D1 Chronocoulometry in Studies of Reactive Species

Electron transfer reactions at electrodes can yield reactive intermediates such as radicals and ion radicals. Electrochemical methods such as chronocoulometry can be used to generate reactive species and study their reac-

Table 12.6 Surface Concentrations and Diffusion Coefficients of Cd(II) from Analysis of Double Potential Step Data [3] Using the Model in Table 12.5

106 D, cmV1 nFT0, fiC cm

a Analysis of one data point/ms in regions t = 4-30 ms and 94-116 ms.

Figure 12.8 Residual plot test for reactant adsorption. The solid line shows the theoretical residual (deviation) pattern for noise-free data following the model in Table 12.5 fit onto the reversible model in Table 12.3. The points represent residual plot from analysis of data in Figure 12.7 with the reversible model. (Reprinted with permission from [2], copyright by the American Chemical Society.)

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Figure 12.9 Residual plot obtained after regression analysis represented in Figure 12.7 onto the adsorption model in Table 12.5. (Reprinted with premission from [2], copyright by the American Chemical Society.)

tions. Depending on the exact sequence of events in the mechanism of the electrode reaction, the shape of the chronocoulometric response curve will change.

For example, suppose the product of an electrode reaction reacts with water or the solvent (S) to form an electroinactive species. The reaction pathway can be expressed by the equations in Box 12.2.

In Box 12.2, the reactive intermediate B formed at the electrode (eq. (12.7)) undergoes a chemical reaction with S in the solution (eq. (12.8)). Electrochemists call the pathway in Box 12.2 an EC reaction, because an electron-transfer step (E) is followed by a chemical step (C). For such a reaction, if k] is very large relative to 1 It, the charge on the reverse pulse will not decrease as it would in a reversible electron transfer, because no B is present to oxidize. All the B has reacted (eq. (12.8)). The charge on the forward pulse will appear identical to a reversible reaction. If kxr is in the correct range, a partial decrease of charge relative to a reversible reaction will be observed. These data can be analyzed to estimate kx. A wide variety of electrochemical reaction types have been found, and models for many of them have been published [1, 7],

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