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⁺//SO₄ ^2--H₂O and Li⁺,Cs⁺//SO₄ ^2--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⁺//SO₄ ^2--H₂O and Li⁺,Cs⁺//SO₄ ^2--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⁺//SO₄ ^2--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⁺//SO₄ ^2--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.