Forward and reverse scans

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. 

FIGURE 2-3 Concentration distribution of the oxidized and reduced forms of the redox couple at different times during a cyclic voltammetric experiment corresponding to the initial potential (a), to the formal potential of the couple during the forward and reversed scans (b, d), and to the achievement of a zero reactant surface concentration (c). 

Trace crossing upon scan reversal appears for intermediate values of X (Figure 2.9). It results from the fact that the C concentration continues to build up during the forward and reverse scans. More C has thus been formed... 

Example Tandem mass spectrometric experiments in quadmpole ion traps are performed by combining the techniques of resonant ejection, and forward and reverse scanning to achieve an optimum in precursor ion selection, ion activation, and fragment ion scanning (Fig. 4.45). [156]... 

Doroshenko, V.M. Cotter, R.J. Losses of Ions During Forward and Reverse Scans in a QIT Mass Spectrometer and How to Reduce Them. J. Am. Soc. Mass Spectrom. 1997, 8,1141-1146. 

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). 

In cyclic SWV (Fig. 7.25), the typical peak-shaped response is obtained in both the forward and reverse scans for any bulk concentrations of the ion. In contrast to the behavior at macro-interfaces, the forward and backward peaks are not symmetrical with respect to the potential axis, and the peak height of the reverse scan is dependent on the vertex potential. The position of the voltammograms is well described by the analytical solution (7.50) whenoswv < 0.1, with a difference between analytical and numerical results for the peak potentials of less than 8 mV at 7 298 K for typical SWV conditions sw = 10 — 50mV and AEs = 2 — 10mV. 

Figure 9.1 illustrates the electrochemical reduction of 02 at platinum electrodes in aqueous media (1.0 M NaC104). The top curve represents the cyclic voltammogram (0.1 V s-1) for 02 at 1 atm ( 1 mM), and the lower curve is the voltammogram with a rotated-disk electrode (900 rpm, 0.5 V min-1). Both processes are totally irreversible with two-electron stoichiometries and half-wave potentials (EU2) that are independent of pH. The mean of the Em values for the forward and reverse scans of the rotated-disk voltammograms for 02 is 0.0 V versus NHE. If the experiment is repeated in media at pH 12, the mean Em value also occurs at 0.0 V. 

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... 

When the surface activity is stronger, the surface coverage is high throughout the values of Aq calculated (curve 2). The peak becomes dull and there is no big difference between the surface coverages in the forward and reverse scans. Nevertheless, by and large, the results of numerical calculation in Figure 7.5 indicate that the properties of the electrochemical instability under the current flow are essentially the same as those in the thermodynamic equilibrium. 

Figure 4.19 Hysteresis in the current-voltage characteristics of a gas evolving electrode. Forward and reverse scans are done at 10 V/s. Reprinted from [124] with permission from Elsevier.

The three steps of an MS/MS experiment are performed by DIT using an approach substantially different from that employed in 3D IT or linear ITs (Ding, 2004). In those cases, the precursor ion isolation is performed by applying one or more dipole excitation waveforms, with a maximum isolation resolution of -1300 (expressed as the isolation mass divided by the baseline width of the isolation window). In the case of DIT, ion isolation is performed by sequential forward and reverse scans, so as to eject all ions with m/z values lower and higher than that of interest, respectively. This method can provide precursor ion isolation with a resolution >3500. 

Cyclic ac voltammograms for completely nernstian systems are easy to predict on the basis of results from the previous section. The mean surface concentrations, Co(0, Om Cr(0, Om adhere to (10.5.3) and (10.5.4) unconditionally hence at any potential they are the same for both the forward and reverse scans. The cyclic ac voltammogram should therefore show superimposed forward and reverse traces of ac current amplitude dc-We expect a peak-shaped voltammogram that adheres in every way to the conclusions reached in Section 10.5.1 about the general ac voltammetric response to a reversible system at a planar electrode. 

Fig. 4.1 Potentiostatic polarization curves (forward and reverse scan).

Figure 12 illustrates benefits obtained by interpolation of an a.c. cyclic voltammogram obtained using the FT-FAM procedure. The system involves the one-electron reduction of the tetramethylphenyl-enediamine cation (TMPD ) at the platinum-0.10 M tetrabutylammonium perchlorate, acetonitrile interface. Under these conditions the electrode reaction may be characterized as quasireversible on the d.c. time scale. Such systems are predicted to exhibit unequal forward and reverse scan admittance voltammograms which include a cross-over point at a particular d.c. potential.43 The d.c. potential of the crossover point can be used to measure an important... 

C) interpolated data points. Notation o, = original sampled data, forward and reverse scans, respectively ... 

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. 

Note that the area under the curves, i.e. the charges transferred in the forward and reverse scans, are not equal, since not all starting material is regenerated during the reverse scan and some product (ox in the present discussion) escapes from the electrode. 

By extending ac linear sweep voltammetry by a reverse scan, ac-cychc voltammetry is obtained. Surface concentrations of the electroactive species are the same at the same potential for forward and reverse scans. Hence, the peaks for forward and reverse scans are identical. In contrast to classical cyclic voltammograms, ac-cychc voltammograms have a clear baseline that is advantageous for quantitative measurements. 

If the dc process is not fully reversible, the surface concentrations of the electroactive species are different at a given dc potential for forward and reverse scans, that is, for quasi-reversible systems a displacement of the peaks for forward and reverse scan can be observed. This displacement can be used to derive kinetic parameters of the electrode reaction. For... 

When there is a degree of irreversibility this is manifest in the cyclic voltammogram as a larger peak separation and a lowering of the forward and reverse peaks. As might be expected, irreversibility shows up in the a.c. response in a similar way (Fig. 8.19). The peaks on the forward and reverse scans are separated and are of unequal height. Fuller reviews of a.c. methods in analytical chemistry are available [12,13]. 

Figure 5.15. (b) Forward and reverse scans for curve 1 and curve 11. 

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