Electrode Processes irreversible

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

Parry have described procedures to determine the kinetic parameters of totally irreversible electrode processes using various polarographic techniques [6801d]. (Data obtained with these methods are collectively labelled POL.)... 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

If sh 5 10 8 cm s 1 in dc polarography we arrive at a totally irreversible electrode process, where the backward reaction can be neglected we shall treat such a situation for a reduction process as the forward reaction. 

As ksh in this instance is very small, then according to the Butler-Volmer formulation (eqn. 3.5) the reaction rate of the forward reaction, K — 8,he "F(E 0)/flr, even at E = E°, is also very low. Hence Etppl. must be appreciably more negative to reach the half-wave situation than for a reversible electrode process. Therefore, in the case of irreversibility, the polarographic curve is not only shifted to a more negative potential, but also the value of its slope is considerably less than in the case of reversibility (see Fig. 3.21). In... 

Again returning to the diffusion-controlled limiting current, we often meet a considerable influence on its height by catalysis, adsorption or other surface phenomena, so that we have to deal with irreversible electrode processes. For instance, when to a polarographic system with a diffusion-controlled limiting... 

Without going into further detail, we have shown above the great importance of distinguishing between reversible and irreversible electrode processes in order to understand the theory of polarography and its implications. 

The above considerations concern a reversible electrodic process, ox + ne red as instead of 20-100 hz in the sinusoidal technique a fixed frequency of 225 Hz is normally used in the square-wave mode, the chance of irreversibility in the latter becomes greater, which then appears as asymmetry of the bellshaped I curve. Such a phenomenon may occur more especially when the complete i versus E curve is recorded on a single drop, a technique which has appeared useful51 in cases of sufficient reversibility. 

This is the Tafel equation (5.2.32) or (5.2.36) for the rate of an irreversible electrode reaction in the absence of transport processes. Clearly, transport to and from the electrode has no effect on the rate of the overall process and on the current density. Under these conditions, the current density is termed the kinetic current density as it is controlled by the kinetics of the electrode process alone. 

For an irreversible reduction the half-wave potential is determined not only by the standard electrode potential but also by the polarographic overvoltage. For a simple electrode process the metal ion-solvent interaction is mainly responsible for the polarographic overvoltage and hence E[ j of such irreversible reductions may also be considered as a function of the solvation 119f... 

Diagnostic Criteria to Identify an Irreversible Process. In order to characterize an irreversible process it would be necessary to be able to calculate either the thermodynamic parameter E° or the kinetic parameters a. and k°. Unfortunately, we will see below that k° can only be calculated if E0/ is known, and E0 cannot be calculated by voltammetric techniques. Thus, either one knows Eel (for example, by using potentiometric techniques in solutions containing both Ox and Red), or one is limited to give simply the peak potential of the electrode process at a certain rate (usually at 0.1 V s-1 or at 0.2 V s-1). 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V)... 

Radiopolarography measurements for the cathodic reduction of Bk(III) to Bk(0) at a dropping mercury electrode in 0.1 M LiCl at pH 2 give an amalgamation halfwave potential value of 1.63 V versus SHE and an estimated of 2.18 V [169]. Analysis of the electrochemical data leads the authors to conclude that the Bk(III)/Bk(0) electrode process is irreversible. 

The voltammetric data and other relevant kinetic and thermodynamic information are summarized in Table 2. While for X = H the initial ET controls the electrode rate, as indicated by the rather large p shift and peak width, the electrode process is, at low scan rates, under mixed ET-bond cleavage kinetic control (see Section 2) for X = Ph, and CN. Although the voltammetric reduction of these ethers is irreversible, in the case of the COMe derivative, some reversibility starts to show up at 500 Vs in fact, this reduction features a classical case of Nernstian ET followed by a first-order reaction. The reduction of the nitro derivative is reversible even at very low scan rate although, on a much longer timescale, this radical anion also decays. 

Electroanalytical chemists do deal with what they call irreversible reactions because they exist at and interfere with a transport-oriented approach. But the focus of interest in electroanalytical chemistry (rather reasonably), is on the usefulness of electrode processes to analysis and in this case one should aim for an electrode showing the highest ig and hence the least T] for a given current density. 

On the other hand, if the exchange current is very small and a large overpotential is needed for the current to flow, the electrode process is said to be irreversible. In some cases, the electrode process is reversible (the overpotential is small) for small current values but irreversible (the overpotential is large) for large current values. Such a process is said to be quasi-reversible. 

Curves 4 and 4 in Fig. 5.6 show an example of the current-potential relation obtained for an irreversible electrode process. For a reversible electrode process, the reduction wave appears at the same potential as the oxidation wave, giving an oxidation-reduction wave if both Ox and Red exist in the solution (curves 1, 2 and 3 in Fig. 5.6). For an irreversible process, however, the reduction wave (curve 4) is clearly separated from the oxidation wave (curve 4 ), although the limiting currents for the two waves are the same as those in the reversible process. The cur-rent-potential relation for the irreversible reduction process can be expressed by... 

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]. 

Ihe actual from the thermodynamic electrode potential. The polarization is Ihe result of the irreversibility of the electrode process, that is, the activation polarization and the voltage loss, which develops from concentration gradients nf the reactants. This leads to the current-voltage characteristics as shown in Fig. 2,... 

A reversible criterion will be presented in order to clearly establish the experimental conditions for which a charge transfer process can be considered as reversible, quasi-reversible, or fully irreversible. Note that this criterion can be easily extended to any electrochemical technique. This section also analyzes the response of non-reversible electrode processes at microelectrodes, which does not depend on the electrochemical technique employed, as stated in Chap. 2. 

It is important to highlight that the current for a quasi-reversible or fully irreversible electrode process cannot be expressed as a product of functions depending on the different variables, i.e., applied potential and time (or k ) Jt/Do). Unfortunately, the dimensionless parameter % is influenced by time, by the kinetic constants and by the applied potential. 

It is also worth pointing out that a similar result to that shown in Fig. 3.3 is obtained if we analyze the effect of the time in the response of a quasi-reversible charge transfer process, i.e., for a given value of the rate constant k°, a decrease of the time leads to a decrease of the dimensionless rate constant Kplane and therefore to a higher irreversible character of the process. This fact can be used to ascertain at a glance if a particular electrode process behaves in a reversible or non-reversible way, since in the first case no influence of time on the normalized current is observed (see Eq. (2.36)). 

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