Abstract
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 ≤ 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−nn+ (n−5)H+(n = 4–7)
and were fitted empirically as a function of ionic strength and temperature, including the
appropriate Debye-Hückel term.