LIQUID-CHROMATOGRAPHY ELECTROCHEMICAL DETECTION OF HYDROXYLAMINES BY OXIDATION AT A COBALT PHTHALOCYANINE CHEMICALLY-MODIFIED ELECTRODE

Chemically modified electrodes (CMEs) containing a polymeric coating of cobalt phthalocyanine (CoPC) were shown to catalyze the electrooxidation of hydroxylamine (NH2OH) and its N-mono-, N,N-di-, and O-substituted derivatives. All of these compounds were oxidized at unmodified glassy carbon electrodes only at potentials higher than + 1 V (vs. Ag/AgCl) but gave substantial anodic currents between +).25 and +0.55 V at the CoPC-containing surface. On the basis of exhaustive electrolysis experiments, the number of electrons transferred for the oxidations was found to vary between 1.2 and 1.6 depending on the particular hydroxylamine compound and the specific conditions of the electrolysis; and the products included oximes, azoxy compounds, and dimeric species. These observations were consistent with an electrocatalytic mechanism involving oxidation of the hydroxylamine by electrogenerated Co(III)PC and subsequent reaction of the initially formed oxidation products by several pathways. When the CoPC CME was used as the sensor in amperometric detection following liquid chromatography the detection limits obtained at +0.55 V ranged from 0.4 pmol for hydroxylamine itself up to 40 pmol for N,O-dimethylhydroxylamine. By maintaining the applied potential at +0.20 V, the detection could be made selective for hydroxylamine and N-mono-substituted hydroxylamine compounds only.