Working, curves electrode

Thus, in Fig. 4.26 working curves corresponding to the variation of the dimensionless peak current of the catalytic mechanism for spherical and disc electrodes with the parameter < 2 = y/D /ro are plotted. For a given experimental system,... 

For many mechanisms, the steady-state Eia or N tt value is a function of just one or two dimensionless parameters. If simulations are used to generate the working curve (or surface) to a sufficiently high resolution, the experimental response may be interpolated for intermediate values without the need for further simulation. A free data analysis service has been set up (Alden and Compton, 1998) via the World-Wide-Web (htttp //physchem.ox.ac.uk 8000/wwwda/) based on this method. As new simulations are developed (e.g. for wall jet electrodes), the appropriate working surfaces are simulated and added to the system. It currently supports spherical, microdisc, rotating disc, channel and channel microband electrodes at which E, EC, EC2, ECE, EC2E, DISP 1, DISP 2 and EC processes may be analysed. 

The recent availability of working curves and surfaces for a range of conunon mechanisms at a number of electrode geometries (Alden and Compton, 1997a) allows a broad quantitative comparison of the kinetic discrimination of conunon electrode geometries for both first- and second-order homogeneous processes. 

Commercially available microdisc electrodes of radii 0.6-70/xm may be used for steady-state measurements without problems associated with natural convection. Dimensionless rate constants for spherical and microdisc electrode were interpolated from the working curves of Alden and Compton (1997a). 

The manner in which kinetic data are treated in arriving at an electrode mechanism depends primarily upon whether the technique gives a direct measure of the response of the intermediate or an indirect measure, usually the effect of the chemical reaction on the electrode response of the substrate. In the former case, the conventional way of handling the data is to compare the experimental response with theoretical data in the form of a working curve and determine the mechanism from the best fit with theoretical data. The latter case usually involves the calculation of the electrode response to a particular mechanism and then comparing some measurable quantity, for example the sweep rate dependence of the peak potential, with the theoretical value. Which type of analysis is appropriate, direct or indirect, depends upon the... 

It has also been shown that the electrode response of some processes can appear to fit theoretical working curves in which the reaction order in the intermediate differs from the true value (Parker, 1981b). For example, the deprotonation of hexamethylbenzene radical cation studied by derivative cyclic voltammetry gave data which fitted theoretical data for a simple first order decomposition of the intermediate. However, the observed first order rate constants were found to vary significantly with the substrate concentration indicating a higher order reaction. A method was proposed to treat... 

The inadequacy of theoretical working curves in electrode-mechanism analysis has prompted the development of an alternative approach which does not involve the use of theoretical data at all in the determination of the mechanism (Parker, 1981e). Theoretical data are used only after the mechanism has been established and then to evaluate rate constants. 

The reactions of DPAt and radical cations of other aromatic hydrocarbons with pyridine and substituted pyridines are among the most intensively studied electrode reactions of positive ions. The first definitive study of the mechanism of the reaction employed the rotating disk electrode (Manning et al 1969). Data were found to fit ECE working curves (Fig. 21) for the reactions of DPA7 with 4-cyanopyridine, 4-acetoxypyridine, pyridine and 4-methylpyridine. Pseudo first order rate constants of about 3, 10, 30, 300... 

Figure 37. Rotating disk electrode working curves for the eCeh mechanism at different values of the rate constant k for the chemical step.

Application of the RRDE as a diagnostic tool to explore the mechanism of an unknown reaction pathway relies, as most methods do, on the differences between the different working curves. For the RDE the working curves for different mechanisms in many cases do not deviate sufficiently to allow a distinction between two possible pathways [259]. The presence of the ring electrode provides extra information, and thus the RRDE is a more sensitive instrument for mechanism analysis. An example is the distinction between the RR and the RS dimerization mechanisms for CS2 radical anion [269]. It has been pointed out, however, that even with this electrode setup the working curves do... 

Working curves have been calculated for a number of reaction mechanisms. The experimental data are normalized by the absorption measured at the time of electrode... 

Connect the ion-selective electrode and the second reference electrode to the pH meter as shown in Figure 21F-1. Prepare a series of standard solutions of the ion of interest, measure the cell potential for each concentration, plot a working curve of eii versus log c, and perform a least-squares analysis on the data (see Chapter 8). Compare the slope of the line with the theoretical slope of (0.0592 V)/n. Measure the potential for an unknown solution of the ion and calculate the concentration from the least-squares parameters. 

A more realistic approximate theory for the SECM with a tip shaped as a cone or spherical segment was presented in Ref. 9. The surface of the nonplanar tip electrode was considered to be a series of thin circular strips, each of which is parallel to the planar substrate. The diffusional flux to each strip was calculated using approximate equations for a disk-shaped tip over a conductive or an insulating substrate. The normalized current to the nonplanar tip was obtained by integrating the current over the entire tip surface. Two families of working curves for conical tips over conductive (Fig. 8A) and insulating substrates (Fig. 8B) illustrate the effect of the tip geometry. 

With the substrate biased at a potential slightly more positive than E° of A/B couple, B is oxidized to form A for both DISP1 and ECE mechanisms. However, in the latter case the reduction of C also occurs at the substrate. The numerical solution of corresponding diffusion problems (see Ref. 38 for problem formulations) yielded several families of working curves shown in Fig. 12 (DISP1 pathway) and Fig. 13 (ECE pathway). In both cases the tip and the substrate currents are functions of the dimensionless kinetic parameter, K = ka2/D. The normalization of the iT and is for two-electron processes is somewhat problematic. In Ref. 38 both quantities are normalized with respect to the one-electrode steady-state current, which flows at infinite tip/substrate separation (iT,ie.=o = 4FDac°). However, this value is not equal to experimentally measured tip current at d — which... 

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