Redox Reactions in Solution

One of the major potential applications of conducting polymers is as mediators or catalysts for electrochemical sensors and electrosynthesis. 

There has therefore been much interest in the mediation of redox reactions in solution by conducting polymer-modified electrodes. 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

Rotating-disk voltammetry is the most appropriate and most commonly employed method for studying mediation. In most systems that have been studied, there has been little penetration of the substrate in solution into the polymer film. This can be demonstrated most easily if the polymer film is nonconductive at the formal potential of the substrate. Then the absence of a redox wave close to this potential for an electrode coated with a very thin film provides excellent evidence that the substrate does not penetrate the film significantly.143 For cases where the film is conductive at the formal potential of the substrate, more subtle argu- 

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. 

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Click Coached Problems for a self-study module on redox reactions in solution. 

A phenomenological model for redox reactions in solution application to aquocobalt(III) systems. 

Mediation, and redox reactions in solution, 585 Medical dosage, 371 Mercury... 

Although at first sight, the Citrate Process may not appear to be in any way related to the traditional Claus, it is in fact an H2S/SO2 redox reaction in solution with the activating bauxite, carbon, or metal salt type catalyst replaced by a citrate complex with SO2. The chemistry of the process is clearly interesting and of some importance but for the purposes of this review it is sufficient to draw the analogy indicated above. The Citrate Process is yet another reduction process that may require the ancillary generation of H2S from natural gas and product sulphur if the effluent gas stream is solely SO2 as far as sulphur content is concerned. 

Weiss published a paper in 1954 giving a number of modeling ideas concerning the molecular-level mechanism of redox reactions in solution. In 1956 Marcus published (independently) a paper containing similar ideas, but also applied them to electrode reactions. From these works there followed an equation ... 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

Theoretical treatments of charge transfer at electrodes were developed by Gurney, Horiuti, and Eyring and the more recent work of Gerischer, Marcus, Hush, and Levich, among others, permitted the study of simple redox electrode reactions under the same theoretical framework developed for homogeneous redox reactions in solution. 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

Redox electrode reactions on metal electrodes constitute the simpler case for a theoretical approach to the problem. In particular, outer sphere redox electrode reactions not involving specific adsorption interactions have been treated successfully in analogy with homogeneous redox reactions in solution [54, 56], Approximate extension of the theoretical approach to the case of inner sphere redox reactions at electrodes has been done [56, 57b]. 

The Marcus treatment for heterogeneous electron transfer at electrodes is analogous to the homogeneous case of redox reactions in solution discussed in Vol. 2, Chap. 4. The free energy of activation can also be expressed by... 

The dye radical formed by reduction of the dye molecule would have an additional electron, would not have the same electronic configuration, and possibly not the same geometric configuration compared to the excited dye molecule. Moreover, the electrochemical measurements contain contributions from solvation energy differences between the parent dye and its reduced or oxidized radicals (43). These contributions do not appear in the dye s optical transition energy. In addition, many cyanine dyes undergo irreversible redox reactions in solution and the potentials, as commonly measured, are not strictly reversible. Nevertheless, Loutfy and Sharp (260) showed that the absorption maxima of more than 50 sensitizing dyes in solution conformed approximately to the equation... 

Another approach has been to use a photogalvanic cell with a rotating optically transparent Sn02 electrode. Light is used to drive a redox reaction in solution, e.g. equation (48) 42... 

The neutralisation of acids with bases provides many valuable volumetric methods of chemical analysis and redox titrations are useful as well. But here we encounter an important difference between acid/base and redox reactions in solution. Acid/base reactions which involve the transfer of protons are very fast indeed they are usually instantaneous for all practical purposes. In protonic solvents, polar H-X bonds are very labile and undergo rapid proton exchange. For example, if B(OH)3 - a very weak acid - is recrystallised from D20, we obtain a fully-deuterated product. Redox reactions, on the other hand, are often very slow under ordinary conditions. To return to the analogy between acid/base and redox titrations, many readers will be familiar with the reaction between permanganate and oxalic acid the reaction is very slow at room temperature and, for titrimetric purposes, should be carried out at about 60 °C. The mechanism whereby a redox reaction takes place tends to be... 

The mechanism described above is also correct for electron transfer in homogeneous solution except that, instead of the reaction site being an electrode, it is the point where the two ions meet in the interior of the solution. In the equations for energy changes a factor of 2 relative to electrode reactions appears, since whole reactions rather than halfreactions are being considered. Theoretical and experimental comparisons between electrode reactions and redox reactions in solution have been made with satisfactory results3. 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

J. Blumberger, Y. Tateyama, and M. Sprik (2005) Ab initio molecular dynamics simulation of redox reactions in solution. Comp. Pys. Comm. 169, p. 256... 

In the case of a single-step reaction such as the reduction of Fe to Fe " (in the absence of diffusion control), no assumptions are required about a rate-determining step in the usual sense (although microscopically, for such redox reactions in solution, consideration can be given to solvent reorganization in the formation of the transition state associated with electron transfer). Correspondingly, no intermediate (except the transition state itself ) need be considered in the reaction mechanism scheme. 

On the other hand, the cathodic oxidant reduction is an electron transfer process across the metal-solution interface, and electrons transfer from the Fermi level of the metal to the Fermi level of the redox reaction in solution, involving the reorganization of the hydrated structure of the redox particles. 

In indirect electrochemical reactions, the heterogeneous reaction between the substrate and the electrode is replaced by a homogeneous redox reaction in solution between the substrate and an electrochemicaUy activated and regenerated redox catalyst (Fig. 9-4). 

Equations for redox reactions in solution are best balanced via the appropriate half equations. These are combined in such away as to eliminate the electrons in them. For example, for the dissolution of aluminium in acid, the half equations are... 

Bockris, Khan, and Matthews pointed out that if one plots the value of calculated on the solvent fluctuation theory for a number of redox reactions in solution, against the corresponding value, calculated from experiment, with the transmission coefficient assumed to be unity, one obtains no correlation, except that the values theoretically predicted are generally below those observed experimentally. 

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