Mercury, cathode oxidation-reduction behavior

Owing to the large number of oxidation states possible for osmium species in solution one would expect controlled-potential coulometry to be well suited to the determination of this element. Unfortunately, much of the evidence concerning the solution chemistry and electrolytic behavior of osmium is incomplete and contradictory. Meites (182) has investigated the reduction of osmium (VUI) both polarographically and at large mercury cathodes and found reduction to osmium (VI) and osmium (IV) in 0.1 10 M sodium hydroxide at potentials of —0.35 V and —1.00 V vs. SCE, respectively. In 0.1 and 1.0 M potassium cyanide solution, osmium (VI) was further reduced to osmium (III) and osmium (II) at potentials of —0.62 V and -1.00 V vs. SCE these proeesses are well suited for analytical purposes. In a later study, Cover and Meites (183) reported the transient formation of osmium (V) and reduction of osmium (VT) to osmium (III) in an apparent three electron step. These processes are not directly useful for analytical purposes owing to disproportionation and secondary chemical reactions, however. Coulometric studies of related oxidation processes have not, as yet, been reported in the literature. 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

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