Experimental study of the solubilities of salts in the systems Na2B4O7–NaBr–H2O and Na2B4O7–Na2SO4–NaBr–H2O at 323 K

The solid–liquid equilibria in quaternary system Na2B4O7–Na2SO4–NaBr–H2O and its ternary subsystem Na2B4O7–NaBr–H2O at 323 K were studied with the method of isothermal solution saturation. The solubilities of salts and equilibrium solid phases in the two systems were determined. The solubility diagram of the ternary system has one invariant point, two univariant curves and two fields of crystallization [Na2B4O7·10H2O and NaBr·2H2O]. The solubility diagram of the quaternary system has one invariant point, three univariant curves, and three crystallization fields, where the equilibrium solids are Na2B4O7·10H2O, Na2SO4 and NaBr·2H2O, respectively. According to our experimental data, the solubility of NaBr·2H2O is highest among the three salts, and that of Na2B4O7·10H2O is lowest. Therefore, the field of crystallization of Na2B4O7·10H2O(s) in ternary or quaternary system is larger than others. The addition of sodium bromide leads to rapid decrease of the solubility of Na2B4O7·10H2O(s).