Cyclic voltammetry irreversible

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Electrochemically active compounds can be evaluated using a potentiometer to generate a cyclic voltammogram for the analyte. Cyclic voltammetry will allow the analyst to determine whether the compound can be oxidized or reduced, to choose the appropriate potential to use in the electrochemical detector, and to establish whether oxidation or reduction is irreversible. Irreversible oxidation or reduction of the analyte could be predictive of problems with electrode poisoning and reduced sensitivity of the electrochemical detector over time. Turberg et al. used EC detection at an applied potential of -1-600 mV to analyze for ractopamine. 

The redox chemistry of several phosphaferrocenes,31,50 l,l -diarsaferro-cene (7),13 the complete series of 2,2, 5,5 -tetramethyl-l,r-diheteroferro-cenes (89, 26, 29, 32),22 and octamethyl-1,1 -diheteroferrocenes (90, 44, 48, 49)22 has been investigated by cyclic voltammetry. These compounds undergo quasi-reversible one-electron oxidations (0/+) to their radical cations and irreversible one-electron reductions (0/-) to their radical anions. The data are summarized in Table VI. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

Since the values of for many organic acceptors are generally unobtainable (in organic solvents), an alternative measure of the electron-acceptor property is often based on the irreversible cathodic peak potential F (in cyclic voltammetry). Thus for a series of related compounds, Fig. 6 shows that the values of Fred are linearly related to gas-phase electron affinities (EA).70... 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Ag+/Ag, in the range of the oxidation potential of 3-methylthiophene, (14.) due to the irreversible oxidation of the monomeric thiophene unit. Figure 1 shows the typical cyclic voltammetry upon repeatedly scanning the potential of a Pt electrode between 0.0 V and 1.5 V vs. Ag+/Ag in a solution of 0.2 M 1. 

The cyclic voltammetry of polysilanes adsorbed on the electrode surface has also been investigated [65]. The oxidation potentials depend upon the nature of the organic groups on silicon. The electrochemical oxidation is irreversible to give soluble products which are liberated from the surface of the anode. 

The redox characteristics, using linear sweep and cyclic voltammetry, of a series of (Z)-6-arylidene-2-phenyl-2,3-dihydrothiazolo[2,3-r][l,2,4]triazol-5(6//)-ones 155 (Figure 24) have been investigated in different dry solvents (acetonitrile, 1,2-dichloroethane, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO)) at platinum and gold electrodes. It was concluded that these compounds lose one electron forming the radical cation, which loses a proton to form the radical. The radical dimerizes to yield the bis-compound which is still electroactive and undergoes further oxidation in one irreversible two-electron process to form the diradical dication on the newly formed C-C bond <2001MI3>. 

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

FIGURE 1.17. Cyclic voltammetry of slow electron transfer involving immobilized reactants and obeying a Butler Volmer law. Normalized current-potential curves as a function of the kinetic parameter (the number on each curve is the value of log A ) for a. — 0.5. Insert irreversible dimensionless response (applies whatever the value of a). 

FIGURE 2.5. EC reaction scheme in cyclic voltammetry. Mixed kinetic control by an electron transfer obeying the Butler-Volmer law (with a = 0.5) and an irreversible follow-up reaction, a Variation of the peak potential with the scan rate, b Variation of the peak width with scan rate. Dots represent examples of experimental data points obtained over a six-order-of-magnitude variation of the scan rate. 

In cyclic voltammetry, the current-potential curves are completely irreversible whatever the scan rate, since the electron transfer/bond-breaking reaction is itself totally irreversible. In most cases, dissociative electron transfers are followed by immediate reduction of R, as discussed in Section 2.6, giving rise to a two-electron stoichiometry. The rate-determining step remains the first dissociative electron transfer, which allows one to derive its kinetic characteristics from the cyclic voltammetric response, ignoring the second transfer step aside from the doubling of the current. 

Although cyclic voltammetry could fruitfully be applied to the kinetic analysis of these catalytic systems, it has mostly been investigated by means of rotating disk electrode voltammetry (Section 1.3.2). The simplest case is that of an irreversible catalytic reaction at a monolayer coating. The next section is devoted to the analysis of these systems by the two techniques. 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

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