Abstract: The solubilities and densities of equilibrium solutions in the quaternary system Na₂CO₃-Na₂SO₄-Na₂B₄O₇-H₂O were studied at 273 K by isothermal evaporation method. On the basis of the solubility data, the phase diagram of the system was plotted. The phase diagram consists of two invariant point, five univariant curves and four crystallization fields, which correspond to the salt Na₂CO₃·10H₂O, Na₂SO₄·10H₂O, Na₂B₄O₇·10H₂O and 2Na₂SO₄·Na₂CO₃. In this system, the double-salt(2Na₂SO₄·Na₂CO₃) was formed, and this double-salt also exists in the other quaternary or higher-component systems which contain the Na₂CO₃-Na₂SO₄-H₂O ternary system. The crystallization forms of sodium carbonate and sodium sulfate are Na₂CO₃·10H₂O and Na₂SO₄·10H₂O, respectively. The inextenso molecular fomula of sodium borate is Na₂B₄O₅·(OH)₄·8H₂O. Na₂CO3 has salt-out effect to Na₂B₄O₇ in the solution.

Key words: Salt-water system, Metastable phase equilibrium, Sodium salt