Isopiestic Determination of the Osmotic and Activity Coefficients of Li2SO4(aq) at T = 298.15 and 323.15 K, and Representation with an Extended Ion-Interaction (Pitzer) Model

Isopiestic vapor-pressure measurements were made for Li2SO4(aq) from 0.1069 to 2.8190 mol.kg at 298.15 K, and from 0.1148 to 2.7969 mol.kg at 323.15 K, with NaCl(aq) as the reference standard. Published thermodynamic data for this system were reviewed, recalculated for consistency, and critically assessed. The present results and the more reliable published results were used to evaluate the parameters of an extended version of Pitzer's ion-interaction model with an ionic-strength dependent third-virial coefficient, as well as those of the standard Pitzer model, for the osmotic and activity coefficients at both temperatures. Published enthalpies of dilution at 298.15 K were also analyzed to yield the parameters of the ion-interaction models for the relative apparent molar enthalpies of dilution. The resulting models at 298.15 K are valid to the saturated solution molality of the thermodynamically stable phase Li2SO4.H2O(cr). Solubilities of Li2SO4.H2O(cr) at 298.15 K were assessed and the selected value of m(sat.)=3.13+/-0.04 mol.kg was used to evaluate the thermodynamic solubility product K s(Li2SO4.H2O, cr, 298.15 K) = (2.62+/-0.19) and a CODATA-compatible standard molar Gibbs energy of formation Deltaf G m(Li2SO4.H2O, cr, 298.15 K) = -(1564.6+/-0.5) kJ.mol.