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

Abstract

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-Hückel 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.