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

• Published in: Inorganica Chimica Acta 2012-03, Vol.383, p.26-32
• Main Authors: Holclajtner-Antunović, I., Bajuk-Bogdanović, D., Popa, A., Uskoković-Marković, S.
• Format: Article
• Language: English
• Subjects: 12-Tungstophosphoric acid; 31P NMR spectroscopy; FTIR; Methanol/water; Micro Raman; Stability
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2011.10.035

Micro Raman spectra of evaporated WPA from methanol/water solutions: (a) 25% methanol (v/v) and (b) 50% methanol (v/v). [Display omitted] ► 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. 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 pH8, 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.