Speciation in vanadium bioinorganic systems 6. Speciation study of aqueous peroxovanadates, including complexes with imidazole

Using a combination of potentiometry (glass electrode) and quantitative V-51 MMR spectroscopy, the full speciation in the vanadate-peroxide and vanadate-peroxide-imidazole systems was determined in the pH range 1-10 (0.150 M Na(CI) medium, 25 degrees C). Using the computer program LAKE, the pK(a) value of imidazole and the formation constants for 10 peroxovanadate species and also for three more species where a single imidazole moiety is also bound, have been calculated. The experimental data show a good fit to the calculated speciation model, even for the less abundant species. The species are either monomeric or dimeric in vanadium, and four resonances of the dimeric species have been unambiguously assigned via 2D V-51 NMR. Diperoxovanadates are the favoured species at pH 2-10, when sufficient peroxide is present. Imidazole: is found to bind strongly to them at pH 6-9. The equilibrium conditions are illustrated in distribution diagrams. (C) 2000 Elsevier Science Inc. All rights reserved.