A Study on the Equilibrium System of CdBr2–l-Proline (trans-4-OH-l-Proline)–H2O at 298.15 K and the Standard Molar Enthalpies of Formation of Their Double Salts

Two-phase equilibrium of ternary systems of CdBr2–l-proline/(4-OH-l-proline)–H2O at T = 298.15 K is studied. Their isothermal solution equilibrium phase diagrams have been drawn based on the solubility data. Two double salts (Cd(C5H9NO2)Br2·H2O and Cd(C5H9NO3)Br2) have been found by the Schrienemakers method in phase diagrams, and they are all congruently soluble in water. Both of them have been synthesized and characterized by powder XRD and TG-DTG. For the first time, enthalpies of solution of Cd(C5H9NO2)Br2·H2O and Cd(C5H9NO3)Br2 are measured in water, and their standard molar enthalpies of formation at 298.15 K are calculated by designing thermochemical cycles according to the Hess law. By comparison, it is found that the region of infinite dissolution appears in the ternary system of CdBr2–l-proline–H2O at 298.15 K. If the composition of the solution is in this region, there is no crystal when the water evaporates and only a viscous mixture can be obtained.