Characterization of decavanadate and decaniobate solutions by Raman spectroscopy

The decaniobate ion, (Nb-10 = [Nb10O28](6-)) being isoelectronic and isostructural with the decavanadate ion (V-10 = [V10O28](6-)), but chemically and electrochemically more inert, has been useful in advancing the understanding of V-10 toxicology and pharmacological activities. In the present study, the solution chemistry of Nb-10 and V-10 between pH 4 and 12 is studied by Raman spectroscopy. The Raman spectra of V-10 show that this vanadate species dominates up to pH 6.45 whereas it remains detectable until pH 8.59, which is an important range for biochemistry. Similarly, Nb-10 is present between pH 5.49 and 9.90 and this species remains detectable in solution up to pH 10.80. V-10 dissociates at most pH values into smaller tetrahedral vanadate oligomers such as V-1 and V-2, whereas Nb-10 dissociates into Nb-6 under mildly (10 > pH > 7.6) or highly alkaline conditions. Solutions of V-10 and Nb-10 are both kinetically stable under basic pH conditions for at least two weeks and at moderate temperature. The Raman method provides a means of establishing speciation in the difficult niobate system and these findings have important consequences for toxicology activities and pharmacological applications of vanadate and niobate polyoxometalates.