Peak current in cyclic voltammetry

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. 

The usual expressions for the current peak-height in cyclic voltammetry (CV) (such as the Randles-Sevcik equation) are simply inserted into Eq. (40) to obtain the corresponding DCVA relations. 

Perhaps the most unusual behavior is that, even with reversible systems, semimicro electrodes show little or no reverse current in cyclic voltammetry. The product formed in the forward sweep diffuses away from the electrode surface so rapidly that it cannot be caught on the reverse cycle. This effect, too, is relative—reverse peaks were observed even at 1-pm electrodes by Millar et al. (1981) with potential sweep rates of ca. 120 V/sec. 

Useful experimental parameters in cyclic voltammetry are (i) the value of the separation of the potentials at which the anodic and cathodic peak currents occur, A = Pia — PiC, and (ii) the half wave potential, 1/2, the potential mid-way between the peak potentials. A value of AE of c. 0.057 V at 25°C is diagnostic of a Nernstian response, such as that shown in Figure 2.87. More generally, if n electrons are transferred from R, then the separation will be 0.057/n V. It should be noted that the expected value for AE of 0.57/nV has no relationship to the usual Nernstian slope of RT/nF = 0.059/n V at 25UC. 

The abrupt transition to the 6,7 orientation manifests itself in cyclic voltammetry as a sharp current spike (Figure 6). The cathodic spike was found to contain a charge of 2.9 fiC cm, while its anodic counterpart contained 3.3 pC cm". The peak separation was 100 mV this large value is due to the large iR losses suffered in the thin layer cell. 

FIGURE 1.25. Successive reversible electron transfers in cyclic voltammetry of attached reactants. Normalized charge (a) and current (b) as a function of the separation between the standard potentials, at 25°C, from right to left A ° — E — E = 0.4, 0.1, 0.0356, —0.2 V. The middle of each curve corresponds to — )/2. (c) Variation of the normalized peak current with AE° in the range where a single wave is observed. 

FIGURE 2.4. EC reaction scheme in cyclic voltammetry. Derivation of the rate constant from the anodic-to-cathodic peak current ratio in zone KO. In this example the scan is reversed 200 mV (at 25° C) after the peak. 

FIGURE 2.8. CE reaction scheme in cyclic voltammetry. Kinetic zone diagram showing the competition between diffusion and preceding reaction as a function of the equilibrium constant, K, and the dimensionless kinetic parameter, X [equation (2.1)]. The boundaries between the zones are based on an uncertainty of 5% at 25°C on peak of plateau currents. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

To leam that the magnitude of the current peak in cyclic voltammetry (after suitable correction for baseline drift, where applicable) is proportional to analyte concentration according to the Randles-Sev5ik equation. 

Conductivity and anodic charge density, current density and potential of peak recorded by cyclic voltammetry on a platinum electrode (scan rate 50 mV s-1) in commercial and homemade creams... 

The DSCVC response has a peak-shaped feature similar to that obtained in Cyclic Voltammetry. Indeed, the most appropriate way of analyzing the DSCVC response is to divide Qdscvc by the pulse amplitude AE in order to obtain the (Qdscvc/AE) — E response, since the following relationship between the continuous current-potential curve corresponding to CV and the (QdscvcM-E) — curve obtained from a discrete staircase potential sequence can be established for AE < RT/F ... 

The electrochemical reduction of 02 in aptotic media is dramatically changed by the presence of electroinactive metal cations.2 Figure 9.5 illustrates the effect of a fivefold excess of [Znu(0H2)6](C104)2, [Znn(dimethylurea)6] (C104)2, and [Znu(bpy)3](C104)2 on the cyclic voltammetry of 02 in DMF at a platinum electrode. Prior to each reductive scan the electrode has been repolished a second scan yields a much reduced peak current. In the presence of an excess concentration of Zn(II) cations the reduction of 02 is a totally irreversible process, and the electrodes (Pt, Au, and C) are passivated after the initial negative scan. 

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