Isopiestic determination of the osmotic coefficients and Pitzer model representation for the Li2B4O7+LiCl+H2O system at T=298.15 K

Isopiestic molalities and water activities have been measured for the Li2B4O7+LiCl + H2O system at T=298.15 K using an improved isopiestic apparatus. Two types of osmotic coefficients, phi(S) and phi(E), were determined, where phi(S) is based on the stoichiometric molalities of the solute Li2B4O7(aq) and phi(E) is based on equilibrium molalities calculated by consideration of the equilibrium speciation of Li2B4O7 to partially form H3BO3, B(OH)(4)(-) and B3O3(OH)(4)(-). The stoichiometric equilibrium constant K-m for the aqueous speciation reaction was estimated. Two representations of the osmotic coefficients of Li2B4O7+LiCl+H2O were made with Pitzer's ion-interaction model. Model (1) involved representing the phi(S) values with six parameters based on considering the ionic interactions between Li+, Cl-, and B4O72- ; and model (2) involved representing the phi(E) values based on the calculated equilibrium speciation. Reasonable agreements were obtained between the experimental osmotic coefficient data and those calculated using the above models, with standard deviations of 0.075 and 0.0229, respectively, for these two models. The thermodynamic osmotic coefficients for the complex system containing polymeric boron anions and lithium cation was modelled and explained by use of Pitzer's ion-interaction model, with minor modifications in combination with speciation reaction equilibria.