Hydrolysis of Molybdophosphoric Acid in Concentrated Aqueous Solutions and Effect of Phosphoric Acid
1986 Volume 1986 Issue 2 Pages 113-119
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1986 Volume 1986 Issue 2 Pages 113-119
12-Molybdophosphoric acid (PMo12) in concentrated aque ous solutions decomposes on heating to precipitate molybdenum oxides. The decomposition scheme is explained in terms of Eqs. (6) and (5):
2H3PMO12O40+6H2O→H6P2Mo18O62 6(MoO3·H2O) (6)
MoO3·H2O→MoO3↓+H2O (5)
The addition of 1wt%H3PO4 to 67wt% PMo12 solution inhibited the formation of precipitates over a period of 3000 h at 70°C, although precipitates were observed when the H3PO4 added was less than O.5 wt%. Such a marked effect of H3PO4 was elucidated by Eq. (1).
3H3PMo12O40+H3PO4⇔2H6P2Mo18O62 (1)
Studies using Raman spectra and 31P-NMR spectra revealed 18-molybdodiphosphoric acid (P2Mo18) was formed from PMo12 via 9-molybdophosphoric acid (PMo9) as expressed in Eq. (2) and Eq. (3).
3H3PMo12O40+H3PO4+12H2O⇔4H3PMo9O31(OH2)3 (2)
2H3PMo9O31(OH2)3→H6P2Mo18O62+6H2O (3)
Eq. (2) is in rapid equilibrium even at room temperature, while the dimerization of PMo9 (Eq. (3)) proceeds slowly.
On the other hand, 12 -tungstophosphoric acid (PW12) remained unchanged even in the presence of H3PO4 at 70°C.
The prohibition of 18-tungstodiphosphoric acid (P2W18) formation even at elevated temperature can be attributed to stability of PW12 against the degradation into 9-tungstophosphoric acid (PW9) in strong acidic solutions.
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