Metastable Phase Equilibrium in the Quaternary System NaCl + KCl + Na2CO3 + K2CO3 + H2O at 273 K

The metastable phase equilibrium in the quaternary system NaCl + KCl + Na 2 CO 3 + K 2 CO 3 + H 2 O was studied at 273 K using an isothermal evaporation method. The solubility and the density of the equilibrated solution were determined. On the basis of the solubility data, the metastable phase diagram and the water diagram were plotted. The phase diagram consists of three invariant points, seven univariant curves, and five metastable crystallization fields. Among the three invariant points, points E 1 and E 3 are a commensurate invariant point type, while point E 2 is a incommensurate invariant point type. The three cosaturated salts for points E 1, E 2, and E 3 are NaCl + KCl + Na 2 CO 3 ·10H 2 O, KCl + Na 2 CO 3 ·10H 2 O + KNaCO 3 ·6H 2 O, and KCl + K 2 CO 3 ·3/2H 2 O + KNaCO 3 ·6H 2 O, respectively. The five metastable crystallization fields correspond to the single salts potassium chloride (KCl), sodium chloride (NaCl), sodium carbonate decahydrate (Na 2 CO 3 ·10H 2 O), potassium carbonate sesquihydrate (K 2 CO 3 ·3/2H 2 O), and a hexahydrate double salt of potassium and sodium carbonate (KNaCO 3 ·6H 2 O). Sodium carbonate decahydrate (Na 2 CO 3 ·10H 2 O) is the only crystallization form of sodium carbonate at 273 K in this system; the monohydrate (Na 2 CO 3 ·H 2 O) and heptahydrate (Na 2 CO 3 ·7H 2 O) are not formed.