Electron transfer kinetics chronoamperometry

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

Chronoamperometry was used to determine the electron-transfer kinetics and it could provide information regarding the dynamic model of an electrochanical process. In chronoamperometry, the current is integrated over relatively long time intervals thus, it gives a better signal to noise ratio in comparison to other voltammetric techniques. The Faradaic current, which is due to electron-transfer events and is... 

These expressions are designed for cyclic voltammetry. The expressions appropriate for potential step chronoamperometry or impedance measurements, for example, are obtained by replacing IZT/Fv by the measurement time, tm, and the inverse of the pulsation, 1/co, respectively. Thus, fast and slow become Af and Ah I and -C 1, respectively. The outcome of the kinetic competition between electron transfer and diffusion is treated in detail in Section 1.4.3 for the case of cyclic voltammetry, including its convolutive version and a brief comparison with other electrochemical techniques. 

Kinetic studies of ECE processes (sometimes called a DISP mechanism when the second electron transfer occurs in bulk solution) [3] are often best performed using a constant-potential technique such as chronoamperometry. The advantages of this method include (1) relative freedom from double-layer and uncompensated iR effects, and (2) a new value of the rate constant each time the current is sampled. However, unlike certain large-amplitude relaxation techniques, an accurately known, diffusion-controlled value of it1/2/CA is required of each solution before a determination of the rate constant can be made. In the present case, diffusion-controlled values of it1/2/CA corresponding to n = 2 and n = 4 are obtained in strongly acidic media (i.e., when kt can be made small) and in solutions of intermediate pH (i.e., when kt can be made large), respectively. The experimental rate constant is then determined from a dimensionless working curve for the proposed reaction scheme in which the apparent value of n (napp) is plotted as a function of kt. 

The chronocoulometry and chronoamperometry methods are most useful for the study of adsorption phenomena associated with electroactive species. Although less popular than cyclic voltammetry for the study of chemical reactions that are coupled with electrode reactions, these chrono- methods have merit for some situations. In all cases each step (diffusion, electron transfer, and chemical reactions) must be considered. For the simplification of the data analysis, conditions are chosen such that the electron-transfer process is controlled by the diffusion of an electroactive species. However, to obtain the kinetic parameters of chemical reactions, a reasonable mechanism must be available (often ascertained from cyclic voltammetry). A series of recent monographs provides details of useful applications for these methods.13,37,57... 

Saveant SM, Vianello E. Potential-sweep chronoamperometry theory of kinetic currents in the case of a first order chemical reaction preceding the electron-transfer process. Electrochim Acta 1967 ... 

Saveant JM, TaneUo E (1965) Potential-sweep chronoamperometry kinetic currents for first-order chemical reaction parallel to electron-transfer process (catalytic currents). Electrochim Acta 10 905-920... 

These nanoporous electrodes can be utilized for the same type of electrochemical studies as planar electrodes but have some distinct differences in electrochemical response due to their nanoscale geometry. The kinetics at the electrode surface can be significantly altered as the pore sizes are often on the same scale as the electrical double layer thickness and thus overlapping double layers can be achieved. Voltammetric responses of nanoporous electrodes are distinctly different from their nonporous counterparts as the surface features can provide acceleration of proton or electron transfer steps of electrochemical reactions. " Despite these differences in electrochemical response, nanoporous electrodes are often utilized to perform chronoamperometry and voltammetry to achieve the detection of various electroactive analytes. 

The kinetically best characterized system for which a bimolecular reductive elimination has been proposed is a neutral hydrido porphyrin derivative of Ru(III) [4]. Cyclic voltammetry and double potential step chronoamperometry afford data that are more consistent with a second order than with a first order decay for the 17-electron RuH(OEP)(L) (OEP = octaethylporphyrin L = THE, 1 -tert-bytyl-S-phenylimidazole) complexes in THF as solvent. The second order dependence of the rate constant and the independence on the parent 18-electron anion concentration exclude the proton transfer mechanism. The possibility of a disproportionation mechanism (which would afford the same second order dependence, see section 6.5.2), however, has not been considered, nor were studies in solvents other than THF carried out. In the light of the gathered information, the mechanism shown in Scheme 17 was proposed. 

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