Mercury nucleation on glassy carbon electrodes

The amount of mercury, deposited on a glassy carbon electrode at various potentials during the first 200 ms of electrolysis, has been determined by constant current coulometric stripping potentiometry. Eight microliter samples, containing approximately 4 M chloride, were placed on top of a combined glassy carbon, Ag counter and AgAgCl reference electrode positioned up-side-down under a stereo microscope. It was found that, if the electrode was conditioned for 200 ms at −1.80 V. mercury was reversibly deposited during the following electrolysis stripping cycle. If, however, the conditioning cycle was omitted overpotentials of the order of magnitudes 0.50 V were needed in order to initiate mercury nucleation. If the electrode was vigorously anodised at 1.80 V for one minute, 8–10 conditioning pulses were required to restore the electrode sensitivity. Under the microscope it was observed that, provided that the electrode was potentiostatically conditioned, mercury was deposited homogeneously on the glassy carbon surface as shown by a monochromatic sky blue colour of the electrode surface. If not conditioned, the electrode became multi-coloured indicating an inhomogeneous deposition of mercury droplets. Electrode conditioning was found to have a profound effect on the coulometric stripping potentiometry determination of copper(II). On a conditioned electrode, copper(II) could be determined in a five decades concentration range, 1–10000 μM, with a relative accuracy varying only between 96 and 103%. If not conditioned, the accuracy varied between 70–90%. Mechanisms for the deposition and stripping of mercury and copper on glassy carbon electrodes are proposed.