Solid-Liquid Phase Equilibria of the Quaternary System Na+//Cl–, SO42–, SeO32––H2O at 298.15 and 323.15 K

The solubility data of the quaternary system Na⁺//Cl^–, SO₄^2–, SeO₃^2––H₂O at 298.15 and 323.15 K were determined using the isothermal solution equilibrium method. On the basis of the measured data, a stereoscopic phase diagram, dry salt projection diagram, and water content diagram were constructed. The results show that the phase diagrams of the quaternary system Na⁺//Cl^–, SO₄^2–, SeO₃^2––H₂O at 298.15 and 323.15 K differ significantly. At 298.15 K, the phase diagram includes one invariant point, four univariate curves, and four crystallization zones, corresponding to the four single salts Na₂SO₄·10H₂O, Na₂SO₄, NaCl, and Na₂SeO₃. In contrast, the phase diagram at 323.15 K includes one invariant point, three univariate curves, and three crystallization zones, corresponding to Na₂SO₄, NaCl, and Na₂SeO₃. Comparative analysis of the phase diagrams at both temperatures reveals that with rising temperature, the crystallization areas of Na₂SO₄ and Na₂SeO₃ expand, whereas that of NaCl decreases.