The EC Process

Initially, the simple case of a irreversible first-order chemical reaction step (Cirrev) following a reversible heterogeneous charge transfer process (Erev) is considered. The reaction scheme for this type of process is given in Eq. II. 1.21 and 

In the case given, a chemical reaction B C with k = 200 s is considered and the effect of varying the scan rate is shown. With a chemical rate constant of k = 200 s the reaction layer thickness [90] can be estimated from 

Reaction = A / to extend ca. 2.2 pm from the electrode surface into the solution 

Reaction- given approximately by (D/kh) - (kh denotes a first-order rate constant). 

The simple use of a chemically irreversible chemical reaction step representing a chemical process is physically unrealistic, because the law of microscopic reversibility or detailed balance [94] is violated. More realistic is the use of an reaction scheme (Eq. 11.1.22, Fig. 11.1.21b). Even for the relatively simple reaction scheme, interesting additional consequences arise when the possibility of reversibility of the chemical step is considered. In Fig. II. 1.2lb, cyclic voltammograms for the case of a reversible electron transfer process coupled to a chemical process with kf = 10 s and fcb = 10 s are shown. At a scan rate of 10 mV s a well-defined electrochemically and chemically reversible voltammetric wave is found with a shift in the reversible half-wave potential E1/2 from Ef being evident due to the presence of the fast equilibrium step. The shift is AEi/2 = RT/F ln(X) = -177 mV at 298 K in the example considered. At faster scan rates the voltammetric response departs from chemical reversibility since equilibrium can no longer be maintained. The reason for this is associated with the back reaction rate of ky, = 10 s or, correspondingly, the reaction layer, Reaction = = 32 pm. At Sufficiently fast scan rates, the product B is irre- 

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A similar role is played by irreversible follow-up reactions, but the possibility of a mixed kinetic control by the two steps of the EC process should then be taken into account. A simplifying assumption is that the follow-up reaction is so fast that the conditions of zone KP prevail. It corresponds to the maximal influence of the coupled chemical step. The dimensionless expression of the cathodic trace of the irreversible voltam-mogram is then given by (see Section 6.2.1)... 

When the EC process is forced to occur within two opposite insulating flat substrates the technique is termed CEC and leads to thin single crystals, their thickness ultimately defined by the separation between both substrates (Thakur et al., 1990). This technique can be universally applied, as the parent standard EC technique, to the synthesis of a variety of thin single crystals of conducting and insulating molecular materials. 

By comparing the problem given by Eqs. (3.186b)-(3.193b) for aCE mechanism with Eqs. (3.186c)-(3.193c) for an EC mechanism, it can be easily inferred that the current corresponding to the EC process can be deduced with a similar procedure to that followed for a CE one (see Appendix D). So, the solution is ... 

The voltammetric response for the reaction scheme (3. XI) depends on the difference between the formal potentials of both electrochemical steps, ACt°, and on the equilibrium and kinetic constants of the intermediate chemical reaction. If AE = Ef.2 — Ef j [Pg.217]

The latest contribution to the theory of the EC processes in SECM was the modeling of the SG/TC situation by Martin and Unwin [86]. Both the tip and substrate chronoamperometric responses to the potential step applied to the substrate were calculated. From the tip current transient one can extract the value of the first-order homogeneous rate constant and (if necessary) determine the tip-substrate distance. However, according to the authors, this technique is unlikely to match the TG/SC mode with its high collection efficiency under steady-state conditions. 

In the EC process, the destabilization mechanism of the contaminants, particulate suspension, and breaking of emulsions may be summarized as follows. (1) Compression of the diffuse double layer around the charged species by the interactions of ions generated by oxidation of the sacrificial anode. (2) Charge neutralization of the... 

The EC process has the advantage of treating the water with low temperature and low turbidity. In this case, the chemical coagulation has difficulty in achieving a satisfying result. 

Turning to the EC process, it is evident from Fig. 1 that the chemical step... 

EC). In any of these processes the atomic number of the nucleus changes one unit. In decay electrons (e ) and in P" positrons (e ) are emitted from the nucleus. In the EC process an electron is captured from the K (or higher) shell. In some cases the same nucleus may undergo P decays of different types (O Fig. 2.37). The P decay may lead to the ground state of the daughter nucleus and/or to excited state(s). 

Figure 2.42c shows the (Kill )o branching ratios for allowed transitions. The EC process always accompanies the P decay, but the electron captme usually becomes dominating only in... 

The EC process is characterized by compact size, ease of operation, low sludge generation, high water recovery, and avoidance of chemical addition. 

A special and very important case of EC-type processes is denoted as the catalytic or EC (or ErgyC y) electrode reaction. In this reaction sequence (see Eq. II. 1.24), the heterogeneous electron transfer produces the reactive intermediate B, which upon reaction with C regenerates the starting material A. The redox system A/B may therefore be regarded as redox mediator or catalyst, and numerous applications of this scheme in electroorganic chemistry are known [97]. Furthermore, a redox mediator technique based on the EC process has been proposed by Sav ant et al. [98] allowing the voltammetric time scale for the study of very fast ECj ev processes to be pushed to the extreme. 

This type of reaction scheme is readily identified in steady-state or cyclic voltammetric [99] investigations based on the effect of the substrate concentration on the current [100]. The reaction layer thickness for the EC process is given by reaction = (f / h[C]buik)° where [C]buik denotes the concentration of C (Eq. II. 1.24) and fch is a second-order rate constant. The ratio of diffusion and reaction layer thickness yields an important kinetic parameter A = dpeak/Reaction = ([fch[C]buik RT]l[v nF]y- which describes the competition between diffusion and chemical reaction in the diffusion layer. With A < 0.1, diffusion is fast compared to the chemical reaction (e.g. fast scan rate) and the limiting case of a process... 

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