Voltammetric Behaviour and Determination of 8-Hydroxyquinoline Using a Glassy Carbon Paste Electrode and the Theoretical Study of its Electrochemical Oxidation Mechanism

The 8-Hydroxyquinoline (8HQ) oxidation process has been investigated by Cyclic Voltammetry using a Glassy Carbon Paste Electrode (GCPE) as a working electrode. The theoretical study of the mechanism of electrochemical oxidation of 8HQ has been based on the AM1 semi-empirical quantum chemical computations of the heats of formation of the reaction intermediates, taking into account the influence of pH and solvation effects. We proposed that a two-electron irreversible process, controlled by diffusion of electroactive species, is responsible for an oxidation peak of 8HQ that appears in all cyclic voltammograms recorded on a clean electrode in the solutions of pH in the range 2-12 with a supporting electrolyte of Britton-Robinson Buffer/methanol. A single-electron oxidation of 8HQ leads, depending on pH, to the formation of various free radical species that combine to make dimers which, after being oxidized once more, give quinonoid-type compounds. Recording continuous cyclic voltammograms on the GCPE, pre-peaks appear as a consequence of dimer and quinonoid compounds formation. By applying Differential Pulse Voltammetry for 8HQ determination it was calculated that the limit of detection was 5.2x10(-8) mol/L. For more sensitive quantitative determination of the investigated substance Adsorptive Stripping Differential Pulse Voltammery can be used since it was found that after 300s-deposition time at 0.0 V vs. Saturated Calomel Electrode, a 2.1 times higher peak current than without deposition was obtained.