Solubility of B-Nb2O5 and the Hydrolysis of Niobium(V) in Aqueous Solution as a Function of Temperature and Ionic Strength

B-Nb2O5 was recrystallized from commercially available oxide, and XRD analyses indicated that it is stable in contact with solutions over the pH range 0 to 9, whereas solid polyniobates such as Na8Nb6O19⋅13H2O(s) appear to predominate at pH>9. Solubilities of the crystalline B-Nb2O5 were determined in five NaClO4 solutions (0.1≤I m /mol⋅kg−1≤1.0) over a wide pH range at (25.0±0.1) °C and at 0.1 MPa. A limited number of measurements were also made at I m =6.0 mol⋅kg−1, whereas at I m =1.0 mol⋅kg−1 the full range of pH was also covered at (10, 50 and 70) °C. The pH of these solutions was fixed using either HClO4 (pH≤4) or NaOH (pH≥10) and determined by mass balance, whereas the pH on the molality scale was measured in buffer mixtures of acetic acid + acetate (4≤pH≤6), Bis-Tris (pH≈7), Tris (pH≈8) and boric acid + borate (pH≈9). Treatment of the solubility results indicated the presence of four species, \(\mathrm{Nb(OH)}_{n}^{5-n}\) (where n=4–7), so that the molal solubility quotients were determined according to: $$0\mathrm{.5Nb}_{2}\mathrm{O}_{5}\mathrm{(cr)+0}\mathrm{.5(2}n-5\mathrm{)H}_{2}\mathrm{O(l)}_{\leftarrow}^{\to}\mathrm{Nb(OH)}_{n}^{5-n}+(n-5)\mathrm{H}^{+}\quad (n=4\mbox{--}7)$$ and were fitted empirically as a function of ionic strength and temperature, including the appropriate Debye-Huckel term. A Specific Interaction Theory (SIT) approach was also attempted. The former approach yielded the following values of log 10 K sn (infinite dilution) at 25 °C: −(7.4±0.2) for n=4; −(9.1±0.1) for n=5; −(14.1±0.3) for n=6; and −(23.9±0.6) for n=7. Given the experimental uncertainties (2σ), it is interesting to note that the effect of ionic strength only exceeded the combined uncertainties significantly in the case of log 10 K s6 to I m =1.0 mol⋅kg−1, such that these values may be of use by defining their magnitudes in other media. Values of Δ f G o, Δ f H o, S o and \(C_{p}^{\mathrm{o}}\) (298.15 K, 0.1 MPa) for each hydrolysis product were calculated and tabulated.