3 research outputs found
Solubility of B-Nb2O5 and the hydrolysis of niobium(V) in aqueous solution as a function of temperature and ionic strength.
International audienceB-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·kgâ1 †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·kgâ1, whereas at Im = 1.0 mol·kgâ1 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 â„ 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 â 7), Tris (pH â 8) and boric acid+borate (pH â 9). Treatment of the solubility results indicated the presence of four species, Nb(OH)5ân n (where n = 4â7), so that the molal solubility quotients were determined according to: 0.5Nb2O5(cr) +0.5(2nâ 5)H2O(l)ââ Nb(OH)5ân n + (nâ5)H+ (n = 4â7) and were fitted empirically as a function of ionic strength and temperature, including the appropriate Debye-HĂŒckel term. A Specific Interaction Theory (SIT) approach was also attempted. The former approach yielded the following values of log10Ksn (infinite dilution) at 25 °C: â(7.4 ± 0.2) for n = 4; â(9.1 ± 0.1) for n = 5; â(14.1 ± 0.3) for n = 6; and â(23.9 ± 0.6) for n = 7. Given the experimental uncertainties (2Ï), 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·kgâ1, 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 tabulate