Examples of redox couples

The table below shows the redox potentials (in volts) of a number of redox eouples used in lithium batteries, quoted versus the normal hydrogen electrode (NHE)  

Mn204 / LiMn204 Lio 5C0O2 / C0O2 Ni02/LiNi02 

17 The Normal Hydrogen Electrode (NHE) is defined in footnote 56 in Chapter 2. 

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Table 14.1 Examples of redox couples involving complexation or precipitation equilibria...

An oxidation half-reaction is a conceptual way of reporting an oxidation the electrons are never actually free. In an equation for an oxidation ha If-reaction, the electrons released always appear on the right of the arrow. Their state is not given, because they are in transit and do not have a definite physical state. The reduced and oxidized species in a half-reaction jointly form a redox couple. In this example, the redox couple consists of Zn2+ and Zn, and is denoted Zn2+/Zn. A redox couple has the form Ox/Red, where Ox is the oxidized form of the species and Red is the reduced form. 

This half-reaction, too, is conceptual the electrons are not actually free. In the equation for a reduction half-reaction, the electrons gained always appear on the left of the arrow. In this example, the redox couple is Agf/Ag. 

Metal Translocation Based on the Fe "/Fe" Couple The first example of redox-driven translocation of a metal center was based on the Fein/Fen couple and took place in ditopic ligands containing (i) a tris-hydroxamate compartment and (ii) a tris-(2,2 -bipyridine) compartment.5... 

In principle, interfacial recombination processes can be inhibited by modifying the interface. The use of t-butylpyridine in the DSSC electrolyte solution to increase its photovoltage is one example [2,97]. We wished to explore general methods for passivating interfacial recombination sites in DSSCs that might allow the use of a variety of redox couples and therefore facilitate making a viable solid-state DSSC. 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

In Table 1 are summarized representative examples of self-exchange rate constant data for a variety of different types of redox couples based on metal complexes, organometallic compounds, organics and clusters. Where available the results of temperature dependence studies are also cited. For convenience, data obtained from temperature dependence studies are presented as enthalpies and entropies of activation as calculated from the reaction rate theory expression in equation (14). 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

From this statement it is seen that the eventual practical interest of the method deals with series of redox couples that involve a too chemically unstable member for experimental E° to be deteraiined easily. An example of such a case is given by the series of alkylbenzenes [29] presented in Fig. 2. Indeed, owing to the rather small size of the n system compared with the large aromatics in Fig. 1, as well as to the considerable variations in size with the number and nature of the alkyl substituent, it seems clear that the difference in solvation energies in Eq. (25) must vary within the series. 

In many circnmstances, photovoltages are found to be almost constant for redox conples whose formal potentials span a range exceeding the band gap of sihcon as shown for example in Table 6.5. ° A similar effect has been observed in pure acetonitrile and acetonitrile-acid mixtures in which a large number of redox couples with a large span of formal potentials exhibit a similar photovoltage. The departure from the ideal situation described by Eq. (1.96) is often attributed to Fermi level pinning... 

Consecutive oxidation and reduction processes would make the metal center M shuttle back and forth, between A and B, along a determined route. The rate of the translocation process should depend on the nature of the coordinative interactions between M and receptors A and B, whether labile or inert, and on the feasibility of the stereochemical rearrangement which may accompany the metal displacement. Examples of redox-driven metal ion translocation within a two-component system have been recently investigated, and refer to the Fe(III)/Fe(II) and Cu(II)/Cu(I) couples. 

A number of redox couples simultaneously function in wetlands, making it difficult to quantitatively use thermodynamic equilibrium relationships in predicting the activities of a redox pair (Rowell, 1981). However, if any of the redox pair is sufficiently high in concentration, it is possible to predict the behavior of that redox couple (assuming that the effect of other redox couples is minimal). The Eh values are good indicators of redox couples with reversible reactions at the electrode surface. Eor example. Eh may be useful in determining the concentrations of a redox system dominated by Ee(OH)3/Ee and MnOj/Mn. But electrodes poorly respond to O2/H2O, NO3/N2, SO4 /H2S, and CO2/CH4 redox couples. Thus, caution should be exercised while interpreting the Eh values. 

Unfortimately, concentrations of redox-couples are either not calculated or are determined summarily. This concerns first of all of polyvalent metals (Fe, Mn, Cu, etc.). When summary concentrations are available, ratio of metals with different degrees of oxidation may be evaluated if Eh value of the solution is known (see example 2.4). 

It is important to know the thermodynamic characteristics of redox couples in order to understand and qualitatively draw the current-potential curves . Mentioned here are some particular cases which are frequently seen in practice. When protons or hydroxide ions take part in the redox half-reaction and when the solution is buffered, one can assume that the proton or hydroxide ion activity remains constant whatever extent the reaction has reached. This is also the case when components present in large quantities take part in the half-reaction. In this instance therefore, the Nernst law is expressed by defining an apparent standard potential for this specific medium. These types of example are frequently found in systems involving highly acidic or highly basic solutions where the pH can be considered constant. Here apparent standard potentials are therefore defined, corresponding to aqueous solutions with fixed pH. 

Corrosion, in the presence of a high concentration of oxygen but at a metal where the exchange current for oxygen reduction is low, is also enhanced by the presence in solution of redox couples with an equilibrium potential in the range 0 to +1.23 V (at pH 0). For example in the presence of ferrous ions, the cathodic reaction in the corrosion process can be a part of the catalytic cycle... 

For systems where the outer layer does not form, or where the outer layer presents little impediment to the transport of species to or from the barrier layer/outer layer (bl/ol) interface, the specific impedance in the absence of redox couples is very high ( 10 -10 Qcm for NiO on Ni, for example) but in the presence of redox couple [e.g. Fe(CN)6 ] the impedance is often low. This demonstrates that the barrier layers may be good electronic conductors but, generally, are poor ionic conductors. 

This book was initially prepared as lecture notes for an electrochemistry course which has been presented regularly in Southampton and elsewhere during the past fifteen years. The course seeks to develop an understanding of electrochemical experiments and to illustrate the applications of electrochemical methods to, for example, the study of redox couples, homogeneous chemical reactions, and surface science. In many studies, several of the techniques will be equally applicable, but there are situations where one technique has a unique advantage and hence the course also seeks to discuss the selection of method and the design of experiments to aid the solution of both chemical and technological problems. 

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