Preceding chemical reactions

1 Preceding Chemical Reactions. It is conceivable that if the species Ox is the product resulting from a chemical reaction preceding the electron transfer, such a reaction will influence the amount of Ox able to be reduced, so essentially perturbing the forward peak. 

For a preceding chemical reaction two mechanisms are possible, depending on whether the electron transfer is reversible or irreversible. 

1 First-order chemical reaction preceding a reversible electron transfer. The process in which a homogeneous chemical reaction precedes a reversible electron transfer is schematized as follows  

A similar process is indicated as a CE mechanism, or more specifically as a CrEr mechanism (where the subscript r indicates the reversibility of the respective process). 

As the supply of the electroactive species Ox results from the chemical reaction, it will be important to know, at least qualitatively, how much of Ox is formed during the time scale of the cyclic voltammogram. In this connection it must be taken into account that the time scale of cyclic voltammetry is measured by the parameters  

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Conversely, the use of elevated temperatures will be most advantageous when the current is determined by the rate of a preceding chemical reaction or when the electron transfer occurs via an indirect route involving a rate-determining chemical process. An example of the latter is the oxidation of amines at a nickel anode where the limiting current shows marked temperature dependence (Fleischmann et al., 1972a). The complete anodic oxidation of organic compounds to carbon dioxide is favoured by an increase in temperature and much fuel cell research has been carried out at temperatures up to 700°C. 

There are several variants of this method for more complicated reactions. If the reacting species is produced by a preceding chemical reaction, deviations from Eq. (14.6) may be observed for large in, when the reaction is slower than mass transport. From these deviations the rate constant of the chemical reaction can be determined. As an example we consider hydrogen evolution from a weak acid HA, where the reacting protons are formed by a preceding dissociation reaction ... 

In addition, though less common, there are cases of preceding chemical reactions (preceding, naturally, the electron transfer). In this case, the reagent Ox is the product of a preliminary chemical reaction of a species that is not itself electroactive. For example, the reduction of acetic acid proceeds through the two microscopic stages ... 

Figure 12 Cyclic voltammogram for a CrEr mechanism in which the preceding chemical reaction is fast, but the equilibrium is shifted towards the reagents...

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

The forward peak current measured under the effect of the kinetic reaction, ik (kinetic current), is a fraction of the peak current id (diffusive current) that one would record at the same scan rate in the absence of the preceding chemical reaction, according to the relationship ... 

Also in this case, if the kinetics of the preceding chemical reaction are very slow (kf + kT < a na-F-v/R-T), the process appears as a simple irreversible electron transfer. The peak height of the process depends on the equilibrium constant because, as mentioned previously, the concentration of the active species C0x is a fraction of the amount C (= Cox + Cy) put in solution ... 

If AT is large, again the response appears as if the preceding chemical reaction would be absent. However, the peak potential is shifted towards more negative values than those that would be recorded in the absence of the chemical complication by a factor equal to ... 

Preceding chemical reactions. The typical case of a preceding chemical reaction is that of the species Ox in equilibrium with an electro inactive substance Y, which follows the sequence of events already seen in Section 1.4.1.1, relative to cyclic voltammetry ... 

Since the effects of the preceding chemical reaction substantially affect the forward response, the single potential step experiment can be adequately used. 

The value of /k//d at a given time represents the ratio of the kinetic current, /k, recorded experimentally and the value of diffusion current, /d, that one would have if the process was not complicated by the preceding chemical reaction. 

For the sake of simplicity, hereafter the charge of the species is omitted. The preceding chemical reaction is assumed to be a chemically reversible process attributed with first-order forward (s ) and backward kb (s ) rate constants. In the real experimental systems, the forward chemical reaction is most frequently a second-order process ... 

The overall effect of the preceding chemical reaction on the voltammetric response of a reversible electrode reaction is determined by the thermodynamic parameter K and the dimensionless kinetic parameter . The equilibrium constant K controls mainly the amonnt of the electroactive reactant R produced prior to the voltammetric experiment. K also controls the prodnction of R during the experiment when the preceding chemical reaction is sufficiently fast to permit the chemical equilibrium to be achieved on a time scale of the potential pulses. The dimensionless kinetic parameter is a measure for the production of R in the course of the voltammetric experiment. The dimensionless chemical kinetic parameter can be also understood as a quantitative measure for the rate of reestablishing the chemical equilibrium (2.29) that is misbalanced by proceeding of the electrode reaction. From the definition of follows that the kinetic affect of the preceding chemical reaction depends on the rate of the chemical reaction and duration of the potential pulses. 

Although it is difficult to generalize the dependence of the peak potential on e, in general, for an oxidative electrode mechanism, the position of the peak shifts to positive potentials by increasing the rate of the preceding chemical reaction. At the same time, the half-peak width is largely insensitive to the chemical reaction. If log( ) < -2, Ap vs. log(e) is a linear function with a slope of about 30 mV. 

In this case, besides the thermodynamic and kinetic parameters of the preceding chemical reaction, the response depends on the kinetics of the electrode reaction represented by the electrode kinetic parameter k = (see Sect. 2.1.2) [60], Figure 2.29 shows the variation of A Fp with e for various k. It is obvious that there is... 

Experimental studies of CE mechanisms with SWV are scarce. Santos et al. [65] studied two experimental systems, i.e., the reduction of Cd + ion in the presence of nitrilotriacetic acid (NTA) and aspartic acid (ASP). For the first experimental system, the preceding chemical reaction is described by the scheme ... 

Similar to the diffusion controlled CrE mechanism (Sect. 2.4.1) the preceding chemical reaction (2.108) is characterized by the eqnilibrinm constant K=, where kf and kb are the first-order rate constants of the forward and backward cliemical reactions, respectively. The surface CrE mechanism is represented by (2.92) and the following differential eqnations ... 

Fig. 39. Complex plane diagram of the faradaic impedance in the case of a preceding chemical reaction (CE) with an equilibrium constant of KA = 1. Solid line feA — °° broken lines feA has a finite value, decreasing from top to bottom.

Fig. 41. Potential dependence of log aa = log (Ao o + (JR) in the case of a preceding chemical reaction (CE). Numbers indicate values of KA. Vertical bars indicate the halfwave potential at the corresponding d.c. polarograms. Numerical data as in Fig. 40(a) except that kAt — 3 X 10s (see text). From ref. 175.

The preceding chemical reaction causes the concentration of the electroactive A, to be lower than otherwise the effect of this will be largest when K is large, when a purely kinetic current dependent on the value of fe] will be observed. 

Fig. 13. Diagnostic plots for electrode reactions with coupled homogeneous reactions, illustrated for the RDE. (a) CE mechanism. Curve A, no effect from chemical reaction (5k = 0) curve B, effect of preceding chemical reaction (5k >0). (b) Catalytic mechanism. Curve A, in the absence of parallel chemical reaction curve B, experimental dependence predicted from eqn. (175).

Values of A and a are the same as in Table 4. This immediately shows us the effect of the preceding chemical reaction on the half-wave potential. We can also, by analogy with Sect. 4.3, write... 

In the CE mechanism (or preceding chemical reaction) the electroactive species is generated from an electroinactive species by a chemical reaction ... 

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