Molecular Speciation of Isopolyoxomolybdates and Isopolyoxotungstates with Silicic Acid in Aqueous Solution Using ESI–MS

There have been few reports on the characterization of molybdate and tungstate species in pure water using ESI–MS. Hexameric molybdate anions were reported to be unstable in neutral solution, but to exist in acidic solutions. The monomolybdate anion was found to be stable in pure aqueous solution in the form of [HMoO 4 ] − . Monomeric tungstate was observed as [WO 4 ] 2− and [HWO 4 ] − in acidic solution. At low pH, [Mo 4 O 13 ] 2− , [HMo 4 O 13 ] − and [HW 3 O 10 ] − were the main molecular species observed. At high pH, hydrolyzed [HMoO 4 ] − and [HWO 4 ] − were detected by mass spectometer. When silicic acid was added to the molybdate solution, [SiMo 12 O 40 ] 4− and [HSiMo 12 O 40 ] 3− were formed from the tetrameric molybdate anion. In addition, monomeric, trimeric, hexameric, heptameric, undecameric and dodecameric polyoxomolybdates were detected in the supernatant solution by ESI mass spectrometry. When silicic acid was added to a tungstate solution, [HSiW 12 O 40 ] 3− , composed of the trimeric tungstate anion was detected. In addition, two pentameric polyoxotungstates as well as tetrameric, hexameric and heptameric polyoxotungstates were present in the supernatant solution, while no monomer or dimer was detected.