Phase Equilibria in the Quaternary Systems NaOH + Na2CO3 + Na2SO4 + H2O, Na2CO3 + NaOH + NaCl + H2O, and NaOH + Na2SO4 + NaCl + H2O at 363.15 K.

The solid-liquid equilibrium data of the aqueous NaOH-Na2CO3-Na2SO4-H2O, NaOH-Na2CO3-NaCl-H2O, and NaOH-Na2SO4-NaCl-H2O quaternary systems at 363.15 K were measured. The equilibrium solid phases and solubilities of salts in the three systems and its subsystems were determined. The densities of the saturated solutions were also determined. The experimental data are used to plot the solubility diagrams and water content diagrams of the systems. It was found that the NaOH-Na2CO3-Na2SO4-H2O system contains the solid solution of γ-salt (mNa2SO4·nNa2CO3) and the other two systems Na2CO3-NaOH-NaCl-H2O and NaOH-Na2SO4-NaCl-H2O have the complex salts S1 (Na2SO4·NaOH) and S3 (Na2SO4·NaCl·NaOH). On the basis of Xu's activity coefficient model, a model was constructed for the correlation of solid-liquid equilibrium in electrolyte solutions to calculate the solubilities of salts in these systems at 363.15 K. The calculated solubilities are in agreement with the experimental values.