Platinum positive potential limit

Figure 4.8 shows the potential windows obtained at a bright platinum electrode, based on the Fc+/Fc (solvent-independent) potential scale. Because of the overpotentials, the window in water is 3.9 V, which is much wider than the thermodynamic value (2.06 V). The windows for other solvents also contain some overpotentials for the reduction and the oxidation of solvents. However, the general tendency is that the negative potential limit expands to more negative values with the decrease in solvent acidity, while the positive potential limit expands to more positive values with the decrease in solvent basicity. This means that solvents of weak acidity are difficult to reduce, while those of weak basicity are difficult to oxidize. This is in accordance with the fact that the LUMO and HOMO of solvent molecules are linearly related with the AN and DN, respectively, of solvents [8]. 

During the anodic polarization of platinum to potentials of about 3.0 V (RHE), one or several layers (but no more than three) of chemisorbed oxygen are formed, which sometimes are called the a-oxide of platinum. The limiting thickness of these layers is about 1.3 nm. They can be studied both by electrochemical methods and by ellipsometry. At more positive potentials phase-oxide surface layers, the p-oxides are formed. The quantitative composition and structure of these layers and the exact limits of potential for their formation depend on many factors composition of the electrolyte solution, time of polarization, surface history, and often remain unknown. 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

Using the HMDE covered by calomel, Kemula and coworkers studied the oxidation of several other substances [67]. Kublik studied the oxidation of various ions at the HMDE covered by a film of HgO [68]. These electrodes may be used at even more positive potentials than platinum electrodes in the same solutions. Although the mechanism of oxidation at these electrodes is very interesting, platinum and carbon electrodes are more useful in practical work since their properties and stability are not as dependent on the nature of the sample. When using such electrodes at positive potentials, some current due to oxidation of mercury is inevitable. This limits the application of passivated mercury electrodes to rather concentrated sample solutions. 

The potential limits are particularly dependent on the eluent pH. In principle, more negative potentials may be applied in alkaline solution than in acidic solution and, conversely, in acidic solution more positive potentials may be selected than in alkaline solution. For oxidation reactions at high positive potentials, glassy carbon and platinum electrodes are suited reduction reactions at very negative potentials can be performed at glassy carbon, silver, and gold electrodes. 

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