Linear potential sweep voltammograms

Figure 2.19 Linear potential sweep voltammograms for chalcopyrite electrode.

Figure 6.12 shows a theoretical linear potential sweep voltammogram for a planar electrode, using the data of Table 6.3. The current drops beyond the peak ip shown in Fig. 6.12 because the species getting oxidized (or reduced) is depleted, in turn because the diffusion of analyte from bulk solution has not kept apace with the electrochemical process at the electrode. 

Upon evaluating the convolution integral from the experimental current-potential (time) curve and its limiting values (Eq. 77), kinetic analysis can be performed with the help of Eq. (76). Conversely, Eq. (76) or similar equations can be used to calculate the theoretical current-potential curve, e.g., for the linear potential sweep voltammogram, provided that the values of all the parameters are known. Some illustrative examples were provided by Girault and coworkers [183]. 

Laviron, E., General expression of the linear potential sweep voltammogram in the case of diffusionless electeochemical systems. J. Electroanal. Chem. 1979,101, 19-28. 

Figure 6.2.1 Linear potential sweep voltammogram in terms of dimensionless current function. Values on the potential axis are for 25°C...

Figure 6.2.3 Effect of double-layer charging at different sweep rates on a linear potential sweep voltammogram. Curves are plotted with the assumption that Cd is independent of E. The magnitudes of the charging current, ic, and the faradaic peak current, /p, are shown. Note that the current scale in (c) is lOX and in (d) is lOOX that in (a) and (b).

From the data in Table 6.3.1 plot the linear potential sweep voltammograms, that is,... 

Figure 5.4 Voltammograms for a pyrite electrode in solutions at different pH conditions modified by CaO and NaOH (Linear potential sweeps at 20 mV/s)...

Fig. 15. A normalized potential sweep voltammogram showing the linear projection of theoretical (X) and experimental (V) electrode potential data onto the X—Y plane.

Figure 3.9 Linear voltage-sweep voltammogram with reversal of sweep direction to give a cyclic voltammogram. Initial sweep direction to more negative potential.

Fig. 16.6. Examples of BIA voltammetry, illustrated for the oxidation of 2 mM K4Fe(CN)6 in 0.4 M K2S04 electrolyte at a Pt electrode, dispension flow rate 24.5 p.Ls , cell parameters as in Fig. 16.5. (a) Consecutive injections of 16 p.L during a linear potential sweep, scan rate 10 mVs l (b) Background-subtracted cyclic voltammogram recorded during injection, scan rate 2 Vs-1 (c) Background-subtracted square wave (SW) voltammogram recorded during injection SW amplitude 50 mV, SW increment 2 mV, frequency 100 Hz.

A steady state is independent of the details of the experiment used in attaining it. Thus, under conditions where a steady state is attained, e.g., under convective conditions in an - electrochemical cell, the application of a constant current leads to a constant potential and similarly the application of a constant potential leads to the same constant current. Voltammetric steady states are most commonly reached using linear potential sweeps (or ramps) in a single or cyclic direction at a UME or RDE. A sigmoidally shaped current (l)-potential (E) voltammogram (i.e., a steady-state voltammogram) is recorded in the method known as steady-state voltammetry as shown in the Figure. Characteristics of the... 

Fig. 18 Simulated first derivative cyclic voltammogram for a reversible charge transfer showing the measurement of the derivative of the forward current (//), the backward current (/ b) and the switching potential ( sw)- The plot is for currenttime for a linear potential sweep. (Ahlberg and Parker, 1981b)...

Linear potential sweep and cyclic voltammetry are at their best for qualitative studies of the reactions occurring in a certain range of potential. In Fig. 5L, for example, we see the cyclic voltammogram obtain on a mercury-drop electrode in a solution of p-nitrosophenol in acetate buffer. Starting at a potential of 0.3 V versus SCE, and sweeping in the cathodic direction, one observes the first reduction peak at about - 0.1 V. This potential corresponds to the reduction of... 

Numerous excellent texts exist on the fundamentals of cyclic voltammetry. The reader is referred especially to the recent text by Bond, which provides an excellent treatment of fundamentals as well as applications. The important aspects of cyclic voltammetry are illustrated by the diagram shown in Figure 1 of a typical voltammogram of a soluble, reversible couple subjected to a linear potential sweep (and return scan) between applied voltages E and E2- The characteristic curve shown in Figure 1 provides peak potentials ( p and E° ) as well as peak currents 1° and Note that... 

Fig. 14. Peak voltammograms of dopamine at graphite electrodes. (A) Linear potential sweep (200 mV/sec), 500 pM dopamine in pH 7.4 phosphate buffer (B) cyclic voltammogram of same solution as A. Dotted line—see text.

A useful adjunct of linear potential sweep methods is called cyclic voltammetry. Rather than stopping an oxidative voltammogram at, say, + 0.8 V, the potential is reversed and scanned backward, i.e., a triangular wave potential is applied. The oxidation product formed is present at and close to the electrode surface. With fairly rapid potential sweeps (ca. >4 V/min) it is almost completely re-reduced back to the starting material on the reverse potential sweep. Figure 14B shows a typical cyclic voltammogram for a reversible system (solid line). The ratio of forward to reverse peak currents is unity. If, however, some rapid process removes the product(s), litde or no reverse current is obtained (dotted lines of Fig. 14B). This happens if the overall oxidation is totally irreversible, or fast chemical reactions intervene. We will also see later that a peculiar property of very small electrodes can eliminate most of the reverse current in a cyclic voltammogram. 

E2. Typical current-potential responses (note that the potential axis is also a time axis), which are called as potential sweep voltammograms or cyclic voltammograms, for infinite linear diffusion conditions are shown in Fig. 5b. 

Typical linear potential sweep (LPS) voltammograms, obtained at different pH as indicated, are given in Figure 8.10. They contain two well-defined maxima... 

Figure 8.15 Changes in Cu(ll) surface concentration under linear potential sweep conditions. Transform of the LPS voltammograms by Eq. (8.14).

The basic idea of this technique is that a linear potential sweep (of low rate) is applied to the disk electrode, while cyclic voltammograms (of a relatively high sweep rate) are measured at the ring. The results of such experiments can lead to the creation of a 3D map , which may reveal the electroactive intermediates or products that are formed in the electrode process(es) taking place on the disk. The applicability of the proposed technique is demonstrated here by considering the oxygen reduction process at the gold 0.5 mol dm sulphuric acid electrode as an illustrative model reaction. 

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. 

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