Study on production of free hydroxyl radical and its reaction with salicylic acid at lead dioxide electrode

The formation of free hydroxyl radical (^.OH) in oxygen transfer reactions at PbO₂ electrodes and the reaction between the electrochemically produced ^.OH and salicylic acid (SA) were studied by cyclic voltammetry and capillary electrophoresis-amperometric detection (CZE-AD) in this paper. Experiments showed that ^.OH was formed in the potential range of 1.0-1.4 V corresponding to the anodic discharge of H ₂O at the PbO₂ electrode, while this phenomenon was not obvious at the Pt electrode. Optimum CZE-AD conditions for indirect determination of ^.OH were explored. It was also found that when salicylic acid was used to trap ^.OH produced on the PbO₂ electrode surface, the amount of 2,5-dihydroxybenzoic acid produced is much more than that of 2,3-dihydroxybenzoic acid. This phenomenon is explained by the proposal that SA first forms an active aromatic radical on the PbO₂ electrode surface before trapping ^.OH, where ^.OH performs a stronger preference in attacking at the 5-position than for attacking at the 3-position of the aromatic radical.