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.