Solid–Liquid Phase Equilibrium for the Quaternary System Na⁺, K⁺//VO₃^–, Cl^––H₂O at 298.15 K

The stable phase equilibrium of the reciprocal quaternary system of Na+, K+//VO3–, Cl––H2O at 298.15 K was investigated by an isothermal solution equilibrium method. The equilibrium liquid phase solubility data and physical property data [density (ρ), refractive index (nD), and pH] were determined, and the composition of the equilibrium solid phase was verified by X-ray diffraction. The phase diagram and physical property–composition diagrams were constructed based on the equilibrium solubility data and physical property data. The phase diagram contains three invariant points, seven isothermal univariant curves, and five crystalline phase regions. The study found that this quaternary system belongs to a complex system with the presence of the double salts. By comparing the experimental results with literature data, it was confirmed that at 298.15 K, the chemical formulas of the complex salt which formed by the quaternary system Na+, K+//VO3–, Cl––H2O were consistent with the complex salt chemical formula (Na2mKn(VO3)2m+n) proposed by the Druzynski group, thus verifying the reliability of this experiment. The results of this study will provide new experimental data for the thermodynamics of vanadium–water systems.