The Compton-Coles cell

When assembled, the channel has cross-sectional dimensions of 0.4 mm x 6 mm and is 30 mm long. The upstream edge of the working electrode is 

The distribution of concentration c(x, t) of a species flowing in the channel can be described by the following (time-dependent) convective-diffusion equation  

The dependence of the steady-state ESR signal (5) on the current (/) and the flow rate was investigated theoretically for the same cell, using approximations akin to those made to derive the Levich equation, in order to be able to solve Eq. (15). For a stable electrogenerated radical, it was deduced that the following equation held  

Two different strategies exist for the determination of radical kinetics. Either the steady-state ESR signal can be measured as a function of the electrode current and the solution flow rate, or else a transient ESR signal can be recorded after the working electrode has been open-circuited once a steady-state has been established. In the latter method, one has a direct measure of 

Let us consider the steady-state method first and illustrate this with reference to a radical reacting via first-order kinetics. The results of calculations for this case are most conveniently expressed in terms of the ESR detection efficiency, Mx, given by 

The high sensitivity of the Allendoerfer cell makes it of great value in the detection of unstable radicals but, for the study of the kinetics and mechanism of radical decay, the use of a hydrodynamic flow is required. The use of a controlled, defined, and laminar flow of solution past the electrode allows the criteria of mechanism to be established from the solution of the appropriate convective diffusion equation. The uncertain hydrodynamics of earlier in-situ cells employing flow, e.g. Dohrmann [42-45] and Kastening [40, 41], makes such a computational process uncertain and difficult. Similarly, the complex flow between helical electrode surface and internal wall of the quartz cell in the Allendoerfer cell [54, 55] means that the nature of the flow cannot be predicted and so the convective diffusion equation cannot be readily written down, let alone solved Such problems are not experienced by the channel electrode [59], which has well-defined hydrodynamic properties. Compton and Coles [60] adopted the channel electrode as an in-situ ESR cell. 

For the channel electrode, Levich [61] calculated the diffusion-limited current-flow rate behaviour to be 

The dependence of the steady-state ESR signal upon diffusion-limiting current and flow rate was predicted to obey the equation 

The successful verification of the predicted behaviour for the channel electrode s use as an in-situ cell with stable species subsequently led to a study of unstable radical species. For the determination of the kinetics and mechanism of radical decay, two possible strategies exist, viz. 

The use of both methods has been shown to be successful with the Comp-ton-Coles cell [65, 66]. 

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Fig. 17. ESR signal/current/volume flow rate behaviour for the reduction of fluorescein at pH 13 in the Compton-Coles cell.

Use of the Compton-Coles cell in quantitative studies has been presented for EC [65], ECE [65, 67], and DISP1 [67] processes. The use of this in-situ cell in the study of radical kinetics and mechanism is well illustrated by the... 

The Compton-Coles cell has been used for quantitative study of Ce (85,87) processes. An equivalent cell design has been used... 

We next consider an example of the determination of the mechanism and kinetics of an electrode reaction. This concerns the apparent two-electron reduction of the molecule fluorescein (F) to leuco-fluorescein (L) in buffered aqueous solution, pH range 9-10. Experiments by Compton et al. using the Compton-Coles cell revealed strong ESR signals (Figure 23) attributable to semifluorescein (S ) where ... 

Figure 1.100 The Compton-Coles in situ flow cell.

As with the Compton-Coles channel electrode, Albery s cell should obey... 

FIGURE 24. The transport-limited current (/um) for the reduction of fluorescein at the Compton-Coles channel electrode cell. The solid lines show the predicted flow rate (V) (cm s" ) behavior for simple one- and two-electron reductions. 

Bond and coworkershave developed a small-volume (0.2 ml) variable-temperature EPR spectroelectrochemical cell that enables simultaneous rapid-scan voltammetry and EPR measurements to be made. The performance of this cell is compared to that of a flow-through cell designed by Coles and Compton. The small-volume cell permits cyclic voltammetric studies at variable temperatures but has significantly lower sensitivity compared to the flow-through cell, which is not amenable to low-temperature work. 

The electrochemistry of poly(./V-vinylcarbazole)-modified platinum electrodes has been investigated by Davis and co-workers [85] using the in-situ cell of Compton and Coles [60]. On oxidation of the modified electrode, the ESR spectrum as shown in Fig. 37 was observed. The broad symmetrical single line produced with peak-to-peak linewidth of 3.8 G is indicative of an organic radial "powder spectrum strong exchange interactions between... 

We next consider cells designed with the intention of studying the kinetics and mechanism of radical decay, as well as identifying the presence of particular radicals through their ESR spectra. We have noted (see above) that for this end, it is desirable not only to have a hydrodynamic flow over the electrode surface, but also that this flow be well defined and calculable so that the distribution of radicals in space and time may be calculated by solving the relevant convective-diffusion equation. We suggested above that this process could be expected to be difficult for the flow cells of Dohrmann and of Kastening because of the uncertain hydrodynamics of those cells. Likewise, the nature of the flow in Allendoerfer s cell cannot be confidently predicted, since it involves a complex flow between the inside of a silica tube and the surface of a coiled helix. We now describe the flow cell due to Compton and Coles, which was shown to display predictable and calculable hydrodynamics. 

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