Some observations on carbon paste electrodes in ac voltammetry

Phase-sensitive alternating current (a.c.) voltammetric techniques were used to examine the background current at carbon paste electrodes. Several different mulling agents were used for the pastes. The available potential span depends on the mulling agent; paraffin oil gives the largest span. Peaks in the background current obtained by cyclic a.c. voltammetry are attributed to current arising from surface functional groups. Chemical or electrochemical oxidation of the electrode surface enhanced the peak current. Little evidence of this current could be found in d.c. cyclic voltammograms because it is obscured by charging current. The anodic potential limit is determined by these surface currents. On the cathodic side, dissolved oxygen is reduced in a process which produced two peaks in the a.c. scan. It was shown that the second peak is due to the reduction of hydrogen peroxide to water and a method for the quantitation of H₂O₂ was developed which had a detection limit of a few mg l^?1.