A Closer Look into Solubility in the Binary NaVO3-H2O and NH4VO3-H2O Systems from 298.15 to 333.15 K and 0.1 MPa

The solubilities of anhydrous sodium metavanadate (NaVO3), its hydrated form (NaVO3 center dot 2H(2)O), and ammonium metavanadate (NH4VO3) in water were measured in the temperature range from 298.15 to 333.15 K using the isothermal saturation method. Previous literature data on the solubility in these binary systems were surveyed, and its reliability was evaluated. Empirical equations were fitted to the accepted data to obtain analytical expressions for the solubilities of the three salts as functions of temperature. Only a few previous papers, mostly published over 60 years ago, were found to report experimental points on the solubility curve of the dihydrate NaVO3 center dot 2H(2)O. We herein report new experimental determinations of the dihydrate solubility. Finally, it was investigated whether the solubility of NH4VO3 at 298.15 K can be predicted using only thermodynamic data appearing in the literature. Although there seems to be a consensus on the identity of the major species in dilute V(V) solutions, discrepancies in the values of infinite dilution equilibrium constants (I = 0) exist. Accordingly, the solubilities predicted using different sets of equilibrium constants were somewhat scattered but of approximately correct magnitude. The neglect of ion-pair/complex formation and the lack of an accurate model for the activity coefficients are probably the most serious flaws limiting the accuracy of the current models. It is clear that much work remains to be done if temperature-dependent thermodynamic models extending to high vanadium concentrations are to be developed.