Stable Phase Equilibria of Ternary Systems Li+,Rb+//SO42––H2O and Li+,Cs+//SO42––H2O at 273.2 K

The stable phase equilibria of ternary systems Li+,Rb+//SO42--H2O and Li+,Cs+//SO42--H2O at 273.2 K are studied herein by the isothermal dissolution equilibrium method. The solubility and density of the systems are measured experimentally, and the related phase diagrams, density versus composition diagrams, are plotted. Results show that the ternary systems Li+,Rb+//SO42--H2O and Li+,Cs+//SO42--H2O are both complex systems with two kinds of double salts formed at 273.2 K. The stable phase diagrams of these two ternary systems all consist of one unsaturated solution region, three co-crystallization regions, three invariant points, four invariant curves, and four crystallization regions. By comparing the phase diagrams of the ternary system Li+,Rb+//SO42--H2O at 273.2 and 298.2 K, it is found that the crystallization regions of the two double salts (3Li2SO4·Rb2SO4·2H2O and Li2SO4·Rb2SO4) and the single salt Li2SO4·H2O all decrease, while that of Rb2SO4 increases when the temperature drops. Also, it can be seen from the phase diagram of the ternary system Li+,Cs+//SO42--H2O at 273.2 and 298.2 K that the crystallization regions of the double salt 3Li2SO4·Cs2SO4·2H2O and the single salt Cs2SO4 increase, while those of the double salt Li2SO4·Cs2SO4 and the single salt Li2SO4·H2O decrease as the temperature drops.