Solid–Liquid Phase Equilibrium for the Quaternary System Na ⁺ //BO ₂ – , HCO ₃ – , Cl – –H ₂ O at 298.15 K

The stable phase equilibrium of the quaternary system of Na + //BO 2 –, HCO 3 –, Cl – –H 2 O at 298.15 K was investigated using the isothermal dissolution method. The equilibrium phase diagram of the system under consideration contains two co-saturation points, five univariant curves, and four crystallization zones, corresponding to the solid phases NaCl, Na 2 B(OH) 4 Cl, NaBO 2 ·4H 2 O, and NaHCO 3 . At the temperature of 298.15 K, a double salt NaCl·NaBO 2 ·2H 2 O was generated in this system, and the crystalline form of the double salt is consistent with literature reports. In addition, the theoretical refractive index values of the system in this study were calculated using empirical formulas. It was found that the theoretical values are consistent with the experimental values, thereby verifying the reliability of the experimental results. The findings of this study provide new experimental data for the thermodynamic study of boron-containing systems.