EC’ reaction

This reaction sequence, as mentioned in the Introduction, is expressed by E 

The case where a first-order chemical reaction follows reversible electron transfer has been considered using the Levich analysis [100]. Results, in 

---

Galceran, J., Taylor, S. L. and Bartlett, P. N. (1999). Application of Danckwerts expression to first-order EC reactions. Transient currents at inlaid and recessed microdisc electrodes, J. Electroanal. Chem., 466, 15-25. 

FIGURE 2.1. EC reaction scheme in cyclic voltammetry. Kinetic zone diagram showing the competition between diffusion and follow-up reaction as a function of the equilibrium constant, K, and the dimensionless kinetic parameter, X. The boundaries between the zones are based on an uncertainty of 3 mV at 25°C on the peak potential. The dimensionless equations of the cyclic voltammetric responses in each zone are given in Table 6.4. 

FIGURE2.2. EC reaction scheme in cyclic voltammetry. Pure kinetic conditions (zone KP in Figure 2.1). a concentration profiles, b dimensionless cyclic voltammogram. 

FIGURE 2.4. EC reaction scheme in cyclic voltammetry. Derivation of the rate constant from the anodic-to-cathodic peak current ratio in zone KO. In this example the scan is reversed 200 mV (at 25° C) after the peak. 

FIGURE 2.5. EC reaction scheme in cyclic voltammetry. Mixed kinetic control by an electron transfer obeying the Butler-Volmer law (with a = 0.5) and an irreversible follow-up reaction, a Variation of the peak potential with the scan rate, b Variation of the peak width with scan rate. Dots represent examples of experimental data points obtained over a six-order-of-magnitude variation of the scan rate. 

A reaction scheme frequently encountered in practice, the so-called square scheme mechanism, consists of the association of two EC reaction schemes as shown in Scheme 2.3 (which may as well be viewed as an association of two CE mechanisms). In the general case, the cyclic voltammetric response may be analyzed by adaptation and combination of the treatments given in Sections 2.2.1 and 2.2.2. A case of practical interest is when the follow-up reactions are fast and largely downhill. A and D are then stable reactants, whereas B and C are unstable intermediates. When the starting reactant is A (reduction process), the reaction follows the A-B-D pathway. The reoxidation preferred pathway is D-C-A. It is not the reverse of the forward... 

We now start examining how competing follow-up reactions control product distribution. The way in which these reactions interfere depends on their rate relative to the diffusion process, or alternatively, on the relative size of the corresponding reaction and diffusion layers (Figure 2.31). For a follow-up reaction with a first (or pseudo-first-order) rate constant, k, occurring in the framework of an EC reaction scheme (see Section 2.2.1), the reaction layer thickness is y/D/k. 

The opposite situation (y/D/k -C <5), where the reaction layer is much thinner than the diffusion layer (as represented in the lower diagram of Figure 2.31) is more specific of electrochemistry, in the sense that the homogeneous follow-up reactions are more intimately connected with the electrode electron transfer step. The same pure kinetic conditions discussed earlier for cyclic voltammetry (Section 2.2.1) apply. In the case of a simple EC reaction scheme, as shown in the figure, the production of C in the bulk solution obeys exactly the same equations (2.32) to (2.34) as for B in the preceding case, as established in Section 6.2.8. 

If electron transport is fast, the system passes from zone R to zone S+R and then to zone SR. In the latter case there is a mutual compensation of diffusion and chemical reaction, making the substrate concentration profile decrease within a thin reaction layer adjacent to the film-solution interface. This situation is similar to what we have termed pure kinetic conditions in the analysis of an EC reaction scheme adjacent to the electrode solution interface developed in Section 2.2.1. From there, if electron transport starts to interfere, one passes from zone SR to zone SR+E and ultimately to zone E, where the response is controlled entirely by electron transport. 

Fig. 9 Typical cyclic voltammograms of an EC reaction system rate of follow-up reaction increases from short-dashed through dotted, dash-dotted and long-dashed to solid curve.

Chemical reactions are designated as C, so if the product of electron transfer undergoes a homogeneous chemical reaction we say that it is an EC reaction. The C terms are often given a superscript or subscript to show why type of chemical reaction occurs, e.g. disproportionation, dimerization or catalytic. Table 6.4 lists many of the commonly encountered Reinmuth terms. 

Remember Reinmuth notation is read from left to right, so an EC reaction occurs with a chemical reaction following an initial electrode (electron-transfer) reaction. 

Following chemical reactions. We will now consider following chemical reactions, which are also called coupled chemical reactions or EC reactions. We will illustrate such a situation with the bromine-bromide couple. 

How can cyclic voltaimnetry detect that an EC reaction is occurring ... 

The return, reverse peak is smaller than the forward peak if the CV of the Brj, Br couple is run with allyl alcohol in solution I pc a [bromine], where pc = peak, cathodic . The fact that Ipc is smaller than Ipo implies that some fraction of the electrogenerated bromine has been consumed by the chemical reaction between Br2 and allyl alcohol the CV thus contains evidence of an EC reaction. Figure 6.19 below shows the cyclic voltammogram for the EC reaction between bromine and allyl alcohol, while Table 6.5 lists a series of simple diagnostic tests for an EC reaction. 

