Electrochemical oxidation of quercetin in aqueous and ethanol-water media with the use of graphite/chemically modified silica ceramic electrode

A mixture of silica chemically modified with 1- n -propyl-3-methylimidazolium chloride and graphite was used to fabricate a ceramic working electrode. The conventional three-electrode cell that included the prepared electrode has been used to investigate the electrochemical behavior of quercetin in aqueous and ethanol-water media by methods of cyclic voltammetry and chronoamperometry. The procedures for the quercetin determination in aqueous and ethanol-water media have been developed. The limits of detection are 0.05 μmol L −1 in aqueous medium and 3–6 μmol L −1 in ethanol solution and ethanol-water mixtures. The applicability of the electrode and the developed procedure has been verified by the analysis of real pharmaceutical. Therefore, the proposed procedures can be implemented in the drug quality control. The mechanisms of the quercetin oxidation depend on the medium. Oxidation at ~ 150 mV occurs according to the well-known mechanism that involves the removal of two electrons and two hydrogen ions from the quercetin molecule. The oxidation of quercetin in ethanol-water media takes place also at ~ 550 mV. The plausible mechanism of this process derived from the electrochemical data and confirmed by the quantum-chemical calculations includes firstly formation of the 7,4′-biradical structure which then transforms into the 7,4′-dione compound.