Coupled homogeneous reactions, double-step

Double-step chronocoulometry is a powerful tool in identifying adsorption phenomena, in obtaining information on the kinetics of coupled homogeneous reactions and for the determination of the capacitive contribution. The double potential step is executed in such a way that after the first step from E to E2, a next step is applied, i.e., the reversal of the potential to its initial value Ei from 2 (see Fig. 3). 

Double-Step chronocoulometry is also extremely useful for characterizing coupled homogeneous reactions. Any deviation from the coulometric responses described by Eqs. (II.4.3) and (II.4.11) - providing that diffusion control prevails -implies a chemical complication. For example, O rapidly reacts with a component of the solution, and this homogeneous chemical reaction results in the formation of an electrochemically inactive species. Qmit > t) falls less quickly than expected or, at complete conversion within the timescale of the experiment, no backward reaction is seen at all. A quick examination of this effect can be carried out by the evaluation of the ratio of Qm (t = 2x) Q t = x). For stable systems this ratio is between 0.45 and 0.55. 

Chronoamperometry Chronoamperometry involves the study of the variation of the current response with time under potentiostatic control. Generally the working electrode is stepped from a potential at which there is no electrode reaction to one corresponding to the mass-transport-limited current, and the resulting current-time transient is recorded. In double-step chronoamperometry, a second step inverts the electrode reaction and this method is useful in analysing cases where the product of the initial electrode reaction is consumed in solution by a coupled homogeneous chemical reaction. 

Where applicable, the shortsightedness principle can significantly simplify quantitative modeling, especially in networks with coupled parallel steps. Examples are olefin reactions that involve double-bond migration in parallel to conversion to products, as in homogeneous catalytic hydrogenation, hydroformylation, hydro-cyanation, and hydrohalogenation [16]. 

Low amount of theoretical, and maybe even less amount of experimental work has been made on double potential step chronoamperometry that, similarly to cyclic voltammetry (see below) is classified as a reversal technique after the forward step potential, WE is polarized at a value at which the electrogenerated species is reoxidized to the starting one. Such a technique is quite effective in studies of electrode reaction mechanisms. As an example, very accurate quantitative data about the kinetics accounting for the stability of electrogenerated species can be gained. However, the issue of how the data should be treated in order to obtain similar information about different homogeneous kinetics coupled to the charge transfer is far beyond the scope of the present book. 

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