2 ·2H 2 O, respectively. Five crystallization fields corresponding to CsCl, CsCuCl 3 , Cs 3 Cu 2 Cl 7 ·2H 2 O, Cs 2 CuCl 4 and CuCl 2 ·2H 2 O were formed in the system CsCl + CuCl 2  + H 2 O. The ternary system RbBr + CuBr 2  + H 2 O had three crystalline regions corresponding to RbBr, Rb 2 CuBr 4 ·2H 2 O and CuBr 2 . In the system CsBr + CuBr 2  + H 2 O, the solid-phases of CsCuBr 3 , Cs 2 CuBr 4 , besides CsBr and CuBr 2 , 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 Rb 2 CuCl 4 ·2H 2 O, Rb 2 CuBr 4 ·2H 2 O, CsCuCl 3 , Cs 3 Cu 2 Cl 7 ·2H 2 O, Cs 2 CuCl 4 , CsCuBr 3 and Cs 2 CuBr 4 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.