Speciation in the aqueous H+/H2VO4−/H2O2/phosphate system

Abstract

The speciation in the aqueous H⁺/H₂VO₄⁻/phosphate (dihydrogen phosphate, P⁻) and H⁺/H₂VO₄⁻/H₂O₂/P⁻ systems has been determined in the physiological medium of 0.150 M Na(Cl) at 25 °C. A combination of multinuclear NMR integral and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and treated simultaneously by the computer program LAKE. The pK_a-values for phosphoric acid have been determined by potentiometric and ³¹P NMR chemical shift data, and have been found to be 1.85 ± 0.02, 6.69 ± 0.02 and 11.58 ± 0.07. The errors given are 3σ. Altogether nine vanadate–phosphate species have been found in the ternary H⁺/H₂VO₄⁻/P⁻ system in the pH region 1–11, with the following compositions: VP, VP₂ and V₁₄P. Equilibrium is very slow in acidic solutions, requiring more than 3 months for the formation of V₁₄P species. On the other hand, less than 15 min are needed for equilibration at neutral and alkaline pH. In the quaternary H⁺/H₂VO₄⁻/H₂O₂/P⁻ system, four new species have been found in addition to all binary and ternary complexes. They are of VXP and VX₂P compositions, where X denotes the peroxo ligand. ⁵¹V and ³¹P 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. Biological and medical relevance of the species is also discussed and physiological conditions are modelled.