Phase Equilibria for the Aqueous System Containing Sodium, Potassium, Carbonate, and Sulfate Ions at 273.15 K

The phase equilibria in the quaternary system Na2SO4 + K2SO4 + Na2CO3 + K2CO3 + H2O were studied at 273.15 K using an isothermal evaporation method. The solubilities and densities of the equilibrated solution were determined experimentally. The crystalloid forms of the solid phase were determined using chemical analysis and an X-ray diffraction method. On the basis of the experimental data, the isothermal equilibrium phase diagram, water content diagram, and density composition diagram of the quaternary system were plotted. The phase diagram consists of four invariant points, nine univariant curves, and six crystallization fields corresponding to sodium sulfate decahydrate (Na2SO4·10H2O), potassium sulfate, sodium carbonate decahydrate (Na2CO3·10H2O), potassium carbonate hydrate (K2CO3·3/2H2O), potassium and sodium carbonate double salt (K2CO3·Na2CO3·12H2O), and sulfate and carbonate double salt of sodium (2Na2SO4·Na2CO3). This quaternary system is of a complex type. Two kinds of double salts are formed. Na2CO3·10H2O is the only crystalloid form of sodium carbonate in the given system at 273.15 K. Na2CO3·7H2O and Na2CO3·H2O are not formed.