Voltammogram characteristics

The measurement of the current for a redox process as a function of an applied potential yields a voltammogram characteristic of the analyte of interest. The particular features, such as peak potentials, halfwave potentials, relative peak/wave height of a voltammogram give qualitative information about the analyte electrochemistry within the sample being studied, whilst quantitative data can also be determined. There is a wealth of voltammetric techniques, which are linked to the form of potential program and mode of current measurement adopted. Potential-step and potential-sweep... 

At increased scan rates, a transition is observed from the thin layer regime, where the peak current varies linearly with scan rate, to semi-infinite diffusion control, where a v dependence is observed. Under semi-infinite diffusion conditions, voltammograms are characteristic of those obtained for solution species. The general voltammogram characteristics are... 

For an array of isolated disc ultramicroelectrodes the diffusion-concentration profiles develop in time in a characteristic way [43]. At very short time interval, the diffusion is linear. During the extended time of electrolysis the contribution of the radial diffusion is increased and a sigmoidal voltammogram characteristic for the steady state appears. Finally, after long-time experiment the individual diffusion layers overlap and linear diffusion becomes dominant again. 

These studies can successfully be done by utilizing methods of electrochemistry, more specifically, cyclic voltammetry [12]. Following a few potential cycles, voltammograms characteristic of the steady state and of the total electrochemical activity take shape. These are indicators of the saturation concentration of dissolved molecules within the film, representing the result of wetting, hydratation, swelling, and liosorption - to be summarized as break-in. 

The electrochemical properties of the meta/electrolyte interface follow a similar pattern, but involve some complication. Thus, the value of E is found to be different for different crystal faces, and so are the cyclic voltammograms characteristic for UPD formation, as shown in Chapter 12. It has been suggested that E and 0 should be linearly related to each other, and indeed such correlations have been reported in the literature. However, while is a characteristic property of each... 

In view of the limitations referred to above, and particularly the influence of electrode characteristics upon the peaks in the voltammogram, some care must be exercised in setting up an apparatus for stripping voltammetry. The optimum conditions require ... 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

When cyclic voltammograms of an electrode partially or completely covered with an adsorbate in contact with an electrolyte solution are recorded, various characteristic features of the obtained voltammogram can be used to deduce the amount of adsorbed material. This procedure can be repeated at various concentrations of the species to be adsorbed in the solution. From the obtained relationship between... 

Voltammograms with characteristic current maxima are obtained (see Fig. 12.9) when linear potential scans (LPS) which are not particularly slow are apphed to an electrode. The potentials at which a maximum occurs depend on the nature of the reactant, while the associated current depends on its concentration. When several reactants are present in the solution, several maxima will appear in a curve. 

Figure 14. Cyclic voltammograms of /<2c-Re(bpy)(CO)3Cl in acetonitrile-0.1 M Bu4NPF6 at a Pt electrode.144 Scan rate 0.2 V/s. The lower voltammograms show the switching potential characteristics A and F, reversible one-electron wave B and D, redox couple due to a dimer of the complex C, the second metal-based wave. The upper curves show the effect of C02 on the voltammogram. See also Figure 15.

Figure 3.55(b) shows the first sweep of the voltammogram obtained in the presence of C02 and also in Ar saturated solution for comparison. The authors concluded that the reductive scan showed that little or no catalytic enhancement occurs at the Bipy-based reduction potential and that the onset of the catalytic current occurs at -1.4V continuing through the region characteristic of the metal-based reduction process (and the 2e reduction of the dimer) in direct contrast to the results obtained by the majority of other workers (see below). 

The cyclic voltammograms in Figure 3.73 show an interesting characteristic in that, at potentials greater than the anodic peak potential, the current tends towards a constant value. The charge passed, Q, in charging a capacitor C to a potential V is (see section 2.1.1.) ... 

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