Reductants soluble, electrode reaction

Electrochemical corrosion involves the simultaneous occurrence of (at least) two electrode reactions at the same interfacial potential between a metal and a solution. One of these is the reduction of some reducible species (e.g. 02 or H+) and the other is the anodic oxidation of the metal M to its ion Mz+, either to a soluble ionic species or to an insoluble compound (e.g. the metal oxide). In the stationary state, the reduction current Ic and the oxidation current /a compensate each other, i.e. — Jc = Ja, and the net current Ja + Jc is equal to zero at the so-called corrosion potential Ecori. 

The electrochemical reduction of nitrobenzene to produce p-aminophenol has attracted industrial interest for several decades. However, some limitations may be met in this process regarding overall reaction rate, selectivity and current efficiency using a two-dimensional electrode reactor. These restrictions are due to the organic electrode reaction rate being slow and to the low solubility of nitrobenzene in an aqueous solution. In this example, a packed bed electrode reactor (PBER), which has a large surface area and good mass transfer properties, was used in order to achieve a high selectivity and a reasonable reaction rate for the production of p-aminophenol. The reaction mechanism in an acid solution can be simplified as... 

Throughout this discussion, we consider the electrode reaction A + e P to take place at a mercury-coated electrode in a solution of A that also contains an excess of a supporting electrolyte. We have again left the charges off of A and P for clarity. We further assume that the initial concentration of A is while that of P is zero and that P is not soluble in the mercury. Finally, we assume that the reduction reaction is rapid and reversible so that the concentrations of A and P in the layer of solution immediately adjacent to the electrode are given at any instant by the Nernst equation ... 

The rate of a corrosion reaction is affected by pH (via H reduction and hydroxide formation), the partial pressnre of O, (the solubility/concentration of oxygen in solution), fluid agitation, and electrolyte condnctivity. Corrosion processes are analyzed using the thermodynamics of electrode reactions, mass transfer of the cathode reactants O2 and/or H, and the kinetics of metal dissolution reactions [157, 158]. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

Chronopotentiometry is an important molten salt technique because it can be used with electrodes of relatively large areas, such as simple flag electrodes without an insulating seal By using current-reversal chronopotentiometry, preliminary diagnostic work to determine whether the electrode reaction product is soluble or insoluble, and whether the electrode reaction is reversible or irreversible has proven to be convenient, especially for coitplex reactions such as the reduction of chromate (30). The important... 

At the electrode surface there is competition among many reduction reactions, the rates of which depend on iQ and overpotential q for each process. Both /0 and q depend on the concentration of the electroactive materials (and on the catalytic properties of the carbon surface). However, the chemical composition of the SEI is also influenced by the solubility of the reduction products. As a result, the voltage at... 

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