Electroactive substance

Cathode material Electroactive substance Reaction potential — E (V) Electrical yield (%) benzene (molar yield) (%) aniline (molar yield) (%) 1,4-cyclo-hexadiene (molar yield ) (%)... 

Most electrochemical reactions occur at an interface between an electronic conductor system and an ionic conductor system. An interface has three components the two systems and the surface of separation. The electronic conductor stores one of the required chemicals electrons or wide electronic levels. The ionic conductor stores the other chemical needed for an electrochemical reaction the electroactive substance. A reaction occurs only if both components meet physically at the interface separating the two systems. 

The analysis of thermodynamic data obeying chemical and electrochemical equilibrium is essential in understanding the reactivity of a system to be used for deposition/synthesis of a desired phase prior to moving to experiment and/or implementing complementary kinetic analysis tools. Theoretical and (quasi-)equilibrium data can be summarized in Pourbaix (potential-pH) diagrams, which may provide a comprehensive picture of the electrochemical solution growth system in terms of variables and reaction possibilities under different conditions of pH, redox potential, and/or concentrations of dissolved and electroactive substances. 

Another interesting method of amperometric detection for LC is dualelectrode electrochemical detection. Instead of a single WE, one can place two WEs in series, parallel to or opposite each other. The series configuration is mostly used, mainly in the collection mode, i.e., the electroactive substance entering the detector is converted at the upstream (generator) electrode into a product that either is or is not detected at the downstream (indicator) electrode, depending on the potential of the latter. Hoogvliet et al.137,162 were easily able... 

A more fundamental classification considers the character of the charge transfer between the electrode and the electroactive substance ... 

These processes have various characteristic properties when they occur at metallic or semiconductor electrodes and if they occur between partners (electroactive substances or electrons or holes in the electrode) that are in the ground or the excited state. 

The basic condition for electron transfer in cathodic processes (reduction) to an electroactive substance is that this substance (Ox) be an electron acceptor. It must thus have an unoccupied energy level that can accept an electron from the electrode. The corresponding donor energy level in the electrode must have approximately the same energy as the unoccupied level in the substance Ox. 

On the other hand, in oxidation processes, the electroactive substance Red must have the character of an electron donor. It must contain an occupied level with energy corresponding to that of some unoccupied level in the electrode. Oxidation occurs through transfer of electrons from the electroactive substance to the electrode or through the transfer of holes from the electrode to the electroactive substance. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The oxidized and reduced forms can be dissolved in the electrolyte but can also be present as a solid phase (metal, insoluble compound). In the latter case, the concentration of these electroactive substances is constant and is set by convention equal to unity. The reduced form can also be dissolved in the form of an amalgam in mercury representing the electrode material. 

The above-described theory, which has been extended for the transfer of protons from an oxonium ion to the electrode (see page 353) and some more complicated reactions was applied in only a limited number of cases to interpretation of the experimental data nonetheless, it still represents a basic contribution to the understanding of electrode reactions. More frequently, the empirical values n, k° and a (Eq. 5.2.24) are the final result of the investigation, and still more often only fcconv and cm (cf. Eq. 5.2.49) or the corresponding constant of the Tafel equation (5.2.32) and the reaction order of the electrode reaction with respect to the electroactive substance (Eq. 5.2.4) are determined. 

According to Faraday s law, the current passing through the electrode is equivalent to the material flux of electroactive substances. The disappearance of electroactive substances in the electrode reaction is considered as their transport through the electrode surface. Consequently, only diffusion and migration but not convection flux need be considered at the electrode surface, as the electrode is impenetrable to the solution components. 

In general, for a larger number of electroactive substances, the partial current densities corresponding to the individual substances are additive. 

