Chronoamperometric determination of antioxidant capacity using iron complex with 2,2’-bipyridine

The proposed approach to the determination of antioxidants with different hydrophilicity is based on the use of iron(III) complex with bipyridine as an oxidant model and chronoamperometric registration of the analytical signal. The oxidant was chosen due to its solubility in aqueous, organic and aqueous-organic media. The following conditions for registration of chronoamperograms were chosen: background composition (acetonitrile-acetate buffer solution with pH 3.6 (9 : 1)), background electrolyte (LiClO4), potential (E = 1.25 V), current registration time (80 s). Antioxidants soluble in organic and aqueous-organic media were studied: α-tocopherol, quercetin, catechin, caffeic acid. The ranges of determined concentrations are (0.5-4) × 10-4 M. The antioxidant capacity (AOE) of ethanolic extracts of medicinal plant raw materials was determined. A high correlation of AOE values obtained by chronoamperometric and spectrophotometric methods is observed, but only for the objects whose intrinsic coloration does not contribute to the absorption value of Fe(II)-bipyridine complex. Application of the proposed approach and the potentiometric method using the system K3[Fe(CN)6]/K4[Fe(CN)6] showed that the values obtained by the potentiometric method are significantly lower for most of the investigated infusions. Thus, when analyzing multicomponent objects containing substances with different hydrophilicity, it is advisable to use oxidizing agents with different solubility, such as Fe(III)-bipyridine complex.