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The redox potentials of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and cobalt tetraaxaammlene (CoTAA), irreversibly adsorbed on pyrolytic graphite, were investigated as a function of the pH by cyclic voltammetry. The reduction of oxygen was studied under the same conditions with the rotating disc electrode technique. At high pH, where the reduction to H,Or on the cobalt-containing chelates is more reversible, the value of the redox potential is less important. At low pH, where this reduction is irreversible, there is a clear correlation between the redox potential of the central metal ion and the Oa reduction behaviour. In acid media, a 470 mV more positive redox potential of CoTAA compared to that of CoPc, resulted in a half-wave potential, Elfl, for the Or reduction being 400 mV more positive than the El12 of CoPc.

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