Solid-liquid equilibria for the ternary systems of RbX + CdX2 + H2O (X = Cl, Br) at T = 298.15 K and the thermodynamic properties of the new solid compounds
Solid-liquid equilibria in the ternary systems RbX + CdX 2 + H 2 O (X = Cl, Br) at T = 298.15 K and under 101.3 kPa were investigated by the method of isothermal solution saturation, and the corresponding phase diagrams were constructed. It was found that there were five crystallization regions corresponding to CdCl 2 ·2.5H 2 O, RbCd 2 Cl 5 ·H 2 O, RbCdCl 3 , Rb 4 CdCl 6 and RbCl in the system (RbCl + CdCl 2 + H 2 O). Four crystallization regions corresponding to CdBr 2 ·4H 2 O, RbCdBr 3 , Rb 4 CdBr 6 and RbBr were found in the system (RbBr + CdBr 2 + H 2 O). The new solid compounds RbCdCl 3 , RbCdBr 3 and Rb 4 CdBr 6 were congruently soluble in water, and new double salts RbCd 2 Cl 5 ·H 2 O and Rb 4 CdCl 6 were incongruently soluble in water. The new solid-phase compounds RbCd 2 Cl 5 ·H 2 O, RbCdCl 3 , Rb 4 CdCl 6 , RbCdBr 3 and Rb 4 CdBr 6 were characterized by chemical analysis, XRD and TG-DTG techniques. The standard molar enthalpies of solution of RbCd 2 Cl 5 ·H 2 O, RbCdCl 3 , Rb 4 CdCl 6 , RbCdBr 3 and Rb 4 CdBr 6 in water were measured to be (20.74 ± 0.16) kJ·mol −1 , (28.00 ± 0.39) kJ·mol −1 , (96.56 ± 0.50) kJ·mol −1 , (35.38 ± 0.33) kJ·mol −1 and (110.64 ± 0.62) kJ·mol −1 by microcalorimetry in the condition of infinite dilution, and their standard molar enthalpies of formation were determined to be −(1545.3 ± 0.6) kJ·mol −1 , −(856.5 ± 0.7) kJ·mol −1 , −(2180.1 ± 1.3) kJ·mol −1 , −(727.1 ± 0.7) kJ·mol −1 and −(1920.5 ± 1.3) kJ·mol −1 , respectively.
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