Speciation in the aqueous H+/H2VO4−/H2O2/citrate system of biomedical interest

Abstract

The speciation in the quaternary aqueous H⁺/H₂VO₄⁻/H₂O₂/citrate (Cit³⁻) and H⁺/H₂VO₄⁻/Cit³⁻/L-(+)-lactate (Lac⁻) systems has been determined at 25 °C in the physiological medium of 0.150 M Na(Cl). A combination of ⁵¹V NMR integral intensities and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and evaluated with the computer program LAKE, which is able to treat multimethod data simultaneously. The pK_a-values for citric acid have been determined as 2.94, 4.34 and 5.61. Altogether six vanadate–citrate species have been found in the ternary H⁺/H₂VO₄⁻/Cit³⁻ system in the pH region 2–10, only two of which are mononuclear. Reduction of vanadium(V) becomes more pronounced at pH < 2. Solutions, in which reduction occurred to any extent, were excluded from all calculations. In the quaternary H⁺/H₂VO₄⁻/H₂O₂/Cit³⁻ system, eight complexes have been found in addition to all binary and ternary complexes over the pH region 2–10, including three mononuclear species. Equilibria in general are fast, but the significant and rapid decomposition of peroxide in acidic solutions limited the final model to pH > 4. In the quaternary H⁺/H₂VO₄⁻/Cit³⁻/Lac⁻ system, two mixed-ligand species have been determined, with the compositions V₂CitLac²⁻ and V₂CitLac³⁻ (pK_a = 5.0). To our knowledge, this is the first time such complexes have been reported for vanadium(V). ⁵¹V NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. When suitable, structural proposals are given, based on ¹³C NMR measurements and available literature data of related compounds.