In a previous study I the oxidation of I-and Br-ions on a bright Pt electrode in acid solution was studied in the presence of various concentrations of phenol. Strong inhibition was observed even at very low concentrations of phenol. In the case of I-oxidation, a well-defined initial inhibition potential E i was observed, which depended both on the concentration of phenol and on pH. A theoretical interpretation of the observed phenomena was given 2'3 in terms of the known potential-dependence of electrosorption of neutral organic molecules 4'5. The inhibition effect of phenol on Br-oxidation was sustained up to high anodic potentials, in the range 2.5-3.0 V (SCE). Under these conditions copious oxygen evolution occurred (at a rate which was not substantially affected by the presence of phenol), while the rate of oxidation of Br-remained essentially zero. Thus the species adsorbed on the surface* inhibits selectively the oxidations of Br-and I-, while it has no comparable effect on the oxidation of water molecules.
Self-inhibition was observed in the case of oxidation of hydroquinone and other easily oxidisable isomers of di-and tri-hydroxybenzene 6, showing that the phenomenon of inhibition was not unique to phenol and could be expected when various phenol derivatives were used instead.
Results of the investigation of two further aspects of the same problem are presented below : different reactions were used to test the selectivity of inhibition by phenol ; the inhibition of I-oxidation by several derivatives of phenol was studied. Inhibition was observed in almost all cases, but the decline of the current at E > Ei was not generally as sharp as in the case of phenol and I-. EXPERIMENTAL All experimental techniques were identical with those reported earlier 1. Current/potential curves were taken potentiostatically. Steady-state, point by point measurements were taken in some experiments, while in others the potential was swept very slowly (ca. 15 mV min-1). The results were independent of which of these * This was shown to be an essentially complete monolayer of phenoxy radicals chemisorbed on the surface 1.