Irreversible electrode reaction study

In the case of both reversible and irreversible electrode reactions, methods are now available for studying the steady-state and transient currents, and there has been much progress in the analysis of these currents in terms of the kinetic processes involved. 

Certain advantages arise in the use of electrochemical techniques compared to chemical redox titrations in the determination of biological molecule thermodynamic parameters. Electrochemical techniques which couple the principle of mediation between electrodes and biological molecules to overcome irreversible electrode reactions with optical monitoring of the redox state of the sample have recently been developed. The following two sections address the principles and applications of these techniques in the study of biological molecule thermodynamics. 

As with U and Np ions, flow coulometry experiments were conducted to further study the electrode reactions of Pu ions in acidic aqueous solutions [49]. The results from these studies confirm the reversibility of the one-electron couples Pu02 " /Pu02 and Pu" /Pu " " in nitric, perchloric, and sulfuric acid solutions. The further reduction of Pu02 resulted in an irreversible two-electron transfer yielding Pu +. The flow coulometry results in the mixed phosphoric-nitric acid solutions confirm the overall conclusions that have been reached from the stationary and rotated working electrode experiments described previously, in which PuO + is the primary Pu(IV) product from Pu02 reduction. 

However, the peak current in AC polarography markedly depends on the reversibility of the electrode process, being very small for an irreversible process. We can apply this dependence to study the kinetics of the electrode reactions. 

Reversible, quasi-reversible and irreversible electrode processes have been studied at the RDE [266] as have coupled homogeneous reactions without [267] and with the effect of electrode kinetics [268], The theoretical results are very similar to those of a.c. polarography, being very phase-angle sensitive to coupled chemical reactions in the rotation speed range where convection can be neglected, the polarographic results may be directly applied [269]. 

As shown in Chap. 2, attaining analytical explicit solutions is considerably more complex for nonplanar geometries. This section studies quasi-reversible and irreversible processes when a potential step is applied to a spherical electrode, since this solution will be very useful for discussing the behavior of these electrode reactions when steady-state conditions are addressed in the next section. Moreover, the treatment of other electrode geometries seldom leads to explicit analytical solutions and it is necessary in most cases to use numerical treatments. 

The theoretical study of other electrode processes as a reduction followed by a dimerization of the reduced form or a second-order catalytic mechanism (when the concentration of species Z in scheme (3.IXa, 3.IXb) is not too high) requires the direct use of numerical procedures to obtain their voltammetric responses, although approximate solutions for a second-order catalytic mechanism have been given [83-85]. An approximate analytical expression for the normalized limiting current of this last mechanism with an irreversible chemical reaction is obtained in reference [86] for spherical microelectrodes, and is given by... 

Studies of the electrode reactions of aqueous [Pd(CN)4]2 by d.c. polarography have shown the presence of an irreversible two-electron step, while the osdllopolaro-gram showed three anodic waves.106 It was concluded that cyano-complexes of Pd° and Pd1 were formed, the former of which decomposed rapidly into Pd metal (amalgam) and free cyanide. 

Mechanistic studies can employ CPE if the coupled chemical reactions are slow. Conventional bulk electrolyses require typically 10-30 min for completion, longer than the typical longest time for voltammetric techniques (ca. 20 s maximum for cyclic voltammetry, CV, ca. 8 s for polarography, etc.). This is important to recall when comparing CPE with voltammetry data. An electrode reaction that is chemically reversible in a slow CV experiment may be irreversible in bulk electrolysis if the electrode product has a half-life of, e.g., a minute or two. Conversely, an electron transfer that is quasi- or irreversible in a relatively fast voltammetric experiment may be electrochemically reversible in the long timescale of bulk electrolysis. 

This chapter focuses on the fundamental aspects of electrode reactions and how modern electrochemical techniques can be employed to study them. Examples from inorganic or organometallic electrochemistry are illustrative of some of the problems and opportunities inherent in this area. In using electrochemical techniques irreversible waves are no less interesting than reversible ones, and in fact, there may be more chemistry going on in the former. Careful mechanistic work will elucidate these more complex electrochemical processes. 

Theory (SIT) for more accurate consideration of activity coefficients [5, 49, 50b]. Kihara et al. [49] calculated values of 0.956 0.010 V for PiiOr+/Pu()2+ and 1.026 0.010 V for Pu" +/Pu + from the cyclic voltammetry data of Riglet et al. [50] in perchlorate solutions. In an earlier study Capdevila and Vitorge determined of0.938 0.010 V for PuO2 +/PuO2+ and 1.044 0.010 V for Pu +/I u + also from voltammetric data in perchlorate solutions [116]. Because of the irreversibility of the PuO22" /Pu " electrode reaction limited formal potential data exist in the literature for calculation of the standard potential. However, a value of0.867 V has been calculated from the existing data and appropriate correction factors [49],... 

As already mentioned before, mainly irreversible reactions with organic compounds have been investigated at semiconductor particles. When organic molecules, for example alcohols, are oxidized by hole transfer, O2 usually acts as an electron acceptor or in the case of platinized particles, protons or H2O are reduced. A whole sequence of reaction steps can occur, which are frequently difficult to analyze because cross-reactions may also be possible at particles and a new product could be formed. Concerning the primary electron and hole transfer, certainly there should be no difference between particles and compact electrodes. Since sites at which reduction and oxidation occur are adjacent at a particle, the final product may be different. An interesting example is the photo-Kolbe reaction, studied for Ti02 electrodes and for Pt-loaded particles. Ethane at extended electrodes and methane at Pt/Ti02 particles have been found as reaction products upon photo-oxidation of acetic acid [56, 57]. The mechanism was explained by Kraeutler et al. as follows. 

In view of the numerous oxidation states, an extensive oxidation-reduction chemistry of technetium is expected. Polarographic reductions of pcrtechnetate in aqueous and in non-aqueous solutions, supplemented by coulometric and cyclic voltammetric measurements, were conducted to study the electrochemical behavior of technetium, to identify some oxidation stales and to synthesize new technetium compounds. Electrode reactions frequently proved to be irreversible and therefore not adequate for calculating thermodynamic data. The electrochemistry of technetium is reported in detail in several review articles [11-13]. 

There have been few direct electrochemical studies of peroxidase and catalase due to the highly irreversible nature of these electrode reactions. Horseradish peroxidase was found to be electroinactive at the dropping mercury electrode. Tarasevich and co-workers observed a cyclic voltam-metric response for the electron transfer of horseradish peroxidase at an amalgamated gold electrode. However, this response was ascribed to the disulfide bonds of the protein at neutral pH and not to the heme group. No response was detected at pyrolytic graphite electrodes. ... 

In electrochemical systems, on the other hand, electrodes act as both electron acceptors and electron donors, and are considered a simple model system for mimicking a charged interface of the physiological binding domain. The heterogeneous ET reactions between electrodes and various ET proteins in solutions have been extensively studied, as described in previous chapters. The electrode reactions of cytochrome c at mercury, platinum, silver, and gold electrodes have been reported to be irreversible. On the other hand, the electrode reactions of cytochromes C3 (cyt. C3) have... 

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