Solubility of B-Nb2O5 and the hydrolysis of niobium(V) in aqueous solution as a function of temperature and ionic strength.
Résumé
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 ≤ Im/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 Im = 6.0 mol·kg−1, whereas at Im = 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, Nb(OH)5−n n (where n = 4–7), so that the molal solubility quotients were determined according to: 0.5Nb2O5(cr) +0.5(2n− 5)H2O(l)→← Nb(OH)5−n n + (n−5)H+ (n = 4–7) 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 log10Ksn (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 log10Ks6 to Im = 1.0 mol·kg−1, such that these values may be of use by defining their magnitudes in other media. Values of fGo, fHo, So and Co p (298.15 K, 0.1 MPa) for each hydrolysis product were calculated and tabulated