Mercury oxidation-reduction relationships

Voltammetry is a part of the repertoire of dynamic electrochemical techniques for the study of redox (reduction-oxidation) reactions through current-voltage relationships. Experimentally, the current response (i, the signal) is obtained by the applied voltage (.E, the excitation) in a suitable electrochemical cell. Polarography is a special form of voltammetry where redox reactions are studied with a dropping mercury electrode (DME). Polarography was the first dynamic electrochemical technique developed by J. Heyrovsky in 1922. He was awarded the Nobel Prize in Chemistry for this discovery. 

Wiesner assumed that the enediol grouping in L-ascorbic acid is primarily oxidized at the dropping-mercury electrode to a non-hydrated, electroactive, diketo grouping, thus forming a reversible system he partially proved this oscillopolarographically. This problem was solved, theoretically, by Kem and Kouteck - as an instance of a chemical reaction occurring subsequent to the electrode process. They explained what properties must be exhibited by the reduction wave of dehydro-L-ascorbic acid under the assumption that the simple depolarization scheme usually accepted for dehydro-L-ascorbic acid is valid. It follows from the relationships derived by Koutecky that, if the cathodic wave is small compared to the anodic one, the half-wave potentials of both waves should be equal, and, on the other hand, if the cathodic-wave current has diffusion character, then the value of its half-wave potential should be identical with the normal redox potential. The results of experimental work do not correspond to these theoretical conclusions. 

The adsorption of ions and molecules on the surface of mercury electrodes is a thoroughly investigated phenomenon [51 ]. Surface-active substances are either electroactive [52] or electroinactive [53]. The former can be analyzed by adsorptive stripping voltammetry [54]. This is the common name for several electroanalytical methods based on the adsorptive accumulation of the reactant and the reduction, or oxidation, of the adsorbate by some voltammetric technique, regardless of the mechanisms of the adsorption and the electrode reaction [55, 56]. Frequently, the product of the electrode reaction remains adsorbed to the electrode surface. Hence, the term stripping should not be taken literally in all cases. Besides, some adsorbates may be formed by electrosorption reactions, so that their reduction includes covalently bound mercury atoms. The boundary between adsorption followed by reduction, on the one hand, and electrosorption, on the other, is not strictly defined. Moreover, it is not uncommon that, upon cathodic polarization, the current response is caused by a catalytic evolution of hydrogen, and not by the reduction of the adsorbate itself [57]. However, what is common to all methods is a hnear relationship between the surface concentration of the adsorbate and the concentration of analyte at the electrode surface ... 

---