Raman Studies on the Identification of Isopolymolybdates in Aqueous Solution

Molybdenum(VI) solutions were investigated in detail by Raman spectroscopy with a view to establishing the speciation of the predominant polynuclear species (isopolymolybdates) formed in aqueous solution. The most intense Raman frequencies due to asymmetric stretching vibration of Mo=O were very sensitive to even small changes of the structure of the isopolymolybdates. As the pH was lowered, the major Raman line showed no gradual shift but exhibited a discrete change. Evidence was obtained for the existence of Mo7O246− (pH 4—6), Mo3O102− (pH 3.5—5), α-Mo8O264− (pH 2—5), β-Mo8O264− (pH 1.5—4), and Mo36O1128− (pH 0.5—1.5). On the other hand, neither of Mo6O192− and γ-Mo8O264− existed in any appreciable amount in aqueous solution.