Square-wave voltammetry peak potential

Square-wave voltammetry The potential-time waveform and current measuring scheme for this technique is shown in Fig. 10. The waveform consists of a symmetrical square-wave (peak to peak amplitude 2Es ) superimposed on a staircase wave of step height AE and a period t. The response current is sampled at the end of both the forward (If) and reverse (If) half cycle. A difference current dl is determined as... 

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

Table 2.1 Square-wave voltammetry of fast and reversible electrode reaction (1.1). The dimensionless net peak current, the ratio of peak currents of the forward and backward components, the peak potentials of the components and the half-peak width as functions of SW amplitude ...

Fig. 3.9 Square-wave voltammetry of simvastatin microparticles the dependence of peak potential on the logarithm of frequency. All other data are as in Fig. 3.8 (reprinted from [188] with permission)...

Nevertheless, the mid-peak potentials determined by cyclic voltammetry and other characteristic potentials obtained by different electroanalytical techniques (such as pulse, alternating current, or square wave voltammetries) supply valuable information on the behavior of the redox systems. In fact, for the majority of redox reactions, especially for the novel systems, we have only these values. (The cyclic voltammetry almost entirely replaced the polarography which has been used for six decades from 1920. However, the abundant data, especially the half-wave potentials, 1/2, are still very useful sources for providing information on the redox properties of different systems.)... 

Typical Tafel plots for different copper materials are shown in Fig. 3.11. In all cases, an excellent linearity was obtained for n i/ip) on E representations in terms of the correlation coefficient for linear fitting. Similar results were obtained for binary or ternary mixtures of such materials where highly overlapping peaks were recorded, both using linear potential scan and square-wave voltammetries. 

TABLE 8.5 Square Wave Voltammetry or Differential Pulse Voltammetry Peak Potentials (in V Versus Fc + /Fc) of Multimetallofullerenes and Metal Carbide Fullerenes... 

Eletrochemical detection has been used for the detection of synthetic dyes. Fogg et al. (226) described a method for the qualitative and quantitative determination of several synthetic dyes using polarographic detection. The system was a stationary mercury drop electrode operated in the differential-pulse mode. Ashkenazi et al. (131) used fast-scan square-wave voltammetry for the polarographic detection of five synthetic dyes. The voltametric mode was observed to be much faster than the differential-pulse method. Another advantage is that the experimental measurement produces, in addition to the peak current, the redox potential of the dye, which can serve to identify the analyte further. 

In the case of Cyclic Square Wave Voltammetry (CSWV), the SWV curve obtained in the second scan is a mirror image to that of the first scan whatever the electrode geometry if the diffusion coefficients of species O and R are assumed as equal. In the contrary case, although the peak potentials of both scans are coincident, differences in the peak heights are observed for nonplanar electrodes. 

As in the case of differential double potential pulse techniques like DDPV, slow electrochemical reactions lead to a decrease in the peak height and a broadening of the response of differential multipulse and square wave voltammetries as compared with the response obtained for a Nemstian process. Moreover, the peak potential depends on the rate constant and is typically shifted toward more negative potentials (when a reduction is considered) as the rate constant or the pulse length decreases. SWV is the most interesting technique for the analysis of non-reversible electrochemical reactions since it presents unique features which allow us to characterize the process (see below). Hereinafter, unless expressly stated, a Butler-Volmer potential dependence is assumed for the rate constants (see Sect. 1.7.1). 

Applications of pulse techniques in electrochemistry have been predominantly in the area of analysis, relying on the linear dependence of peak height on potential, although recently their use in mechanistic studies, particularly square-wave voltammetry, has begun to be exploited. The reason for their use in analysis is intimately linked with the low detection limits that are attainable, particularly in combination with pre-concentration techniques, as will be seen in Chapter 14. Finally, since nowadays the pulse sequences are generally controlled and responses analysed using microprocessors, the development of new waveforms for particular situations is now a much easier task than it was even a decade ago. 

Domenech-Carbo et al. also showed the voltammetry of immobilized microparticles to be valuable in the unambiguous identification of dyes such as Curcuma and Safflower in microsamples of works of art and archaeological artifacts (see also Section 6.4.1) [140]. Here, the use of square-wave voltammetry in aqueous acetate or phosphate buffers led to the appearance of well-defined oxidation peaks ofthe dyes in the potential region of +0.65 to +0.25 V (versus Ag AgCl). 

An attempt to follow by direct electrochemistry the red-ox reactions involving the cofk tors of the RC embedded in lipid films on pyrolytic graphite electrodes has been recendy carried out, allovdng the evaluation of the peaks relative to quinones and the primary donor. Direct electrochemistry of cofactors was also realized for RC in a lipid film on graphite and ITO or sandvdched between polycation layers on gold, permitting the determination of their midpoint potentials by cyclic and square wave voltammetry. In this case evidence of the presence of peaks relative to the bacteriopheophytin was reported for the first time. ... 

Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

For reversible systems there is no special reason to use these techniques, unless the concentration of the electrochemical active species is too low to allow application of DCP or cyclic voltammetry. For a reversible electrochemical system, the peak potentials in alternating current voltammetry (superimposed sinusoidal voltage perturbation) and in square-wave voltammetry (superimposed square-wave voltage... 

The measurement of formal potentials allows the determination of the Gibbs free energy of amalgamation (cf Eq. 1.2.27), acidity constants (pATa values) (cf. Eq. 1.2.32), stability constants of complexes (cf. Eq. 1.2.34), solubility constants, and all other equilibrium constants, provided that there is a definite relationship between the activity of the reactants and the activity of the electrochemical active species, and provided that the electrochemical system is reversible. Today, the most frequently applied technique is cyclic voltammetry. The equations derived for the half-wave potentials in dc polarography can also be used when the mid-peak potentials derived from cyclic voltammograms are used instead. Provided that the mechanism of the electrode system is clear and the same as used for the derivation of the equations in dc polarography, and provided that the electfode kinetics is not fully different in differential pulse or square-wave voltammetry, the latter methods can also be used to measure the formal potentials. However, extreme care is advisable to first establish these prerequisites, as otherwise erroneous results will be obtained. 

Finally, other types of voltammetric experiments may be employed beneficially for the characterisation of the redox properties of redox active compounds. Figure H.l.lbd shows square wave voltammograms [72] for the oxidation and reduction of the binuclear ruthenium complex. Well-defined peak responses indicate the presence of a reversible redox process. In this situation, the peak position corresponds closely to the reversible potential for the process and the peak height is related to the number of transferred electrons. Square-wave voltammetry may be employed to enhance reversible redox processes and to discriminate against irreversible and background processes (see also Chap. II.3). 

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