Electrochemical reversibility defined

For a given adsorption constant, the observed electrochemical reversibility depends on the kinetic parameter defined as (O = Xy, or (o =. This reveals that the inherent properties of reaction (2.208) are very close to surface electrode reactions elaborated in Sect. 2.5. The quasireversible maximum is strongly pronounced, being represented by a sharp parabolic dependence of vs. m. The important feature of the maximum is its sensitivity to the adsorption constant, defined by the following equation ... 

The theory for cyclic voltammetry was developed by Nicholson and Shain [80]. The mid-peak potential of the anodic and cathodic peak potentials obtained under our experimental conditions defines an electrolyte-dependent formal electrode potential for the [Fe(CN)g] /[Fe(CN)g]" couple E°, whose meaning is close to the genuine thermodynamic, electrolyte-independent, electrode potential E° [79, 80]. For electrochemically reversible systems, the value of7i° (= ( pc- - pa)/2) remains constant upon varying the potential scan rate, while the peak potential separation provides information on the number of electrons involved in the electrochemical process (Epa - pc) = 59/n mV at 298 K [79, 80]. Another interesting relationship is provided by the variation of peak current on the potential scan rate for diffusion-controlled processes, tp becomes proportional to the square root of the potential scan rate, while in the case of reactants confined to the electrode surface, ip is proportional to V [79]. 

Redox equilibrium is not achieved in natural waters, and no single pe can usually be derived from an analytical data set including several redox couples. The direct measurement of p thus is usually not meaningful because only certain electrochemically reversible redox couples can establish the potential at an electrode (4, 35). However, p is a useful concept that indicates the direction of redox reactions and defines the predominant redox conditions. Defining pe on the basis of the more abundant redox species like Mn(II) and Fe(II) gives the possibility of predicting the equilibrium redox state of other trace elements. The presence of suitable reductants (or oxidants) that enable an expedient electron transfer is, however, essential in establishing redox equilibria between trace elements and major redox couples. Slow reaction rates will in many cases lead to nonequilibrium situations with respect to the redox state of trace elements. 

Electrochemical studies on 134 and [CoPdPt( -dppm)2(CO)3X]+ (X = I, PPh3) in DMSO show electrochemically reversible, but chemically irreversible, two-electron reductions attributed to the cationic species [Co-PdPt( -dppm)2(CO)3(L)] + (L = DMSO and PPh3) and several ill-defined oxidation steps (203). For X = CO an irreversible reduction is observed, indicating a differing electrochemical mechanism (204). 

Therefore, the ratio / = kt]/ni allows us to define a reversibility criteria for a given current-potential response once the expression of the mass transport coefficient is obtained (see Sects. 3.2.1.4 and 5.3.2). Note that electrochemical reversibility thus considered is not only defined in terms of the intrinsic characteristic of the process (i.e., the particular value of the heterogeneous rate constant and other... 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

A measure of the oxidation/reduction capability of a solution (liquid or solid) measured with an -> inert electrode. For -> electrochemically reversible systems it is defined by the - Nernst equation. For -> electrochemically irreversible systems it is a conditional measuring quantity, i.e., depending on the experimental conditions. See also -> potential, - redox potential. 

Finally, a term that should be clearly defined and one that is often used haphazardly is that of reversibility. One must make a clear distinction between electrochemical reversibility and chemical reversibility. 

Electrochemical reversibility. This term is related to the kinetics of electron transfer at the electrode surface. For a facile electron transfer, equilibrium is achieved rapidly and the system is defined as being electrochemically reversible effectively both the forward and reverse electron transfers are... 

Thus, electrochemical reversibility involving the rate of electron transfer is most conveniently defined in terms of the parameter k, often designated as k . 

At the RDE, mass transport to the electrode is varied by altering the disc rotation speed (W/Hz). The consequences of this on the current-voltage relationship, for the two reactions, are shown schematically in Fig. 1. If the E step is considered to be electrochemically reversible, then the voltam-metric wave is defined by the two parameters Elj2 (the half-wave potential) and Jim (the transport-limited current), as depicted. It is the dependence of these two quantities on the disc rotation speed which allows the deduction of the mechanism, as shown in Fig. 1. For a kinetically uncomplicated reversible electrode reaction, JLIM varies as W112 [2] and EV2 is independent of W [3]. For a CE process, at fast rotation speeds the limiting current is... 

The above treatment assumes that the measured reduction potentials are thermodynamically meaningful. Although redox potentials can be measured by a variety of electrochemical techniques, cyclic voltammetry, differential pulse polarography, and more recently, square wave voltammetry have found the greatest use because of the ability of these techniques to reveal the dynamics of the associated chemical processes, and hence access the chemical and electrochemical reversibility of the couple. Chemical and electrochemical reversibility have been defined and problems associated with the distinction between these terms have been covered in Chapter 2.15 (2.15.2.2.1), however, for the purpose of this discussion it is useful to treat these behaviors separately. 

In this section, pure MOF systems as well as composites comprising MOF-encapsulated redox-active materials or interpenetrated networks are considered (see Interpenetration and Entanglement in Coordination Polymers and Patterning Techniques for Metal-Organic Frameworks). The reversibility and stability of their electrochemical response are discussed. Stability may be defined as a retained electrochemical response on repeated cycling, whereas electrochemical reversibility is linked to the concept of fast electron transfer at the... 

Thus the tendency for an electrochemical reaction at a metal/solution interface to proceed in a given direction may be defined in terms of the relative values of the actual electrode potential E (experimentally determined and expressed with reference to the S.H.E.) and the reversible or equilibrium potential E, (calculated from E and the activities of the species involved in the equilibrium). 

For these reasons a somewhat different approach will be adopted here, and an attempt will be made to show how a corrosion reaction may be represented by a well-defined reversible electrochemical cell, although again there are a number of difficulties. Consider the corrosion of metallic zinc in a reducing acid... 

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

In the Me2dtc complex, a unique, 15-line, epr spectrum was reported (69) that was peculiar only to the tetraphenylborate salt. This suggests a V-V interaction in the lattice. Electrochemical studies on these [CpjVLJ complexes (L = dithiocarboxylato ligand) shows two well defined, polarographic, reduction waves, and, for the process at most positive potential, the reversible formation of a V(III) species was postulated (72-74). 

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