Voltammogram

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

Figure C2.10.2. Cyclic voltammogram of Cu(l 11)/10 mM HCl and in situ measured STM micrographs revealing tire bare Cu(l 1 l)surface (-1.05 V, left) and tire (V3 x A/3)R30°-Cladsorbate superstmcture (-0.6 V, right) (from [39]).

In voltammetry a time-dependent potential is applied to an electrochemical cell, and the current flowing through the cell is measured as a function of that potential. A plot of current as a function of applied potential is called a voltammogram and is the electrochemical equivalent of a spectrum in spectroscopy, providing quantitative and qualitative information about the species involved in the oxidation or reduction reaction.The earliest voltammetric technique to be introduced was polarography, which was developed by Jaroslav Heyrovsky... 

The shape of a voltammogram is determined by several experimental factors, the most important of which are how the current is measured and whether convection is included as a means of mass transport. Despite an abundance of different voltam-metric techniques, several of which are discussed in this chapter, only three shapes are common for voltammograms (figure 11.33). 

In the voltammograms in figures 11.33a and 11.33b, the current is monitored as a function of the applied potential. Alternatively, the change in current following... 

Earlier we described a voltammogram as the electrochemical equivalent of a spectrum in spectroscopy. In this section we consider how quantitative and qualitative information may be extracted from a voltammogram. Quantitative information is obtained by relating current to the concentration of analyte in the bulk solution. Qualitative information is obtained from the voltammogram by extracting the standard-state potential for the redox reaction. For simplicity we only consider voltammograms similar to that shown in Figure 11.33a. 

Determining the Standard-State Potential To extract the standard-state potential, or formal potential, for reaction 11.34 from a voltammogram, it is necessary to rewrite the Nernst equation... 

Potential-excitation signal and voltammogram for normal polarography. 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Potential-excitation signals and voltammograms for (a) normal pulse polarography, (b) differential pulse polarography, (c) staircase polarography, and (d) square-wave polarography. See text for an explanation of the symbols. Current is sampled at the time intervals indicated by the solid circles ( ). 

Potential-excitation signal and voltammogram for anodic stripping voltammetry at a hanging mercury drop electrode. 

The current during the stripping step is monitored as a function of potential, giving rise to peak-shaped voltammograms similar to that shown in Figure 11.37. The peak current is proportional to the analyte s concentration in the solution. 

Ampcromctry The final voltammetric technique to be considered is amperome-try, in which a constant potential is applied to the working electrode, and current is measured as a function of time. Since the potential is not scanned, amperometry does not lead to a voltammogram. 

Two methods are commonly used to correct for the residual current. One method is to extrapolate the total measured current when the analyte s faradaic current is zero. This is the method shown in the voltammograms included in this chapter. The advantage of this method is that it does not require any additional data. On the other hand, extrapolation assumes that changes in the residual current with potential are predictable, which often is not the case. A second, and more rigorous, approach is to obtain a voltammogram for an appropriate blank. The blank s residual current is then subtracted from the total current obtained with the sample. 

When the overlap between the voltammograms for two components prevents their independent analysis, a simultaneous analysis similar to that used in spectrophotometry may be possible. An example of this approach is outlined in Example 11.12. 

Why is it not necessary to remove dissolved O2 from the solution before recording the voltammogram ... 

Electrochemical Reversibility and Determination of m In deriving a relationship between 1/2 and the standard-state potential for a redox couple (11.41), we noted that the redox reaction must be reversible. How can we tell if a redox reaction is reversible from its voltammogram For a reversible reaction, equation 11.40 describes the voltammogram. 

The following data were obtained from the linear scan hydrodynamic voltammogram of a reversible reduction reaction... 

Because of its stability, reduction of the OLp complex is less favorable than the reduction of O. As shown in Figure 11.42, the voltammograms for OLp occur at potentials more negative than those for O. Furthermore, the shift in the voltammo-gram depends on the ligand s concentration. 

The shift in the voltammogram for a metal ion in the presence of a ligand may be used to determine both the metal-ligand complex s stoichiometry and its formation constant. To derive a relationship between the relevant variables we begin with two equations the Nernst equation for the reduction of O... 

A voltammogram for the two-electron reduction of M has a half-wave potential of —0.226 V versus the SCE. In the presence of an excess of the ligand L, the following half-wave potentials are recorded... 

A linear-potential scan hydrodynamic voltammogram for a mixture of Le + and Le + is shown in the figure, where and... 

Fig. 6. Voltammogram showing both the reversible and irreversible regions. Terms are defined in text.

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