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

Solubilities and refractive indices in the ternary system NaCl–NaBO 2 –H 2 O 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 NaBO 2 ·4H 2 O, NaCl·NaBO 2 ·2H 2 O 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 NaBO 2 ·4H 2 O region decreased whereas the areas of NaCl·NaBO 2 ·2H 2 O increased obviously with temperature increasing. The Pitzer binary parameters of NaB(OH) 4 , Pitzer mixing parameters θ Cl - , B OH 4 - and ψ Na + , Cl - , B OH 4 - , and the dissolution equilibrium constants of NaB(OH) 4 ·2H 2 O 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 –H 2 O can’t be neglected. The assumption of only one boron   containing   ionic   species 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 K sp values obtained in this study are capable of solubility calculation for the system containing B(OH) 4 − .