Cyclic voltammetry ultramicroelectrodes

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

FIGURE 2.28. Comparison of high-scan-rate ultramicroelectrode cyclic voltammetry (A), redoc catalysis (A), and laser flash photolysis (x) for the determination of the rate constant of deprotonation of methylacridan cation radical by bases of increasing pKa. Adapted from Figure 6 in reference 20, with permission from the American Chemical Society. 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

R. Dudek, and E. Tabet, Cyclic Voltammetry with Ultramicroelectrodes, ... 

C. Amatore, C. Lefrou, and F. Pfltiger, On-Line Compensation of Ohmic Drop in Submicrosecond Time Resolved Cyclic Voltammetry at Ultramicroelectrodes, J. Electroanal. Chem. 270 43-59 (1989). 

The introduction of ultramicroelectrodes in the field of voltammetric analysis offers access to cyclic voltammetry experiments that are impossible with conventionally sized macroelectrodes. In addition to analyses in small volumes or at microscopic locations, microelectrodes allow measurements in resistive media and make it possible to perform high scan rate voltammetry [9,10]. 

Minimization of time scale measurements Ultrafast undistorted cyclic voltammetry may be performed at ultramicroelectrodes using an ultrafast potentiostat allowing on-line ohmic drop compensation. 

The time domain on a window accessed by a given experiment or technique, e.g., femtosecond, picosecond, microsecond, millisecond. The time scale (or domain) is often characterized by a set of physical parameters associated with a given experiment or technique, e.g., r2 ]/1) (for - ultramicroelectrode experiments) - thus if the electrode radius is 10-7 cm and the - diffusion coefficient D = 1 x 10-5 cm2/s-1 the time scale would be 10 9s. Closely related to the operative kinetic term, e.g., the time domain that must be accessed to measure a first-order -> rate constant k (s-1) will be l//ci the time domain that must be accessed to measure a given heterogeneous rate constant, k willbe /)/k2. In - cyclic voltammetry this time domain will be achieved when RT/F v = D/k2 with an ultramicroelectrode this time domain will be achieved (in a steady-state measurement when r /D = D/k2 or ro = D/k at a microelectrode [i-ii]. 

The development of ultramicroelectrodes with characteristic physical dimensions below 25 pm has allowed the implementation of faster transients in recent years, as discussed in Section 2.4. For CA and DPSC this means that a smaller step time x can be employed, while there is no advantage to a larger t. Rather, steady-state currents are attained here, owing to the contribution from spherical diffusion for the small electrodes. However, by combination of the use of ultramicroelectrodes and microelectrodes, the useful time window of the techniques is widened considerably. Compared to scanning techniques such as linear sweep voltammetry and cyclic voltammetry, described in the following, the step techniques have the advantage that the responses are independent of heterogeneous kinetics if the potential is properly adjusted. The result is that fewer parameters need to be adjusted for the determination of rate constants. 

Unfortunately, it is far from trivial to obtain oxidation potentials for commonly encountered 17-electron metalloradicals M, because many such radicals dimerize at rates approaching diffusion-control, rendering it nearly impossible to observe such species by cyclic voltammetry. The use of ultramicroelectrodes was shown [41] to give a reversible signal for the oxidation of Mn(CO)5 at scan rates of ca 5000 V s , but the fmther oxidation of this radical to the 16-electron cation was not reported. There are, however, certain frequently encountered systems for which such radicals are stable at least on the time-scale of normal voltammetric measurements. Figure 4 shows an example, the oxidation of CpCr(CO)3 in acetonitrile. 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

Although the use of ultramicroelectrodes is not restricted to any specific measurement technique [125,143,178,181], only applications in the context of cyclic voltammetry at high sweep rates are considered here (see also Sec. IV). For the studies of reaction kinetics using ultramicroelectrodes under steady-state conditions the reader is referred to the original literature [182]. 

Only the application of fast cyclic voltammetry (in the range of 10-100 kVs-1) at gold ultramicroelectrodes has resulted recently in obtaining65,107 in DMF reversible one-electron oxidation peaks of p-anisidine (68a), p-chloroaniline (68b) and p-bromoaniline... 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

The use of ultramicroelectrodes for fast measurements has been recognized and developed by others and has been discussed in the literature. For the most part, the existing reports have concentrated on cyclic voltammetry. We believe that potential step methods are more appropriate to the realization of the promise of high speed kinetic measurements at ultramicroelectrodes, so we have concentrated on them. A preliminary report of our activity has appeared. ... 

Fig.6. Cyclic voltammetry of 2,6-anthraquinone disulfonic acid at mercury ultramicroelectrodes. Scan rate = 10240 mV/s. The top voltam-mogram was obtained with a platinum electrode overcoated with mercury. The bottom voltammogram was obtained using a platinum electrode that was etched and silanized before coating with mercury as shown in Fig.5.

Already in the mid-1960s, there was rich potential of applying such experiments to the determination of concentrations but even more to the elucidation of reaction mechanisms and kinetics coupled to electron transfer at an electrode was recognized. Today the resulting Knear sweep or cyclic voltammetries are employed as simple, flexible routine techniques in particular as sophisticated means to solve chemical and mechanistic problems. The combination with computer control, ultramicroelectrodes, and digital simulation has further contributed to their success. 

Linear sweep and cyclic voltammetry are among the most widely used electroana-lytical techniques for analysis of electron transfer related reactions. They are simple to apply, available in modem computer based electrochemical instmments and backed by extensive theoretical treatment. Besides classical applications in mechanistic analysis (see also Volume 8, Chapter 1), advances in data treatment, ultramicroelectrode use, and combination with other techniques allow the study of molecular electrochemical systems in great detail. 

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