Electrode overall reactions

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

One possible reason for the reluctance of non-electrochemists to venture into this field is that in contrast to the electrochemists claim that controlled potential electrolysis offers a method for the selective introduction of energy into molecules, many electrode reactions carried out at a controlled potential have still been reported to give low yields and a diversity of products. The electrode potential is, however, only one of several variables and the lack of selectivity in the electrode process may be attributed to a failure to understand and to control all the parameters of the overall electrode reaction. 

The fact that transport limits the rate of the overall electrode reaction affects the fastest timescale accessible. Once transport controls the rate, faster reaction steps cannot be characterized. It is thus important to enhance mass transfer, for example, by increased convection with high flow rates [37, 38]. 

In order to illustrate the application of LSV in mechanistic analysis we can look at the redox behavior of the formazan-tetrazolium salt system which we studied some years ago [17], 1,3,5-Triphenyl formazane was oxidized at controlled potential in CH3CN-Et4NC104 solution to 2,3,5-triphenyl tetrazolium perchlorate which was then isolated in quantitative yield. Coulometry showed that the overall electrode reaction was a two-electron oxidation. It has been shown that the rate of variation of Ep with log v was 30 mV per decade of sweep rate and that there was no variation of the peak potential with the concentration of 1,3,5-triphenylformazan. According to Saveant s diagnostic criteria (Table 1), four mechanistic schemes were possible e-C-e-p-p, e-C-d-p-p, e-c-P-e-p and e-c-P-d-p. If cyclization is the rate-determining step, then the resulting e-C-e-p-p and e-C-d-p-p mechanisms would not imply variation of Ep with the concentration of base. However, we have observed the 35 mV shift of Ep cathodically in the presence of 4-cyanopyridine as a b e. These observations ruled out the first two mechanisms. The remaining possibilities were then e-c-P-e and e-c-P-d, as shown in Scheme 3. 

An alternative containment scheme is immobilization of active species on a surface" " or within a tethered polymer brush or network. ° Surface immobilization can achieve high surface utilization by locating mediators and biocatalysts within nanometers of conducting surfaces. Immobilization on polymer networks allows for dense packing of enzymes within electrode volumes at the expense of long-distance electron mediation between the enzyme active center and a conductive surface. Such mediation often represents the rate-limiting step in the overall electrode reaction. 

Mathematical analysis of the polarograms indicated that the overall electrode reaction involved one mole of reactant, two electrons, and three moles of product. The mechanism which accounts for the experimental results is illustrated in Figure D. 

The basicity of a number of reagents 1, 5 and related compounds in THF solutions was determined voltammetrically by Chevrot and coworkers . Using platinized platinum and hydrogen electrodes, they measured the cathodic-anodic current as a function of potential for overall electrode reactions (equations 43a and 43b)... 

As in other complex servicing centers, the electrodic reaction may consist of a number of steps. For example, it has just been indicated that the overall electrodic reaction... 

It has been stated that though an overall electrodic reaction may consist of several steps, it is usually possible to single out one step and regard it as the essential cause of the overall electron queue and hence the overpotential 1). What is the justification for this discriminatory attitude toward the one step ... 

Consider an overall electrodic reaction that takes place in n steps (Fig. 7.69). I et it be assumed, for convenience of exposition, that each step is a charge-transfer reaction with an electron acceptor receiving an electron. To simplify the treatment, let it also be assumed that the n individual electronation reactions are only slightly off equilibrium and that therefore, for each reaction, one can use the linear current-den-sity-overpotential law Hq. 7.25]. The rate of any one step in such a case is proportional to its overpotential t y62 ... 

Equation (7.144) is the most general form of the Butler-Volmer equation it is valid for a multistep overall electrodic reaction in which there may be electron transfers in steps other than the rds and in which the rds may have to occur V times per occurrence of the overall reaction. This generalized equation is seen to be of the same form as the simple Butler-Volmer equation for a one-step, single-electron transfer reaction ... 

The mechanism of the overall electrode reaction remains unchanged but accelerates, creating an increased exchange current density i0 (Fig. lb). 

