Forward scan, cyclic voltammetry

Fig. 5 Linear sweep and cyclic voltammetry (a) dotted lines five profiles respectively at various typical excitation signal (b) current response times, increasing time shown by arrows] for a and concentration profiles [(c) forward scan cyclic voltammetric experiment.

With faster scan cyclic voltammetry, a new two-electron anodic peak was detected, at more negative potentials, for the first stage of the oxidation process, with an accompanying cathodic peak on the reverse scan (11). The ratio of the forward to the reverse peak currents increased towards unity as the scan rate was raised to —200 V s 1 (Fig. 15). This behavior was attributed to the initial two-electron process being accompanied by a fairly rapid follow-up chemical reaction and was successfully analyzed in terms of an EqCi process (quasi-reversible electron transfer followed by a first-order irreversible chemical process), with a rate constant for the chemical step, k, = 250 s 1. 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

If k[ = n-F-v/R-T (i.e. if the chemical complication is neither too slow nor too fast and, consequently, the kinetics of the chemical complication are of the same order as the time scale of cyclic voltammetry) the potential of the forward peak, which has been localized at more anodic potentials than E0 by the chemical complication, shifts towards less anodic values with the scan rate according to the relationship ... 

The simplest and most useful case that one can study by single potential step chronoamperometry is that in which " /2> 0/i (z.e. AEor = E2 - E 5 180 mV). This means that the primarily electrogenerated species Red converts to a new species Ox, which is more easily reducible than the initial species Ox. As seen in Section 1.4.3, in cyclic voltammetry such a system exhibits a single reversible process in the forward scan. 

Cyclic voltammetry is often abbreviated CV. In this method, the potential is linearly scanned forward from ) to E2 and then backward from E2 to E), giving a triangular potential cycle (Fig. 5.18). Figure 5.21 shows some examples of cyclic voltammo-grams for the process Ox+ne Red, where only Ox is in the solution. Curve 1... 

Cyclic voltammetry is one such electrochemical technique which has found considerable favour amongst coordination chemists. It allows the study of the solution electron-transfer chemistry of a compound on the sub-millisecond to second timescale it has a well developed theoretical basis and is relatively simple and inexpensive. Cyclic voltammetry is a controlled potential technique it is performed at a stationary microelectrode which is in contact with an electrolyte solution containing the species of interest. The potential, E, at the microelectrode is varied linearly with time, t, and at some pre-determined value of E the scan direction is reversed. The current which flows through the cell is measured continuously during the forward and reverse scans and it is the analysis of the resulting i—E response, and its dependence on the scan rate dE/dt, which provides a considerable amount of information. Consider, for example, the idealized behaviour of a compound, M, in an inert electrolyte at an inert microelectrode (Scheme 1). 

Cyclic voltammetry provides a convenient method of recognizing such processes provided the lifetime of the intermediate is less than a minute or so. Consider an idealized reaction pathway (14) which involves the reversible one-electron reduction of a compound M. Of primary interest in the cyclic voltammetric experiment is the ratio of the back- and forward-peak currents, ipb/ip, and the dependence of this ratio upon the scan rate, v. 

A very useful extension of the voltammetric technique is cyclic voltammetry (Adams, 1969 Cauquis and Parker, 1973) in which one scans the potential of the working electrode in an unstirred electrolyte solution in the anodic (cathodic) direction and records one or several peaks due to oxidation (reduction) of the substrate. At some suitable potential, the direction of the scan is reversed and peaks due to reduction (oxidation) of intermediates and/or products formed during the forward scan are observed. In the simplest case a linear increase (decrease) of the potential with time is employed (triangular cyclic voltammetry) with scan rates in the range 0 01-1000 V s 1. It should be noted that cyclic voltammetry at scan rates above 1 Vs"1 requires the use of a differential cell to reduce the residual current due to charging of the electrified interface (see, for example, Peover and White, 1967). The theory of cyclic voltammetry has been... 

For triangular waves (- cyclic voltammetry) the steady-state current changes from vCd during the forward scan (increasing E) to - vCd during the reverse (decreasing E) scan. 

In cyclic voltammetry, the potential of the working electrode is scanned linearly from an initial to a vertex value, and then the scan is reversed. The electrochemical response of the target species with applied potential during the forward and reverse scans can be obtained from the scan cycle. Figure 1.14 shows... 

Obviously, therefore there must be an intermediate case in which the kinetics of both the forward and reverse electron-transfer processes have to be taken account of. Such systems are described as being quasi-reversible and as would be expected, the scan rate can have a considerable effect on the nature of the cyclic voltammetry. At sufficiently slow scan rates, quasi-reversible processes appear to be fully reversible. However, as the scan rate is increased, the kinetics of the electron transfer are not fast enough to maintain (Nernstian) equilibrium. In the scan-rate region when the process is quasi-reversible, the following observations are made. 

