Investigation of the vanadium(III)/vanadium(II)/ N-(hydroxyethyl)ethylenediaminetriacetic acid system by cyclic voltammetry and differential pulse polarography

Cyclic voltammetry and differential pulse polarography (d.p.p.) are used to study the influence of pH and metal complex concentration on the voltammetric responses of the V(III)/V(II)/ N- (hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) system. The monomer-dimer equilibrium of V(III)HEDTA gives rise to typical patterns on cyclic voltammograms. The formation constant for (V(III)HEDTA)₂ is evaluated by d.p.p. The dimer-monomer concentration ratios representing the equilibrium in the bulk solution can be obtained only when the drop time of the mercury electrode is ?0.2 s. The pH-independent value of the half-wave potential for the reduction of V(III)HEDTA to V(II)HEDTA is used to evaluate the ratio of the formation constants of V(III)HEDTA and V(II)HEDTA. The suitability of V(III)HEDTA as a model complex for V(III) in ascidians (primitive marine organisms) is discussed.