Electrooxidation of chlorophenols at a glassy carbon electrode in a pH 11 buffer

The electrochemical oxidation of three chlorophenols (CPs), namely 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol (2,4,6-TCP) at a glassy carbon electrode in a pH 11 carbonate/hydrogen carbonate buffer electrolyte has been studied by cyclic voltammetry (CV) and impedance measurements. The three CPs show in CV an irreversible oxidation peak whose potential decreased linearly with increasing pKa of the CPs, showing that the oxidation is the more facile the less acidic is the CP. Although the oxidation is not under diffusion control, i.e., the peak potential was not directly related to the rate constant, the same slope was obtained when the potential at which the integrated charge began to increase significantly was plotted instead of the peak potential. A reversible process near −0.45 V is observed for 2,4,6-TCP from the second positive scan, which was attributed to the formation of a soluble (since this couple nearly disappeared upon solution stirring) benzoquinone. Impedance data showed that for 2-CP and 2,4-DCP the formation of the polyphenolic film begins already at 0.0–0.1 V, and that the film is non-porous and non-conducting. On the contrary, with 2,4,6-TCP polyphenol formation began near 0.15 V, and the film is so porous that it allows the oxidation of 2,4,6-TCP to continue.