Waveform, reversible cyclic

Figure 16.7 A reversible cyclic voltammogram obtained with the waveform shown in figure 16.3.

Fig. 5. Applied waveform for linear sweep voltammetry and cyclic voltammetry (a) and a reversible cyclic voltammogram (b).

In cyclic voltammetry, we apply the triangular waveform in Figure 17-22 to the working electrode. After the application of a linear voltage ramp between times t0 and f, (typically a few seconds), the ramp is reversed to bring the potential back to its initial value at time t2. The cycle may be repeated many times. 

As an alternative to a stepwise variation with time, a continuously changing potential may be imposed. Though other possibilities have been used [42, 43], a linearly changing potential—time waveform, known as a potential ramp [Fig. 17(a)], is the most common. The technique has many names, including linear sweep voltammetry [44]. If the direction of the ramp is reversed [Fig. 17(b)], the technique is often termed cyclic voltammetry (see Chap. 3), though this name is more appropriately applied after sufficient ramp reversals [Fig. 17(c)] have caused the experiment to become periodic. 

The term voltammetry refers to measurements of the current as a function of the potential. In linear sweep and cyclic voltammetry, the potential steps used in CA and DPSCA are replaced by linear potential sweeps between the potential values. A triangular potentialtime waveform with equal positive and negative slopes is most often used (Fig. 6.8). If only the first half-cycle of the potential-time program is used, the method is referred to as linear sweep voltammetry (LSV) when both half-cycles are used, it is cyclic voltammetry (CV). The rate by which the potential varies with time is called the voltage sweep (or scan) rate, v, and the potential at which the direction of the voltage sweep is reversed is usually referred to... 

Cyclic Voltammetry. However, experimental use of this technique has been restricted almost exclusively to the analysis of the limiting currents of the signals obtained. One reason for this could be that when a quasi-reversible electronic transfer is analyzed in RPV, two very close waves are obtained, which are difficult to resolve from an experimental viewpoint. This problem can be eliminated by using the triple pulse technique Reverse Differential Pulse Voltammetry (RDPV), proposed in references [80, 84, 85] and based in the application of the waveform presented in Scheme 4.5. 

By inserting the solutions proposed in Eq. (6.189) and condition (6.175) in Eq. (6.185), recurrent expressions for coefficients 8lp) and are deduced [68] and by inserting these expressions into (6.191) the current is calculated. These expressions allow us to obtain limiting cases like the reversible and irreversible ones which have a discrete character which makes them applicable to any multipulse technique by simply changing the potential time waveform, including the continuous limit of Cyclic Voltammetry. Moreover, they are independent of the kinetic formalist considered for the process. 

Scheme 7.4 Potentialtime waveform of square wave voltammetry in cyclic mode. Black and white dots indicate the time at which the forward and reverse currents, respectively, are measured...

The reversibility, or otherwise, of an electrode process is best investigated using the technique of cyclic voltammetry, in which a rapid forward and reverse voltage ramp is applied in triangular form (Fig. 4A) to interact with both the electroactive substance and its reduction product that, for quasi- or fully reversible processes, may be oxidized back to the starting material, giving the characteristic waveform shown in Fig. 4B. The separation between anodic and cathodic peaks indicates whether the electrode process is quasi- or fully reversible. Additional mechanistic investigations can also be made in respect to the number of... 

Fig. 4 Cyclic voltammetry. (A) Voltage waveform showing the rapid forward and reverse voltage sweeps. (B) Typical cyclic voltammogram for completely reversible system.

Figure 1. (Top) Waveform for linear-scan (solid line) and cyclic (dotted line) voltammograms. (Bottom) LSV and CV curves for reversible one-electron reduction of a Pd dithiolate complex.

Cyclic ac voltammetry is a simple extension of the linear sweep technique one simply adds the reversal scan in - dc- This technique retains the best features of two powerful, complementary methodologies. Conventional cyclic voltammetry is especially informative about the qualitative aspects of an electrode process. However, the response waveforms lend themselves poorly to quantitative evaluations of parameters. Cyclic ac voltammetry retains the diagnostic utility of conventional cyclic measurements, but it does so with an improved response function that permits quantitative evaluations as precise as those obtainable with the usual ac approaches. Although this technique is not widely employed, it can be a useful adjunct to dc cyclic voltammetry. 

Figure 10.5.8 is a display of an actual cyclic ac voltammogram for ferric acetylaceto-nate, Fe(acac)3, in acetone containing 0.1 M tetraethylammonium perchlorate. Since this system is very nearly reversible to the dc process, the peak splitting is quite small, but easily detectable. The convenience of the waveform for quantitative work is also readily apparent. 

In cyclic voltammetry (CV), Ihe current response of a small stationary electrode in an unstirred solution is excited by a triangular voltage waveform, such as that shown in Figure 25-2.L In this example, the potential is ftrst varied linearly from -K).8 V to -0,15 V versus an SCH. When the extreme of -0,15 V is reached. Ihc scan direction is reversed, and the potential is returned to its original value of -l O.S V. The scan rate in cither direction is 50 mV/s. This excitation cycle is often repeated several times. The voltage extrema at which... 

