Product properties, electroactive

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

At the electrode surface there is competition among many reduction reactions, the rates of which depend on iQ and overpotential q for each process. Both /0 and q depend on the concentration of the electroactive materials (and on the catalytic properties of the carbon surface). However, the chemical composition of the SEI is also influenced by the solubility of the reduction products. As a result, the voltage at... 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Since the invention of d.c. polarography [10, 11], numerous inorganic and organic compounds have been studied by means of this method in Heyrovsky s school and extensive knowledge gathered about the electrochemical properties of these compounds. Among them, many cases were discovered where the polarographic wave appeared to be influenced by the existence of chemical equilibria between the electroactive substance and other, in most cases electroinactive, species in the electrolyte solution, more particularly by the finite rate at which these equilibria relax after the electrochemical perturbation. In fact, the chemical reaction serves as either a source or a sink to deliver or to consume the electroactive reactants and products, in addition to diffusion. 

An operational definition of thin-layer electrochemistry is that area of electrochemical endeavor in which special advantage is taken of restricting the diffii-sional field of electroactive species and products. Typically, the solution under study is confined to a well-defined layer, less than 0.2 mm thick, trapped between an electrode and an inert barrier, between two electrodes, or between two inert barriers with an electrode between. Diffusion under this restricted condition has been described in Chapter 2 (Sec. II.C). Solution trapped in a porous-bed electrode will have qualitatively similar electrochemical properties however, geometric complexities make this configuration less useful for analytical purposes. The variety of electrical excitation signals applicable to thin-layer electrochemical work is large. Three reviews of the subject have appeared [28-30]. 

Besides charge transfer, the transport of electroactive substance towards the electrode and the transport of reaction product away from the electrode also play an important role3. It is clear that an electrochemical reaction can only occur if the electroactive species is in the vicinity of the electrode surface. However, due to the continuous transformation of electroactive species, this also means that fresh species should be transported towards the electrode surface and that the formed reaction product must be transported away from the surface. The transport conditions and properties will be discussed more in detail in section 1.7. 

The trimethylsilyloxy (TMSO) group is stable under the coupling conditions in acetonitrile (Table 4, number 11). After oxidative dimerization the TMS ether can be mildly hydrolyzed (H and H2O) to the phenol or converted to a dibenzofuran. 1,2-Dialkoxybenzenes have been trimerized to triphenylenes (Table 4, numbers 9, 12, and 13). The reaction product is the triphenylene radical cation, which is reduced to the final product either by zinc powder or in a flow cell consisting of a porous anode and cathode [60]. Dibenzo-crown ethers are converted by anodic oxidation to electroactive polymers. Films of these polytriphenylenes exhibit unusual doping properties 62-64]. 

When the follow-up reaction leads to an electroactive product, the effects of the redox properties of the product (Z, above) on the current-potential curves also must be considered. For more than one electroactive product, the nomenclature for the model system changes from and to A and B, below, each compound having its own E (standard electrode potential) value ... 

Once formed, these colloids may be used for further processing. For example, they can be mixed with paints.22 The products formed are as conductive as those obtained when carbon black is added to paints. However, they are also electroactive and presumably (at least the colloidal component) retain the unique chemical properties of the conducting polymer. 

Voltammetric techniques that can be applied in the stripping step are staircase, pulse, differential pulse, and square-wave voltammetry. Each of them has been described in detail in previous chapters. Their common characteristic is a bell-shaped form of the response caused by the definite amount of accumulated substance. Staircase voltammetry is provided by computer-controlled instruments as a substitution for the classical linear scan voltammetry [102]. Normal pulse stripping voltammetry is sometimes called reverse pulse voltammetry. Its favorable property is the re-plating of the electroactive substance in between the pulses [103]. Differential pulse voltammetry has the most rigorously discriminating capacitive current, whereas square-wave voltammetry is the fastest stripping technique. All four techniques are insensitive to fast and reversible surface reactions in which both the reactant and product are immobilized on the electrode surface [104,105]. In all techniques mentioned above, the maximum response, or the peak current, depends linearly on the surface, or volume, concentration of the accumulated substance. The factor of this linear proportionality is the amperometric constant of the voltammetric technique. It determines the sensitivity of the method. The lowest detectable concentration of the analyte depends on the smallest peak current that can be reliably measured and on the efficacy of accumulation. For instance, in linear scan voltammetry of the reversible surface reaction i ads + ne Pads, the peak current is [52]... 

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