Isopiestic Determination of Water Activities and Solid–Liquid Equilibria in the Mixed Borate System NaB(OH)₄–Na₂B₄O₅(OH)₄–H₂O at 323.15 K

The water activities and osmotic coefficients in the ternary system NaB(OH)4–Na2B4O5(OH)4–H2O at 323.15 K were investigated with the isopiestic method. The solubilities and physicochemical properties in the NaBO2–Na2B4O7–H2O system at 323.15 K were studied with the isothermal dissolution equilibrium method. The phase diagram in the system consists of two single salt regions for NaBO2·4H2O and Na2B4O7·10H2O, one invariant point saturated with the above salts, and two univariant solubility curves. On the basis of the Pitzer model, the Pitzer binary parameters of Na2B4O5(OH)4 and mixing parameters were fitted with the osmotic coefficients and solubilities. The dissolution equilibrium constant of Na2B4O7·10H2O was calculated with the experimental solubility data and parameters. The calculated solubilities and water activities with the parameters and dissolution equilibrium constants of NaBO2·4H2O and Na2B4O7·10H2O agree with the experimental data.