Electrodes, oxidation-reduction volume

In oxidation-reduction titrations, an electrode potential related to the concentration ratio between the oxidized and reduced forms of either of the reactants is determined as a function of the titrant volume. The indicator electrode must be responsive to at least one of the couples involved in the reaction. Indicator electrodes for oxidation-reduction titrations are generally constructed from platinum, gold, mercury, or palladium. The metal chosen must be unreactive with respect to the components of the reaction. The indicator metal is merely a medium for electron transfer. 

For SERS spectroscopy of biomolecules [31], the use of small sample volumes is mandatory. For this purpose, two spectroelectrochemical cells were constructed in which the hquid sample is held in a thin cylindrical volume between a silver rod electrode and the optical access window. The design of the cells (from glass or teflon) and the oxidation-reduction cycle, used for the required roughening of the silver, have been described in detail by Zimmermaim [32]. 

The situation illustrated in Figure 4 allows both species to coexist. Either of the two sets of curves can be considered the oxidized species the other is the reduced species. The choice depends on whether oxidation or reduction is occurring at the surface. Assume the upper curve is the reduced species and the lower curve is its oxidized form. An appHed voltage has maintained fixed surface concentrations for some period of time including and The concentration profile of the oxidized species decreases at the electrode surface (0 distance) as it is being reduced. Electrolysis therefore results in an increase in the concentration of reduced species at the surface. The concentration profiles approach bulk values far from the surface of the electrode because electrolysis for short times at small electrodes cannot significantly affect the concentrations of species in large volumes of solution. 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

The electrochemical detector in the form described above is extremely sensitive but suffers from a number of drawbacks. Firstly, the mobile phase must be extremely pure and in particular free of oxygen and metal ions. A more serious problem arises, however, from the adsorption of the oxidation or reduction products on the surface of the working electrode. The consequent electrode contamination requires that the electrode system must be frequently calibrated to ensure accurate quantitative analysis. Ultimately, the detector must be dissembled and cleaned, usually by a mechanical abrasion procedure. Much effort has been put into reducing this contamination problem but, although diminished, the problem has not been completely eliminated particularly in the amperometric form of operation. Due to potentially low sensing volume the detector is very suitable for use with small bore columns. 

Furthermore, the electrode material must be reasonably stable and not change its volume as a result of reduction or oxidation because such a volume change may cause the electrode to peel off the electrolyte. Also, the thermal expansion coefficient (TEC) of the electrode material must be close to the electrolyte material. Thus, in order to select a proper composition of the doped ceria for a specific electrode application, it is necessary to have knowledge of the ceria chemistry and its relation to the thermal, crystallograhical and electrical properties of the doped ceria. Therefore, the next sections briefly describe the ceria chemistry and the related properties. 

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