Measurement and Application of a Solid–Liquid Equilibrium for the Ternary NaCl + Na2S2O3 + H2O System

Abstract

Solid–liquid equilibrium data for the NaCl + Na2S2O3 + H2O system at 283.15 and 303.15 K were measured using the method of isothermal solution saturation combined with a moist residue method of Schreinemaker, the equilibrium solid phase was identified by powder X-ray diffraction, and finally the solid–liquid phase diagrams of the ternary system were constructed. The ternary NaCl + Na2S2O3 + H2O phase diagrams at 283.15 and 303.15 K turned out to be of the simple eutectic type. One invariant point, two univariant curves, and two crystallization regions exist in each of the phase diagrams. The crystallization region of NaCl is larger than that of Na2S2O3·5H2O at 283.15 K, and the crystallization region of NaCl expands while that of Na2S2O3·5H2O decreases when the temperature rises to 303.15 K. Consequently, the invariant point at 303.15 K is located in the Na2S2O3·5H2O single-salt crystallization region of the phase diagram at 283.15 K. On the basis of this thermodynamic characteristic, a method for separation of the NaCl + Na2S2O3 mixture was proposed, from which both of the pure salts could be obtained.