Figure 6.19 Cyclic voltammograms as a function of scan rate to show the effects of an EC reaction. This figure comprises traces simulated by the DigiSim program. The fastest scan rate is shown outermost note how the reverse peak is essentially absent at slow scan rates. Reprinted with permission from Current Separations, Vol. 18, pp. 9-16, copyright Bioanalytical Systems, Inc., 1999.

Table 6.5 Simple diagnostic tests for a coupled chemical (EC) reaction, carried out by using cyclic voltammetry (after Nicholson, R. S. and Shain, I. Anal. Chem., 36, 706-723 (1994), and Nadjo, L. and Saveant, J. M., J. Electroanal. Chem., 48, 113-145 (1973))...

Figure 6.20 Experimental plot of the ratio of peak currents, /p(back)//p(forward), against log(r) for an EC reaction, where t is the time-scale of the CV.

Figure 6.21 Computed plot of the ratio of peak currents, /p(back//p(focwani), against log (kx) for an EC reaction, where t is the time-scale of the CV and k is the rate constant of the first-order homogeneous reaction. Notice how the plot has a similar shape to that shown in Figure 6.20, but the jc-axis is offset by the amount log k.

While we have discussed the RRDE in terms of an electron-transfer reaction followed by a chemical reaction (an EC reaction), it is such a versatile tool to the electroanalyst that extension to other mechanisms is straightforward and routine, although a further discussion is beyond the scope of this present text... 

However, we must always bear in mind that an electrochemical reaction can sometimes follow a completely different series of steps, and with different variables, and yet by coincidence it can simulate our model quite closely. The usual problem is that our model is too simple. A good example is to recall that an EC reaction always looks like an E reaction (see Section 6.4.4), except at slow scan... 

If we prefer to simulate a CV for a simple E reaction rather than an EC reaction (see Section 6.4.4), then we should type in a ludicrously small value, such as 10 , when the program asks for a value of the rate constant k to be inserted. 

Another complication of the reversible case may be that the reduction product A reacts chemically and is thus not available for reoxidation on the reverse scan, so only a small or no anodic peak is seen. In the usual electrochemical nomenclature, an electron-transfer reaction is called E and a chemical follow-up reaction C. The process in question would thus be an EC reaction the chemical step would after a reduction in most cases be a reaction with an electrophile, including protons, a cleavage reaction, where a nucleophile is expelled, or a dimerization for oxidation reactions with a nucleophile, loss of a proton or dimerization would be the most common follow-up reactions. 

EC reactions at tubular and channel electrodes have been considered [208]. An analytical solution is not possible due to the non-uniformly accessible nature of the electrode. However, an approximate equation for the half-wave potential can be written, for a reduction, as... 

The potential response of the RDE to current steps has been treated analytically [3, 237, 251] and accurately by Hale using numerical integration [252] this enables the elucidation of kinetic parameters [185, 253]. A current density—transition time relationship at the RDE has been established which accounts for observed differences from the Sand equation [eqn. (218)] and which has been applied to EC reactions [254]. Other hydrodynamic solid electrodes have not been considered in detail, although reversible reactions at channel electrodes have been discussed [255, 256]. 

In general, EC reactions are typically observed according to the following general rank order (by relative ease of oxidation) o,p-quinol and o,p-aminophenol > tertiary amine > m-quinol rv phenol rv arylamine > secondary amine thiol > thioether primary amines, aliphatic alcohols. (HDVs) each redox active metabolite are obtained from the response across adjacent EC-Array sensors. These data are a reflection of the kinetic and thermodynamic components of electron transfer reactions. Since chemical structure is a critical determinant of an analyte s redox behavior, the intrinsic generation of an HDV with EC-Array provides qualitative information for each species. 

All the above methods, when hers are present, have one very serious drawback many hers give rise to a compact reaction layer, as described in Chap. 2. The above Reinert-Berg reaction does not, but the EC reaction,... 

In Appendix C, the program CV EC is described, which simulates a CV for a simple EC reaction. 

We can know the information of radicals generated during EC reactions and oxygen vacancies in Sn02 crystal lattice. We also can infer electrocatalytic characteristics from the concentration of oxygen vacancies. 

Coupled homogeneous reactions in electrochemistry — Chemical reactions occurring in an electrochemical cell, in which the electroactive compounds undergo chemical transformation by reacting with the electroinactive compounds, are known as coupled homogeneous reactions. All the compounds in the system are present together in the same phase - EC, CE, and EC reactions. 

If the concentration of Z is much larger than that of O, the chemical reaction is pseudofirst order. The reduction of Ti(IV) in the presence of oxalate and hydroxy-lamine follows this pattern of catalytic chemical reactions in electrochemistry (-> catalytic currents). The typical features of the EC reactions (under conditions of cyclic voltammetry) are reflected in increasing cathodic... 

---