Indirect Electrode Reactions

In electroanalysis, coulometry is an important method in which the analyte is specifically and completely converted via a direct or indirect electrolysis, and the amount of electricity (in coulombs) consumed thereby is measured. According to this definition there are two alternatives (1) the analyte participates in the electrode reaction (primary or direct electrolysis), or (2) the analyte reacts with the reagent, generated (secondary or indirect electrolysis) either internally or externally. 

For the dehydrogenation of CH—XH structures, for example, of alcohols to ketones, of aldehydes to carboxylic acids, or of amines to nitriles, there is a wealth of anodic reactions available, such as the nickel hydroxide electrode [126], indirect electrolysis [127, 128] (Chapter 15) with I , NO, thioanisole [129, 130], or RUO2/CP [131]. Likewise, selective chemical oxidations (Cr(VI), Mn02, MnOJ, DMSO/AC2O, Ag20/Celite , and 02/Pt) [94] are available for that purpose. The advantages of the electrochemical conversion are a lower price, an easier scale-up, and reduced problems of pollution. 

The use of selective electrodes for indirect determinations of species to which the sensor responds by a reaction involving the analyte as the primary... 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

In addition, electrode reactions are frequently characterized by an irreversible, i.e., slow, electron transfer. Therefore, overpotentials have to be applied in preparative-scale electrolyses to a smaller or larger extent. This means not only a higher energy consumption but also a loss in selectivity as other functions within the molecule can already be attacked. In the case of indirect electrolyses, no overpotentials are encountered as long as reversible redox systems are used as mediators. It is very exciting that not only overpotentials can be eliminated but frequently redox catalysts can be applied with potentials which are 600 mV or in some cases even up to 1 Volt lower than the electrode potentials of the substrates. These so-called redox reactions opposite to the standard potential gradient can take place in two different ways. In the first place, a thermodynamically unfavorable electron-transfer equilibrium (Eq. (3)) may be followed by a fast and irreversible step (Eq. (4)) which will shift the electron-transfer equilibrium to the product side. In this case the reaction rate (Eq. (5)) is not only controlled by the equilibrium constant K, i.e., by the standard potential difference be-... 

The free-radicals are generated by discharge of proton, peroxides and easily reducible compounds at the cathode according to Eq. (1—4). The radial-anion of monomer is obtained by both direct and indirect electron transfer process [Eq. (5—6)]. The indirect process means that the active oxidizing species is formed from the electrolytes by electrode reaction, followed by interaction with the monomer. An unstable monomer like a,a -2-trichloro-p-xylene is formed and polymerizes instantaneously [Eq. (7)]. The regeneration of ferrous ion from the pool of used up ferric ion in a redox system is electrolytically successful [Eq. (8)] and an... 

Fig. 6. Indirect method in which the electrode reaction is assumed to be reversible.

The current density has a dramatic influence on the yield of 52 and reveals that more than one electrode reaction is involved in the sequence. When the current density is in the range of 2.8. 7 mA/cm2, 52 is directly obtained in about 30% yield. The rationale for the formation of 52 starts with the direct or indirect generation of phenoxyl radicals at the BDD anode. Since the used conditions will provide concentrations of oxyl spin centers, which exclude a recombination, the transformation has to follow a different mechanistic course. The anodic treatment will cause an Umpolung effect because the electron rich phenol is oxidized [110-113]. Such phenoxyl species are still electrophilic despite the liberation of a proton [114-119]. The electrophilic attack occurs at the most electron rich position of the reaction partner which provides the observed connectivity in 52 (Scheme 21). 

There are many electrochemical reactions known that involve the use of so-called mediators , which are basically redox catalysts. The active form acts as an electron transfer agent to convert the substrate into products. The catalyst is then regenerated at the electrode surface and the cycle can be repeated until all the substrate is consumed. Such an indirect electrode reaction is schematically shown in Figure 2 for the case of an electroreduction. The mediator is reduced to its active form at potentials that can be up to 600 mV more positive than the potential required for direct reduction of the substrate. 

Free halogens in acid media can be reduced to the corresponding halide ions at the mercury electrode or at various solid electrodes. However, this electrode reaction is not convenient for analytical purposes. Therefore halogens are very often determined indirectly after reaction with some reducing agents [3, 16]. Iodine can be oxidized to iodate which gives a well defined cathodic wave. 

Antigen, hapten Antibody Conformation change, weight change Electrode, FET (indirect also by indicator reaction), piezoelectric sensor... 

The final example is concerned with standard Seebeck coefficients of glass melts, which were accessible after zirconia electrodes had been developed [12]. Continuously working glass melting units are characterized by nonisothermal operation, and metals, eg, platinum-type metals, contacting the melt and often short-circuited are subject to electrode reactions, eg, generation and consumption of oxygen, which can indirectly impair the production of the melters. Thermoelectric emfs of such nonisothermal cells. 

In most of these types of electrocatalytic organic electrode reactions, the electron transfer event is involved in production or maintenance of a surface layer of chemisorbed H (cathodic processes) or oxygen species (OH, O, anodic processes), which then participates in electrochemical hydrogenation or oxidation, respectively, of the organic compound often through heterogeneous reactions at the electrode surface between chemisorbed H or OH/O species and the temporarily adsorbed organic reactant. The electrochemical reaction in such cases is hence indirect. 

The generated potential via exergonic reactions in direct fuel cells is partly used to promote electrode reactions (kinetic overpotential), while in indirect fuel cells, the fuel is first processed into simpler fuels (to reduce the kinetic overpotential) via conventional catalytic reactors, or CMRs, in which temperature is used as the key operating parameter to accomplish the desired kinetics and equilibrium conversions. Among the direct fuel cells, e.g., PEMFCs use hydrogen, direct methanol fuel cell (DMFC) uses methanol, while the SOFC can operate directly on natoral gas (Figiue 15.3). However, in indirect fuel cells, a complex fuel must be suitably reformed into simpler molecules such as H2 and CO before it can be used in a fuel ceU. 

CO2 + 2H + 2e- H2C2O4 " = -0.49 V was determined indirectly. The electrode reaction is not reversible, and the rate at which carbon dioxide combines to give oxalic acid is negligibly slow. There is no known electrode system with a potential that varies in the expected way with the ratio of the activities of the reactants and products of this reaction. In spite of the absence of direct measurements, the tabulated potential is useful for computational purposes. 

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