The double-pulse potentiostatic method (Fig. 5.18C) is suitable for studying the products or intermediates in electrode reactions, formed in the A pulse by means of the B pulse. For example, if an electroactive substance is reduced in pulse A and if pulse B is sufficiently more positive than pulse A, then the product can be reoxidized. The shape of the I-t curve in pulse B can indicate, for example, the degree to which the unstable product of the electrode reaction is changed in a subsequent chemical reaction. 

Radioactive tracer techniques. In electrochemistry, the procedure is essentially the same as in studies of chemical reactions the electroactive substance or medium (solvent, electrolyte) is labelled, the product of the electrode reaction is isolated and its activity is determined, indicating which part of the electroactive substance was incorporated into a given product or which other component of the electrolysed system participated in product formation. Measurement of the exchange current at an amalgam electrode by means of a labelled metal in the amalgam (see page 262) is based on a similar principle. 

Figure 5.37 depicts the stationary distribution of the electroactive substance (the reaction layer) for kc—> oo. The thickness of the reaction layer is defined in an analogous way as the effective diffusion layer thickness (Fig. 2.12). It equals the distance [i of the intersection of the tangent drawn to the concentration curve in the point x = 0 with the line c = cA/K,...

So far, several examples have been given of the inhibition of electrocatalytic processes. This retardation is a result of occupation of the catalyti-cally more active sites by electroinactive components of the electrolyte, preventing interaction of the electroactive substances with these sites. The electrode process can also be inhibited by the formation of oxide layers on the surface and by the adsorption of less active intermediates and also of the products of the electrode process. 

Fig. 5.46 The dependence on time of the instantaneous current / at a dropping mercury electrode in a solution of 0.08 m Co(NH3)6C13 + 0.1 m H2SO4 + 0.5m K2S04 at the electrode potential where -7 -/d (i.e. the influence of diffusion of the electroactive substance is negligible) (1) in the absence of surfactant (2) after addition of 0.08% polyvinyl alcohol. The dashed curve has been calculated according to Eq. (5.7.23). (According to J. Kuta and I.

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

The first step is so fast that it can hardly be measured experimentally, while the second step is much slower (probably as a result of the repulsion of negatively charged species, R and R2-, in the negatively charged diffuse electric layer). The reduction of an isolated benzene ring is very difficult and can occur only indirectly with solvated electrons formed by emission from the electrode into solvents such as some amines (see Section 1.2.3). This is a completely different mechanism than the usual interaction of electrons from the electrode with an electroactive substance. 

The orientation of the electroactive substance (the material undergoing electron-transfer at the electrode) with respect to the electrode surface can very substantially affect its electrochemical reactivity. This ought not be surprising electron transfer is a heterogeneous process, and ought therefore to be substantially dependent upon the exact nature of the contact between the electroactive species and the electrode. Orientational effects ought to be particularly important when the electroactive species is adsorbed upon, and hence in intimate contact with, the electrode surface. What kinds of effects are associated with orientation of substances at an electrode surface Generally what one observes is... 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

Electroactive substance(s), when passing through an electrochemical cell, will be electrolyzed resulting in charge (coulomb) transfer between mobile phase and working electrode. 

In principle, the auxiliary electrode can be of any material since its electrochemical reactivity does not affect the behaviour of the working electrode, which is our prime concern. To ensure that this is the case, the auxiliary electrode must be positioned in such a way that its activity does not generate electroactive substances that can reach the working electrode and interfere with the process under study. For this reason, in some techniques the auxiliary electrode is placed in a separate compartment, by means of sintered glass separators, from the working electrode. 

As in the case of cyclic voltammetry, the electrolysis cell can be built with a thermostatic jacket to carry out measurements at low temperatures. In this case, the apparatus is of an isothermic type (i.e. the compartment containing the reference electrode is also cooled). In this case the most suitable reference electrode is the silver/silver chloride electrode filled by the same solution that will be used to dissolve the electroactive substance. One cannot use the saturated calomel electrode or the aqueous Ag/AgCl electrode because the KC1 (or NaCl) solution would freeze. 

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