Decavanadate protonation degrees in aqueous solution

Aqueous solutions of vanadium(V) have been prepared at different OH⁻ vs. vanadium concentrations from both decavanadic acid and metavanadate solutions. These solutions have been extracted with an excess of tetraheptylammonium bromide in benzene. The bromide displaced has been determined in order to gain information on the average charge of the extracting species already known to be variously protonated decavanadate anions. The addition of a constant excess of supporting electrolytes has been avoided, as it is known it could affect the position of equilibria.Besides the already well known H₂V₁₀O₂₈⁴⁻, HV₁₀ O₂₈⁵⁻ and V₁₀O₂₈⁶⁻ species, strong evidence has also been obtained for the existence of H₃V₁₀O₂₈³⁻.Moreover, a slow disproportionation of V₁₀O₂₈⁶⁻ into HV₁₀O₂₈⁵⁻ and V₂O₇⁴⁻ (or HVO₄²⁻) during a first stage, and then into metavanadate, resulted.Therefore, H₆V₁₀O₂₈ proves a strong acid as triprotic instead of tetraprotic, as reported in the literature. Inconsistent results are almost certainly due to the high ionic media usually employed.