Cyclic voltammetric and aqueous equilibria model study of the pH dependant iron(II/III)ethylenediamminetetraacetate complex reduction potential

A mathematical interpretation of the voltammetric half-wave potential vs. pH (E1/2–pH) behavior of the FeII/III ethylenediamminetetraacetate (EDTA) redox system in the pH range of 2–8 is described. In the low pH range (<3.8), the formal potential of the FeII/IIIEDTA redox reaction decreases at 130 mV per pH and is a function of the concentration of EDTA. The latter is a consideration neglected by previous investigators. A Nernst-type equation for this characteristic is provided in Eq. (15). The cyclic voltammograms in the region of 3.8 < pH < 6.8 demonstrated near Nernstian reversibility and the E1/2 was constant at −110 mV vs. Ag/AgCl. This is an indication that the FeII and FeIIIEDTA complexes are involved only with electrochemical oxidation–reduction and no other chemical processes. In the high pH region above 6.8, the half wave potential shifted to more negative at the slope of 44 mV per pH unit. This result is attributable to the hydrolysis of FeIIIEDTA, the Nernst relationship is expressed in Eq. (22). Mass balance relationships are developed for each pH region which allows for the expression of Eqs. (15) and (21).