Spectroscopic identification of molecular species of 12-tungstophosphoric acid in methanol/water solutions

The various molecular species formed by decomposition of 12-tungstophosphoric acid (WPA) in methanol/water solutions of different composition, and over a pH range 1–12, were identified by UV, FTIR, micro Raman and 31P NMR spectroscopy. The dependence of stability on organic solvent amount, pH and concentration of WPA in solution is investigated in detail.It is shown that addition of methanol in aqueous solution of WPA with varied pH causes extension of the pH region where parent Keggin anion is stable. While Keggin anion is preserved in aqueous solution with pH < 2, in solution with 25% of methanol it is stable up to pH 2.5, in solution with 50% of methanol up to pH 3.5, and in solution with 75% of methanol Keggin anion is observed up to pH 4.5.Various molecular species, such as monovacant and trivacant lacunary Keggin structures and dimeric Wells–Dawson derivatives, are observed as a result of Keggin anion destruction with increasing of pH, but speciation in both aqueous and methanol/water solutions is similar. It is also shown that lacunary anions are less soluble in solutions with higher methanol content and with higher pH. In aqueous solution at pH > 8, the solution contains only tungstates and phosphates with no precipitation. In solution with 25% of methanol, precipitate is formed at pH 8.0 and contains trivacant lacunary Keggin anion. In solution with 50% of methanol the precipitation is at pH 6.5, and in solution with 75% of methanol it is observed at pH 4.5, but in both solutions the precipitate contains monovacant lacunary anion as the main component.

Micro Raman spectra of evaporated WPA from methanol/water solutions: (a) 25% methanol (v/v) and (b) 50% methanol (v/v).Figure optionsDownload as PowerPoint slideHighlights
► Addition of methanol in aqueous solution of WPA causes extension of the pH region where parent Keggin anion is stable.
► In solution with 25% of methanol Keggin anion is stable up to pH 2.5.
► In solution with 50% of methanol up to pH 3.5.
► In solution with 75% of methanol Keggin anion is observed up to pH 4.5.