Redox potentials, involved

An Aside Redox Potentials Involved in Photoredox Reactions... 

As the two one-electron redox potentials involved are close to 1V, many aromatic compounds can be oxidized to a radical cation. This is a common source of free radical production during intoxication with xenobiotics [31]. The large difference in redox potential between the two couples O2/H2O2 and H2O2/H2O provides a thermodynamic driving force for H2O2 dismutation catalyzed by catalase ... 

Equilibrium constants have been reported for the association of NO with NO- and the association of the resulting N202 with H+ (281). In principle it would be possible to calculate redox potentials involving N202- and HN202, but because of the current uncertainty in the electronic state of NO in these reactions such a calculation is reserved for the future. 

Experimental determinations of the redox potentials involving Tl2+ were reported in two concurrent independent publications in 1974. In one of them, by Falcinella et al., Tl2+ was generated by flash photolysis... 

This case is an excellent introduction to a key question Is it better to oxidize or to reduce an undesirable substance There is no single answer because the selection of a given treatment depends not only on the redox potentials involved, but also on costs, environmental compatibility of the products, reac-... 

Electrochemical techniques can also be used for specihc monitoring of Cr(V) in complex reaction mixtures (104) and for studying ligand-exchange reactions of Cr(V) (105), but the techniques have not been exploited in any depth. Poten-tiometric techniques are useful for determining the redox potentials involving various oxidation states of Cr (106), but are only applicable to relatively stable systems, which rules out most biologically relevant redox reactions. 

The detection of potentiometric redox potential involves the use of an inert electrode such as platinum that is measured against a suitable reference electrode. Since the measurement is carried out under zero current conditions, the rate of oxidation and reduction at the electrode must be identical. The current amplitude of... 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

The aqueous solution chemistiy of nitrous acid and nitrites has been extensively studied. Some reduction potentials involving these species are given in Table 11.4 (p. 434) and these form a useful summaiy of their redox reactions. Nitrites are quantitatively oxidized to nitrate by permanganate and this reaction is used in titrimetric analysis. Nitrites (and HNO2) are readily reduced to NO and N2O with SO2, to H2N2O2 with Sn(II), and to NH3 with H2S. Hydrazinium salts yield azides (p. 432) which can then react with further HNO2 ... 

It follows from the electrochemical mechanism of corrosion that the rates of the anodic and cathodic reactions are interdependent, and that either or both may control the rate of the corrosion reaction. It is also evident from thermodynamic considerations (Tables 1.9 and 1.10) that for a species in solution to act as an electron acceptor its redox potential must be more positive than that of the M /M equilibrium or of any other equilibrium involving an oxidised form of the metal. 

It is usual to choose a container metal for fused salts sufficiently noble for the displacement reaction (2.16) to be negligible, and the most important aspects of corrosion are, as in aqueous solutions, those which involve reducible impurities, although in a salt melt there is also the additional possibility of a reducible anion (see above). All such factors can be described as controlling the oxidising power of the melt, which can be defined in terms of a redox potential just as in aqueous solutions The redox potential is expressed by relationships of the form... 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

It is evident that the abrupt change of the potential in the neighbourhood of the equivalence point is dependent upon the standard potentials of the two oxidation-reduction systems that are involved, and therefore upon the equilibrium constant of the reaction it is independent of the concentrations unless these are extremely small. The change in redox potential for a number of typical oxidation-reduction systems is exhibited graphically in Fig. 10.15. For the MnO, Mn2+ system and others which are dependent upon the pH of the... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

During oxidation of the MoFe protein the P clusters are the first to be oxidized at about -340 mV. This redox potential was first measured (40) using Mossbauer spectroscopy and exhibited a Nemst curve consistent with a two-electron oxidation process. It is possibly low enough for this redox process to be involved in enzyme turnover (see Section V). No additional EPR signal was observed from this oxidized form at this time. However, later a weak signal near g = 12 was detected and was finally confirmed, using parallel mode EPR... 

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