Speciation in the aqueous H+/H2VO4−/H2O2/picolinate system relevant to diabetes research

Abstract

A detailed study of the quaternary aqueous H⁺/H₂VO₄⁻/H₂O₂/picolinate (Pi⁻) system has been performed at 25 °C in 0.150 M Na(Cl) medium using quantitative ⁵¹V NMR (500 MHz) and potentiometric data (glass electrode). In the ternary H⁺/H₂VO₄⁻/Pi⁻ system, six complexes have been found in the pH region 1–10. In the quaternary H⁺/H₂VO₄⁻/H₂O₂/Pi⁻ system, eight additional complexes have been found. Generally, equilibria are fast in both systems. The rate of peroxide decomposition depends on the species in solution. Chemical shifts, compositions and formation constants for the species are given. Equilibrium conditions and the fit of the model to the experimental data are illustrated in distribution diagrams. Possible formation of mixed ligand species with imidazole, lactic acid and citric acid have been investigated and ruled out under the same experimental conditions. Structural proposals are given, based on ¹³C NMR data and available crystal structures.