Solubility determination and thermodynamic modelling of solid-liquid equilibria in the (NaCl + NaBO2 + Na2B4O7 + H2O) system at 298.15 K

Highlights: • The solubilities in system (NaCl + NaBO₂ + Na₂B₄O₇ + H₂O) were studied at 298.15 K. • The calculated results in the quaternary system agree with the experimental data. • The distribution of four boron species were obtained in the mixed borate solution. The solubility corresponding solution physicochemical properties (densities, refractive indices and pH) in the quaternary system (NaCl + NaBO₂ + Na₂B₄O₇ + H₂O) were investigated at 298.15 K by the isothermal dissolution equilibrium method. The dry-salt phase diagram, water content diagram, refractive index diagram and pH diagram were drawn. There are four crystallization fields corresponding to NaCl, NaBO₂·4H₂O, Na₂B₄O₇·10H₂O, NaCl·NaBO₂·2H₂O, five univariant curves and two invariant points in the phase diagram of the quaternary system. The solubility, concentrations of boron species B(OH)₃, B(OH)₄⁻, B₃O₃(OH)₄⁻, B₄O₅(OH)₄²⁻ and OH⁻ in the system were calculated using the Pitzer model. The variation trends for calculated solubility curves and pH curves are consistent with the experimental results. The distributions of different boron species were obtained in the mixed borate solution. The distributions of four boron species were obtained in the mixed borate solution. The main boron species in the mixed borate solution are B(OH)₄⁻ and B₄O₅(OH)₄²⁻. The mole fractions of the four boron species are mainly affected by m(B) in the solution but less by m(Cl⁻).