Cyclic voltammetry 0 = RT/nFv v, scan rate. Staircase methods 6 = duration of the forward pulse. Steady-state methods S = 5conv Transient methods, 5 =... 

In the type of linear-sweep voltammetry discussed thus far, the potential is changed slowly enough and mass transfer is rapid enough that a steady state is reached at the electrode surface. Hence, the mass transport rate of analyte A to the electrode just balances its reduction rate at the electrode. Likewise, the mass transport of P away from the electrode is just equal to its production rate at the electrode surface. There is another type of linear-sweep voltammetry in which fast scan rates (1 V/s or greater) are used with unstirred solutions. In this type of voltammetry, a peak-shaped current-time signal is obtained because of depletion of the analyte in the solution near the electrode. Cyclic voltammetry (see Section 23D) is an example of a process in which forward and reverse linear scans are applied. With cyclic voltammetry, products formed on the forward scan can be detected on the reverse scan if they have not moved away from the electrode or been altered by a chemical reaction. 

The major use of cyclic voltammetry is to provide qualitative information about electrochemical processes under various conditions. As an example, consider the cyclic voltammogram for the agricultural insecticide parathion that is shown in Figure 23-25. Here, the switching potentials were about —1.2 V and -1-0.3 V. The initial forward scan was, however, started at 0.0 V and not -fO.3 V. Three peaks are observed. The first cathodic peak (A) results from a four-electron reduction of the parathion to give a hydroxylamine derivative... 

Several newer techniques, such as cyclic voltammetry (CV) are now used to identify a proper choice of an antioxidant. CV is an electrolytic method that uses microelectrodes and an unstirred solution, so that the measured current is limited by analyte diffusion at the electrode surface. The electrode potential is ramped linearly to a more negative potential, and then ramped in reverse back to the starting voltage. The forward scan produces a current peak for any analyte that can be reduced through the range of the potential scan. The current will increase as the potential reaches the reduction potential of the analyte, but then falls off as the concentration of the analyte is depleted close to the electrode surface. As the applied potential is reversed, it wiU reach a potential that will reoxidize the product formed in the first reduction reaction, and produce a current of reverse polarity from the forward scan. This oxidation peak will usually have a similar shape to the reduction peak. The peak current, ip, is described by the Randles-Sevcik equation ... 

In cyclic voltammetry, the anodic portion on the reverse scan is not affected as much as the forward response by the coupled reaction (Figure 12.3.2). The ratio of (with /pa measured from the extension of the cathodic curve as described in Section 6.5) increases with increasing scan rate as shown in the working curve in Figure 12.3.6 (25). The actual i-E curves can be drawn using series solutions or a table given by Nicholson and Shain (25) or by digital simulation. 

Cyclic Voltammetry—5 V/min. Figures la-e present the j-U behavior for the first cycle of copper-2% zinc in phosphated saline and in protein solutions. The corrosion potentials (ia—ic) during the forward scans were between -0.35 to -0.40 V in all cases. Two anodic peaks were observed for all protein solutions at —0.25 to —0.10 V and +0.10 to +0.30 V. The first peak in the supporting electrolyte was also observed, whereas j continued to increase, never reaching a peak up to the reversal potential of +0.5 V. Two main cathodic peaks were observed at —0.30 to —0.45 V and —0.65 to —0.75 V in all cases. A prepeak inflection also occurred at -0.2 to -0.3 V for both the albumin and globulin systems, and a small peak at —1.1 to —1.2 V for most systems. Cathodic currents increase sharply below about —1.5 V. Figures 2a-b represent the surface appearances after the first cycle of polar-... 

A technique which is becoming increasingly important in our laboratory is a.c. cyclic voltammetry. This experiment is run on a stationary electrode [Hanging Hg drop (HMDE), Pt, Au, graphite, etc.]. The d.c. potential staircase is swept first in one direction and then the other. The slopes for the forward and reverse scans usually are equal in magnitude, but opposite in sign. The ramp amplitude encompasses one or more admittance peaks. The FT-FAM measurement is performed in this context. 

A second metric that can be tested is the forward peak potential relative to the formal potential of the couple, Ep — Aj ). This is the potential at which the peak current is observed to occur. For a reversible process, this is 28.5 mV which in dimensionless units is 6p = 1.11 at 298 K. By comparing the simulation s output with this peak position and with the Randles-Sevcfk equation for a range of values of a, we can test to see if it is correct. Figure 3.3 demonstrates how cyclic voltammetry varies with scan rate and Figure 3.4 demonstrates the agreement between simulated results and the Randles-Sevcfk equation. 

As discussed for the case of (hemi)spherical microelectrodes in Chapter 4, the response in cyclic voltammetry at microdiscs varies from a transient, peaked shape to a steady-state, sigmoidal one as the electrode radius and/or the scan rate are decreased, that is, as the dimensionless scan rate, a = Y r lv/TZTD, is decreased. The following empirical expression describes the value of the peak current of the forward peak for electrochemically reversible processes [11] ... 

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