Electrochemical oxidation of 6-hydroxyquinoline on a glassy carbon paste electrode: Voltammetric and computational study

Experimental study of the electro-oxidation behavior of 6-hydroxyquinoline (6HQ) has been performed using cyclic voltammetry (CV) on a glassy carbon paste electrode (GCPE). The theoretical study of the 6HQ electrochemical oxidation mechanism, based on the AM1 semi-empirical quantum chemical computations of the heats of formation and ionization energy of reaction intermediates, taking into account influence of pH and solvation effects, has also been conducted. It was established that a two-electron, irreversible process, controlled by diffusion of electroactive species, is responsible for an oxidation peak of 6HQ that appears in all cyclic voltammograms recorded on a clean electrode, in solutions of pH range 2–12, with a supporting electrolyte of Britton–Robinson buffer/methanol. A single-electron oxidation of 6HQ leads, depending on pH, to the formation of various free radical species that combine to make C5–C5 coupled 6HQ dimers which, after being oxidized once more, give quinonoid-type compounds. The irreversible electrochemical oxidation of 6HQ does not stop after the dimerization step but proceeds further to the formation of oligo/poly(6HQ). The optimum conditions for Differential Pulse Voltammetry (DPV) determinations were established and it was shown that for more sensitive quantitative determinations Adsorptive Stripping Differential Pulse Voltammetry (AdSDPV) can be used.