Electrochemical redox reactions

Chemistry and Mechanism of Development. Development has been successfully viewed as an electrochemical redox reaction (275—280), for which the overall reaction can be expressed as follows ... 

Ox and Red are general symbols for oxidation and reduction media respectively, and n and (n-z) indicate their numerical charge (see Section 2.2.2). Where there is no electrochemical redox reaction [Eq. (2-9)], the corrosion rate according to Eq. (2-4) is zero because of Eq. (2-8). This is roughly the case with passive metals whose surface films are electrical insulators (e.g., A1 and Ti). Equation (2-8) does not take into account the possibility of electrons being diverted through a conductor. In this case the equilibrium... 

Table 2-3 Conversion factors and standard potentials for electrochemical redox reactions...

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

The elementary act of an electrochemical redox reaction is the transition of an electron from the electrode to the electrolyte or conversely. Snch transitions obey the Franck-Condon principle, which says that the electron transition probability is highest when the energies of the electron in the initial and final states are identical. 

It follows from the Franck-Condon principle that in electrochemical redox reactions at metal electrodes, practically only the electrons residing at the highest occupied level of the metal s valence band are involved (i.e., the electrons at the Fermi level). At semiconductor electrodes, the electrons from the bottom of the condnc-tion band or holes from the top of the valence band are involved in the reactions. Under equilibrium conditions, the electrochemical potential of these carriers is eqnal to the electrochemical potential of the electrons in the solution. Hence, mntnal exchange of electrons (an exchange cnrrent) is realized between levels having the same energies. 

FIGURE 35.2 Scheme of diabatic (solid line) and adiabatic (dashed line) free-energy curves for a simple electrochemical redox reaction Ox —> Red. 

Figure 1. A plot of free energy of activation, AG (continuum) vs. AG (expt) for electrochemical redox reactions. (Reprinted from Ref 26.)...

Further information on this subject can be obtained by frequency response analysis and this technique has proved to be very valuable for studying the kinetics of polymer electrodes. Initially, it has been shown that the overall impedance response of polymer electrodes generally resembles that of intercalation electrodes, such as TiS2 and WO3 (Ho, Raistrick and Huggins, 1980 Naoi, Ueyama, Osaka and Smyrl, 1990). On the other hand this was to be expected since polymer and intercalation electrodes both undergo somewhat similar electrochemical redox reactions, which include the diffusion of ions in the bulk of the host structures. One aspect of this conclusion is that the impedance response of polymer electrodes may be interpreted on the basis of electrical circuits which are representative of the intercalation electrodes, such as the Randles circuit illustrated in Fig. 9.13. The figure also illustrates the idealised response of this circuit in the complex impedance jZ"-Z ) plane. 

By its chemical nature, the very thin dry layer of glass permits ion exchange without any possibility of electrochemical redox reactions. If the membrane is a sufficiently thin layer, one can achieve millisecond response times to changes in pH, and these properties can be put to full advantage with rapid mixing devices to study chemical reactions with half-lives in the 5-10 msec range. 

The ion formation may occur in the bulk solution before the electrospray process takes place or in the gas phase by protonation or salt adduct formation, or by an electrochemical redox reaction. Polar compounds already exist in solution as ions therefore, the task of the electrospray is to separate them from their counterions. This is the case of many inorganic and organic species and all those compounds that show acidic or basic properties. Proteins, peptides, nucleotides, and many other bio- and pharmaceutical analytes are typical examples of substances that can be detected as proto-nated or deprotonated species. 

Cobalt(II) hexacyanoferrate, formally similar to Prussian blue, exhibits a far more complex electrochemistry. Only recently, Lezna etal. [65] succeeded in elucidating this system by a combination of in situ infrared spectroscopy and electrochemistry, and ex situ X-ray photoelectron spectroscopy. Figure 8 shows the pathways of the three different phases involved in the electrochemistry, and their interconversion by electrochemical redox reactions and photochemical reactions. 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

If a solution, being in contact with an electrode, contains photosensitive atoms or molecules, irradiation of such a system may lead to photoelectro-chemical reactions or, to be more exact, electrochemical reactions with excited particles involved. In such reactions the electrons pass either from an excited particle to the electrode (the anodic process) or from the electrode to an excited particle (the cathodic process). In this case, an elementary act of charge transfer has much in common with ordinary (dark) electrochemical redox reactions, which opens a possibility of interpreting certain aspects of photochemical processes under consideration with the use of concepts developed for general quantum mechanical description of electrode processes. 

Solution studies on the adducts formed by various heterocyclic bases with some nickel porphyrins have been reported.2899-2902 From these studies one can conclude that pyridine and substituted pyridines form predominantly 1 1 adducts while piperidine, imidazole and substituted imidazoles also form 1 2 complexes or a mixture of both 1 1 and 1 2 complexes. Electrochemical redox reactions of nickel porphyrins have been investigated.2903,2904... 

From the retrosynthetic point of view, electrochemical redox reactions are an easy way to accomplish the principle of redox-umpolung (polarity reversal) [2]. As can be seen in Scheme 22.1, oxidation of an electron-rich neutral compound will lead to an electrophilic cation radical, or starting with a nucleophilic anion, anodic oxidation may lead to an electrophilic cation. In the mirror image reductions, an electron-poor neutral compound is transformed to a nucleophilic anion radical, or an electrophilic cation will end up as a nucleophilic anion. 

The band bending at the semiconductor/liquid (electrolyte solution) interface can be understood by considering the potential distribution at this interface. In a case where the electrolyte solution contains a redox couple (R/Ox), which causes an electrochemical redox reaction,... 

Electrochemical interconversion of homo- and heteronuclear gold cluster compounds remains an area that has received scant attention, despite the potential for changing the electron count and hence the metal cage geometries of these clusters by electrochemical methods. The electrochemical redox reactions of [Pt(AuPPh3)8]2+ have been studied, using pulse, differential pulse, and cyclic voltammetric techniques (124, 242) and two reversible, one-electron reduction steps have been... 

A tetrahydrocorphin (126) containing the chromophore system of coenzyme F-430 was synthesized using electrochemical redox reactions (Scheme 15) (84CC1365). Attention is drawn to the original paper for mechanistic details. 

Electrochemical equivalent — Amount of a chemical substance deposited, dissolved, or transformed in an electrochemical redox reaction with exchange of one unit of electric charge. In the SI unit system the electrochemical-equivalent unit is in kgC-1, or alternatively, in molC-1. It means that in a n-electron redox reaction (Ox + ne Red) the electrochemical equivalent is equal to the molar mass M of the reacting compound divided by n times the - Faraday constant (M/nF). In some sources the electrochemical equivalent is defined as the mass of the substance transformed by electric charge corresponding to the Faraday constant. 

Other redox reactions are called electrochemical redox reactions because they either consume or produce electricity. Examples of this type are the following ... 

Electrochemical redox reactions have been reported for triazenide complexes of iron 214), cobalt 214), and rhodium 43). 

---