EXPERIMENTAL DETERMINATION OF SOLUBILITIES OF SODIUM TETRABORATE (BORAX) IN NACL SOLUTIONS TO HIGH IONIC STRENGTHS, AND THERMODYNAMIC MODEL FOR THE NA-B(OH)3-CL-SO4 SYSTEM

In this study, solubility experiments on sodium tetraborate (Na₂B₄O₇•10H₂O, borax) are conducted in NaCl solutions up to 5.0 m at room temperature (22.5 ± 1.5 ^oC). In combination with solubility data of sodium tetraborate in Na₂SO₄ solutions from literature, the solubility constant (log K) for sodium tetraborate regarding the following reaction,

Na₂B₄O₇•10H₂O = 2Na⁺ + 4B(OH)₄^– + 2H⁺ + H₂O (l)

is determined as –24.88 ± 0.10 ^oC based on the Pitzer model. In addition, the lambda parameter (λ_{NaB(OH)4(aq), Na+}) for the interaction between NaB(OH)₄(aq) and Na⁺ is evaluated as 0.09192. It is also discovered that it is necessary to revise θ_{B(OH)4-, SO42-} of the Felmy and Weare (1986) model and include Ψ_{B(OH)4-, SO42-}, _Na+ in order to accurately model solubility of sodium tetraborate in Na₂SO₄ medium. The revised Θ_{B(OH)4-, SO42-} is 0.1697 in comparison with the value of –0.012 in the Felmy and Weare (1986) model, and Ψ_{B(OH)4-, SO42-}, _Na+ is 0.096.

The model developed in this study, which incorporates NaB(OH)₄(aq), Ψ_{B(OH)4‑, SO42-}, _Na+ and revised values for log K for sodium tetraborate and for Θ_{B(OH)4‑, SO42‑}, can precisely model solubility of sodium tetraborate to high ionic strengths. The comparison of the current model with that of Felmy and Weare (1986) demonstrates that there is a significant improvement associated with the current model in predicting solubilities of sodium tetraborate in both NaCl and Na₂SO₄ media. The current model is also validated by comparison of model predicted solubilities of sodium tetraborate in the mixtures of NaCl + Na₂SO₄ to ionic strengths of 8.0 m with independent experimental values from the literature. The validation test indicates that the differences between solubilities predicted by the current model and experimental solubilities are less than 0.05 m with an error less than 30%.

Acknowledgement: Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S Department of Energy.