Redox couple, soluble

Historically, indicator electrodes have been metals which form a redox couple with the analyte, such as a Ag electrode for the determination of Ag", or a chemically inert metal which responds to the activity ratio of a soluble redox couple, such as a Pt electrode for Fe /Fe. Whereas simple indicator electrodes of this type perform well for the analysis of relatively pure samples, they are often subjwt to interferen< when apphed to complex samples such as those of biological origin. 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

This equation applies to all reversible electrode reactions with soluble ox and red, so it includes cathodic chronopotentiometry of ions of amalgamating metals such as Cd2+, Cu2+, Pb2+ and Zn2+ at stationary Hg electrodes. For a redox couple such as Fe3+/Fe2+ the diffusion coefficients DTeA and Dox will not differ much, so that Etl4 is approximately equal to E° (770 V). 

Analogously to eqn. 3.72 for stationary electrodes and a reversible redox couple of soluble ox and red, Kies derived for chronopotentiometry at a dme via insertion in the Nernst equation... 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

There are two thermodynamic factors that must be considered. The first thing to remember is that the product of the redox reaction must be. soluble and electroactive. This is an obvious point, since both the oxidized and reduced forms of the titrant need to comprise a redox couple. Ceric ion, is therefore a good choice of oxidant, since Ce and its Ce ... 

To maximize the current limit that could be shunted by redox additives so that the occurrence of such irreversible processes due to overflowing current could be more efficiently suppressed, the redox additive apparently should be present in the electrolyte at high concentrations, and both its oxidized and reduced forms should be very mobile species. Where the criteria for selecting potential redox additives are concerned, these requirements can be translated into higher solubility in nonaqueous media and lower molecular weight. In addition to solubility and diffusion coefficients, the following requirements should also be met by the potential redox additives (1) the formal potential of the redox couple [R]/[0] should be lower than the onset potential for major decom-... 

The corrosion inhibitor can also be a redox couple presenting a reversible and fast electrochemical behavior that is able to react in place of the metal. This is obtained when its redox couple potential is lower than that of the considered metal. The reversible behavior allows the continuous regeneration of the corrosion inhibitor. These reducing agents are often organic compounds soluble in aqueous solutions. A nonexhaustive list is given in Ref. [5]. 

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

Many anodic and cathodic conversions are not mediated by soluble redox couples or by redox coatings but are essentially initiated by a one-electron charge transfer between the electrode and the organic molecule-generating radical ions or radicals as first, reactive intermediates ... 

Let us now add some soluble redox couple, such as a mixture of Fe2+ and Fe3+ in approximately 10-3 M concentrations and record the current-voltage curve again while stirring (Curve B). This redox couple is defined by the reaction shown in (5.6). 

Another form of redox reference electrode is similar to the electrode of the first kind. In this case the inert metal (e.g., Pt, Au, or C) is used as the inner electrode and a stable and soluble redox couple is placed inside the inner reference electrode compartment. A normal liquid junction is used in this type of reference electrode. Unlike the electrode of the first kind, the redox reference electrode is relatively immune to changes in concentration inside the reference electrode compartment because it is the ratio of the reduced/oxidized form of redox couple that determines the potential and not the absolute concentrations. However, redox reference electrodes are sensitive to changes of concentration of oxygen and other redox species. 

Let us consider this regime for the electrochemical reaction between the oxidized (O) and reduced (R) form of a fast redox couple, when both O and R are soluble and only O is initially present in the solution. The redox couple is... 

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). 

A number of redox couples at platinum electrodes (with both species soluble in the solvent) have been important to the establishment of a relation between potential scales in different solvents. Although they are useful in the comparison of redox potentials, they appear to have no advantages over the silver-silver ion system in the preparation of practical reference electrodes. 

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