Figure 15.34 (a) Excitation waveform and (b) current response for a reversible couple obtained in cyclic voltammetry. 

Typical FFT a.c. cyclic voltammograms of Cr(CN)6 "/Cr(CN)6 " couple at HMDE in aqueous cyanide media. System 1.0 x 10"3 M Cr(CN)63- at Hg-1.0 M KCN, water interface, 25"C. Applied Pseudo-random, odd-harmonic a.c. waveform with 1.5 mV per frequency component, 32 total components superimposed on staircase d.c. scan (5 mV per step) with triangular envelope whose scan rate = 50 mV s-1. Measured Faradaic admittance magnitude at 1840.8 rad s-1 (A,C) and 7877 rad s"l (B,D) obtained on single measurement pass (30 other frequency components measured simultaneously, but not shown). (A,B) Raw data. (C,D) digitally filtered (smoothed) data. (+) Forward scan, ( ) reverse scan. Abscissa = potential vs. Ag/AgCl. 

In this article, the potential waveform employed to run cyclic voltammetry experiments is first described. The current—voltage waveforms for both reversible and irreversible redox reactions are then presented with an emphasis on the various parameters of... 

The triangular potential waveform employed in cyclic voltammetry is shown in Figure 1. Typically, the potential is ramped linearly from an initial potential, Ej, to the switching potential, Emax- The direction of the potential sweep is then reversed and scanning continues until E ,in is reached. The potential sweep may be terminated at the end of the first cycle or it may continue for an arbitrary number of cycles. The primary experimental parameters are the initial potential, the switching potentials, and the potential sweep rate. Typical sweep rates for cyclic voltammetry, employing electrodes of conventional sizes (e.g.. 

Figure 4 (a) Waveform used in cyclic voltammetry and (b) the readout obtained with this technique for a reversible oxidation process of the type A(soiution) B(soiution) +... 

Cyclic, square wave, ac, and differential pulse voltammetry have also been used for bioanalysis, although commerciali2ation of specialized bioassay instruments that exploit the increased selectivity of these methods has not yet occurred. Figure 4 shows the applied waveforms and (reversible) voltammetric responses for each of these techniques. Equations describing the peak currents may be found in most texts of analytical importance is the direct proportionality between peak current magnitude and analyte concentration for all four techniques. 

Fig. 4 Voltammetric waveforms and responses for a reversibly redox-active analyte (a) cyclic voltammetry ...

Linear sweep voltammetry and cyclic voltammetry [23-25] Potential-time waveforms employed for potential sweep measurements are shown in Figs. 4 and 5a. Linear sweep voltammetry involves sweeping potential between two limited values Ej and E2 at a controlled sweep rate v. A more useful method is cyclic voltammetry in which the potential sweep is reversed usually at the same sweep rate on reaching... 

The potential-time waveforms used for sweep measurements are shown in Fig. 6.1. The simplest of these techniques is linear sweep voltammetry (LSV), and this involves sweeping the electrode potential between limits Ei and E2 at a known sweep rate, v, before halting the potential sweep. A generally more useful (and consequently more widely applied) technique is cyclic voltammetry (CV). In this case the waveform is initially the same as in LSV, but on reaching the potential E2 the sweep is reversed (usually at the same scan rate) rather than terminated. On again reaching the initial potential, Ei, there are several possibilities. The potential sweep may be halted, again reversed, or alternatively continued further to a value 3. In both LSV and CV experiments the cell current is recorded as a function of the applied potential (it should be noted, however, that the potential axis is also a time axis). The sweep rates used in... 

In potential sweep methods, the current is recorded while the electrode potential is changed linearly with time between two values chosen as for potential step methods. The initial potential, E, is normally the one where there is no electrochemical activity and the final potential, 2, is the one where the reaction is mass transport controlled. In linear sweep voltammetry, the scan stops at E2, whereas in cyclic voltammetry, the sweep direction is reversed when the potential reaches 2 and the potential remmed to j. This constitutes one cycle of the cyclic voltammogram. Multiple cycles may be recorded, for example, to study film formation. Other waveforms are used to study the formation and kinetics of intermediates when studying coupled chemical reactions (Figure 11.4c). 

Figure llA Waveforms used in linear sweep (a) and cyclic voltammetry (b and c). j, 2 and 3 are the starting and reversal potentials while v is the scan rate. 

Figure 19.3 Charging current and selection of step potential for chronoamperometry. (a) Potential waveform applied to the electrode in chronoamperometry. At i = 0, the potential is stepped from the initial value to a constant value (b) Dependence of faradaic current (if) and charging current (ij on time for a planar macroelectrode, (c) Dependence of if and on time for a UME. (d) Cyclic voltammogram at a UME showing and selection of Ef and E. (e) Cyclic voltammogram of a reversible redox couple at a macroelectrode showing and Ey and selection of and 3,. (J) Cyclic voltammogram of a quasi-reversible redox couple at a macroelectrode showing and and selection of E and...

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