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

Using a combination of potentiometry (glass electrode) and quantitative V-5 1 MMR spectroscopy, the full speciation in the vanadate-peroxide and vanada te-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) va lue of imidazole and the formation constants for 10 peroxovanadate species and also for three more species where a single imidazole moiety is also bou nd, have been calculated. The experimental data show a good fit to the calc ulated speciation model, even for the less abundant species. The species ar e either monomeric or dimeric in vanadium, and four resonances of the dimer ic species have been unambiguously assigned via 2D V-51 NMR. Diperoxovanada tes are the favoured species at pH 2-10, when sufficient peroxide is presen t. Imidazole: is found to bind strongly to them at pH 6-9. The equilibrium conditions are illustrated in distribution diagrams. (C) 2000 Elsevier Scie nce Inc. All rights reserved.