A limiting current insensitive to changes in electrode potential and below the convective-diffusion limiting current indicates that a chemical step is rate-determining and precedes the charge transfer step in the overall electrode reaction (CE mechanism). 

Vetter has extensively used the electrochemical reaction order to establish the mechanism of complex electrode reactions [7]. For instance, for the overall electrode reaction... 

The simplest treatment of this problem considers that, for a given potential, the electrode reaction rate coefficient is a linear function of the coverage by adsorbate. The overall electrode reaction rate coefficient is thus expressed as a weighted linear combination of the rate coefficients at the covered sites, fex, and at the adsorbate free surface, kQ... 

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

The overpotential 77 is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control charge transfer, diffusion, chemical reaction, and crystallization. Charge-transfer reaction involves transfer of charge carriers, ions or electrons, across the double layer. This transfer occurs between the electrode and an ion, or molecule. The charge-transfer reaction is the only partial reaction directly affected by the electrode potential. Thus, the rate of charge-transfer reaction is determined by the electrode potential. 

Mass transport processes are involved in the overall reaction. In these processes the substances consumed or formed during the electrode reaction are transported from the bulk solution to the interphase (electrode surface) and from the interphase to the bulk solution. This mass transport takes place by diffusion. Pure diffusion overpotential t]A occurs if the mass transport is the slowest process among the partial processes involved in the overall electrode reaction. In this case diffusion is the rate-determining step. 

The Cat is either oxidized (see -> oxidation) or reduced (see reduction) at the electrode surface to give a product Cat , which then undergoes chemical -> redox reaction with S. One of the products of the chemical redox reaction between Cat and S is either Cat or an - intermediate of the overall -> electrode reaction in which Cat is produced at the electrode... 

An overall electrode reaction differs from an overall chemical reaction in that at least one... 

In the case of the alkaline copper cyanide bath, the overall electrode reaction is... 

Transfer coefficient for the overall electrode reaction Symmetry factor for an elementary charge-transfer step... 

The overall electrode reactions for the diaphragm and membrane cells are both the same, and given as ... 

The analysis of mass transfer in electrochemical cells requires the use of equations that describe the condition of electroneutrality (which applies for the entire elecnolyte outside the double layer at an electrode), species fluxes, mass conservation, current density, and fluid hydrodynamics. Often, mass transport events are rate limiting, as compared to kinetics processes at the electrode surface, in which case the overall electrode reaction rate is solely dependent on species mass transfer (e.g., during high-rate electroplating of some metals and for those elecnochemical reactions where the concentration of reactant in solution is low). 

Consider an overall electrode reaction, O -h ne R, composed of a series of steps that cause the conversion of the dissolved oxidized species, O, to a reduced form, R, also in solution (Figure 1.3.6). In general, the current (or electrode reaction rate) is governed by the rates of processes such as (1, 2) ... 

The mean surface concentrations enforced by depend on many factors (a) the way in which is varied (b) whether or not there is periodic renewal of the diffusion layer (c) the applicable current-potential characteristic and (d) homogeneous or heterogeneous chemical complications associated with the overall electrode reaction. For example, one could vary sequential potentiostatic manner with periodic renewal of the diffusion layer, as in sampled-current voltammetry. This is the technique that is actually used in ac polarography, which features a DME and effectively constant during the lifetime of each drop. Alternatively one could use a stationary electrode and a fairly fast sweep without renewal of the diffusion layer. Both techniques have been developed and are considered below. The effects of different kinds of charge-transfer kinetics will also be examined here, but the effects of homogeneous complications are deferred to Chapter... 

Now the electrode potential can only be measured against a reference electrode. The standard hydrogen electrode is usually used as the reference electrode, and it is arbitrarily taken to have a potential of zero. The standard hydrogen electrode is platinized (high surface area) platinum immersed in an acid containing ions at an activity of 1 mol dmand dissolved hydrogen gas at atmospheric pressure (Vetter 1967). The overall electrode reaction is... 

Sources of Polarization in Electrolytic Cells Figure 22-8 shows three regions of a half-cell in an electrolytic cell where polarization can occur. These regions include the electrode itself, a surface film of solution immediately adjacent to the electrode, and the bulk of the solution. For this half-cell, the overall electrode reaction is... 

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