Quasireversible reactions

The half-peak width, AEp/2, of the quasireversible reaction increases with decreasing If, but Aiip/2 of totally irreversible reactions is independent of if. 

The quasireversible reaction (2.158) of a dissolved redox conple is described with the following kinetic eqnation ... 

Fig. 2.71 Quasireversible reaction coupled with adsorption of the reactant. Effect of the kinetic...

Similar to the pure surface electrode reaction, the response of reaction (2.146) is characterized by splitting of the net peak under appropriate conditions. The splitting occurs for an electrochemically quasireversible reaction and vanishes for the pure reversible reaction. Typical regions where the splitting emerges are 3 < m < 10 and 0.1 < r < 10 for a = 0.5 and i sw = 50 mV. Contrary to the surface electrode reaction where the ratio of the split peak currents is solely sensitive to a, in the present system this ratio depends additionally on r. For instance, if a = 0.5 and r = 1 the ratio is = 1 for r = 10, > 1 and r = 0.1, < 1. Finally it is worth mentioning when experimentally possible, the electrode mechanism represented by (2.145) to (2.147) has to be simplified to a simple surface reaction (Sect. 2.5.1) in order to avoid the complexity arising from the effect of diffusion mass transport. 

Fig. 2.75 Quasireversible reaction coupled with adsorption of both the reactant and product. Effect of/ on the ratio Conditions of the simulations are /dday = 300 s, = 113 s , (X =...

Nevertheless, the dimensionless net peak current is virtually independent on c . Hence, the real net peak current is a linear funchon of the ligand concentration, which permits apphcahon of this mechanism for analyhcal purposes. A representa-hve theoretical response of a quasireversible reaction (2.205) is shown in Fig. 2.86b. In addition to k and 7, the dimensionless response is a function of the concentration... 

A quasireversible electrode reaction is controlled by the film thickness parameter A, and additionally by the electrode kinetic parameter k. The definition and physical meaning of the latter parameter is the same as for quasireversible reaction under semi-infinite diffusion conditions (Sect. 2.1.2). Like for a reversible reaction, the dimensionless net peak current depends sigmoidally on the logarithm of the thickness parameter. The typical region of restricted diffusion depends slightly on K. For instance, for log( If) = -0.6, the reaction is under restricted diffusion condition within the interval log(A) < 0.2, whereas for log(if) = 0.6, the corresponding interval is log(A) <0.4. 

Due to the similarity with surface electrode processes, a quasireversible reaction in thin-film voltammetry exhibits a quasireversible maximum and splitting of the net peak. The reasons causing these voltammetric features are the same as for surface... 

In the example program EX CV, as mentioned, two kinds of boundary conditions are accommodated those for a quasireversible reaction, and for a fully reversible reaction. The division is made on the basis of the dimensionless heterogeneous rate constant Ko if it exceeds 1000, the reaction is considered reversible. 

This is a simple simulation of a CV experiment, using the explicit method EX, and assuming a quasireversible reaction,... 

The considerable complexity of SECM theory is due to the combination of a cylindrical diffusion to the ultramicroelectrode (UME) tip and a thin-layer-type diffusion space. The time-dependent diffusion problem for a simple quasireversible reaction in cylindrical coordinates is as follows (2,3) ... 

However, Eq. (24) has not been utilized in any calculations. Most published computational results represent the long-time limit of the data obtained by solving time-dependent equations. By fitting these results, several analytical approximations have been obtained for different situations including diffusion-controlled, irreversible, and quasireversible reactions at both tip and substrate surfaces. 

For a quasireversible reaction, however, the forward and backward I(t) curves do not coincide (Figure 6.1.3b). One can regard this effect as a consequence of the shifting of the pa values from their reversible values. The procedure used to obtain the reversible Eii2 value for such a system is shown in Figure 6.1.3b (21). 

In addition, one needs the appropriate i-E characteristic (i.e., for a reversible, totally irreversible, or quasireversible reaction). The resulting nonlinear integral equation must be evaluated numerically. Alternatively, the problem can be addressed by digital simulation techniques. Figures 8.3.1 and 8.3.2 illuminate the effects of different relative contributions of double-layer charging on if (at constant /) and on the E-t curves of a nemstian reaction. The charging contribution is represented there by the dimensionless parameter, K, defined as... 

There is no analytical equation for a quasireversible substrate reaction. In fact, quasireversible reactions have been studied by means of approximate analytical models for the... 

Zoski, C. G., Aguilar, J. C., Bard, A. J. Scanning electrochemical microscopy. 46. Shielding effects on reversible and quasireversible reactions. Anal. Chem. 2003, 75(13), 2959-2966. 

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