Thermodynamic phase equilibria in the aqueous ternary system NaCl–NaBO2–H2O: Experimental data and solubility calculation using the Pitzer model

Abstract

Solubilities and refractive indices in the ternary system NaCl–NaBO2–H2O at 323.15 K were investigated with the dissolution equilibrium method. The phase diagram of this system consists of three single-salt crystallization regions for NaBO2·4H2O, NaCl·NaBO2·2H2O and NaCl, three univariant solubility curves and two invariant points. A comparison of the diagrams of this system from 288.15 to 323.15 K shows that area of NaBO2·4H2O region decreased whereas the areas of NaCl·NaBO2·2H2O increased obviously with temperature increasing. The Pitzer binary parameters of NaB(OH)4, Pitzer mixing parameters θCl-,BOH4-and ψNa+,Cl-,BOH4-, and the dissolution equilibrium constants of NaB(OH)4·2H2O and NaCl·NaB(OH)4 were fitted with the solubility data on the basis of Pitzer model. The variation trends for calculated solubility curves are in accordance with the experimental results. The results show that the other boron species except B(OH)4− in the solution for the system NaCl–NaB(OH)4–H2O can’t be neglected. The assumption of only one boroncontainingionicspecies B(OH)4− in the solution is not very accurate, but the calculated results with the assumption can still describe the experimental values. The Pitzer parameters and Ksp values obtained in this study are capable of solubility calculation for the system containing B(OH)4−.