CYCLIC AND STRIPPING VOLTAMMETRY OF MERCURY AT IMPREGNATED GRAPHITE ELECTRODE IN CHLORIDE AND THIOCYANATE MEDIA

The behaviour of the Hg(II)-Hg(O) system was investigated under voltammetric conditions on the impregnated graphite electrode in thiocyanate and chloride media. The efficiency of anodic dissolution of mercury from graphite is equal to 100% in thiocyanate medium and it is always lower than 100% in chloride medium. This fact was explained on the basis of a mechanism involving the formation of a passive calomel layer. The phenomena of passivation occurring at the mercury film electrode and the HMDE differ clearly. For the passivation of thick mercury films a much larger amount of mercury should be dissolved anodically than for the passivation of the HMDE. On the other hand some passivation phenomena occur also when the deposited amount of mercury is about a hundred times smaller than the amount needed to passivate the HMDE. Such a behaviour confirms the opinions that the mercury film deposited on the graphite substrate does not consist of a uniformly arranged layer of mercury but that it consists of an aggregate of very small mercury droplets. Some calomel crystals formed during the anodic dissolution of the mercury droplets are reduced on graphite with significant overpotential. The results obtained show that it is better to avoid chloride media during the determination of mercury by the anodic stripping method at graphite electrodes. The investigation of the process of anodic dissolution of thin mercury layers in chloride medium may help to estimate the extent of the homogeneity of the mercury deposits. © 1979 Elsevier Sequoia S.A.