Redox reactions in nonaqueous solvents

The analytical chemistry of redox reactions in nonaqueous solvents has received less attention than acid-base reactions in these solvents. It should be a fruitful subject for future study. Thus far the Karl Fischer titration for water has been the most... 

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

Redox-Mediated Metal Deposition. A reduced polyimide surface can function as a reducing substrate for subsequent deposition of metal ions from solution. For metal reduction to occur at a polymer surface, the electron transfer reaction must be kinetically uninhibited and thermodynamically favored, i.e., the reduction potential of the dissolved metal complex must be more positive than the oxidation potential of the reduced film. Redox-mediated metal deposition results in oxidation of the polymer film back to the original neutral state. The reduction and oxidation peak potential values for different metal complexes and metal deposits in nonaqueous solvents as measured by cyclic voltammetry are listed in Table III. 

We observed the redox reactions of different PEO-modified proteins dissolved in other organic solvents as well as in PEO [11-13]. PEO modification had already proved to be an excellent way to make proteins soluble in nonaqueous solvents... 

The rather narrow electrochemical window of water, limited by the discharge of hydrogen and oxygen, has stimulated the use of nonaqueous solvents for electrochemical reactions. Procedures for measuring and reporting electrode potentials in nonaqueous solvents are presented in reference [128]. The solvent influence on the redox properties of cations and anions has been reviewed [172], as have applications of ion-selective electrodes in nonaqueous solvents [129] and the influence of nonaqueous solvents on the polarographic half-wave potentials of cations [173]. 

Most titrations are carried out in aqueous solution, including all those described above. In some circumstances, however, it is advantageous to use other solvents, especially organic solvents. Such nonaqueous titrations are normally used for acid-base reactions, but redox reactions may also be applicable. The Karl-Fischer titration of water, in particular, is based upon redox reactions in a nonaqueous medium. 

The first successful Heyrovsky Discussions were followed by further discussions in the 1970s and 1980s. Their topics were Products and intermediates of electrode reactions (1970), Products and intermediates of redox reactions (1971), New principles in electroanalytical chemistry (1972), Deposition and oxidation of metals (1973), Electrochemistry in nonaqueous solvents (1974), Electrochemical phenomena in biological systems (1975), Redox reactions of coordination compounds (1976), New horizons of polarography (1977), Electrochemical energy conversion expectations, achievements and critical assessment of perspectives (1978),... 

The use of organic solvents as reaction media for biocatalytic reactions can not only overcome the substrate solubility issue, but also facilitate the recovery of products and biocatalysts as well. This technique has been widely employed in the case of lipases, but scarcely applied for biocatalytic reduction processes, due to the rapid inactivation and poor stability of redox enzymes in organic solvents. Furthermore, all the advantages for nonaqueous biocatalysis can take effect only if the problem of cofactor dependence is also solved. Thus, bioreductions in micro- or nonaqueous organic media are generally restricted to those with substrate-coupled cofactor regeneration. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

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]. 

Almost all of the reactions that the practicing inotganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents sud) as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to Ihe inoiganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmctal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the (Jsanovich definition of acid-base reactions. 

Nonaqueous enzymatic redox reactions have been limitedby stability owing to solvents and highly reactive substrates (H202). Here we have shown evidence of methods to alleviate these concerns for reactions with CPO. In experimental systems, the in situ production of H202 by GOx was shown to function equally well and more reproducibly than added H202. In situ production is experimentally easier and prevents enzyme deactivation owing to high peroxide levels. GOx was more solvent stable than CPO therefore, the GOx system may be useful for this and other redox systems. 

The properties of all other hydrogen hahdes are far removed from those of AHF and, therefore, their use as nonaqueous solvents is rather limited. Nevertheless, they are used for physical studies of solutions and for some synthetic purposes, such as in the formation of salts with HCl2 or BCU and related anions. Anhydrous HX can be considered to undergo self-ionization (equation 90) and can therefore be used to perform reactions of an acid-base nature, and solvolytic and redox reactions (equations 91-93). 

A reference electrode is needed to provide a potential scale for E° valnes as all voltages are relative. Any electrochemical reaction with a stable, well known potential can be nsed as a reference electrode. The NHE or standard hydrogen electrode (SHE) (Pt/H2,1.0 M H+) was the first well known reference electrode and is used as a reference in most tables of redox potentials. An NHE is difficult to construct and operate and therefore, is not typically used experimentally. Since the NHE is widely accepted, potentials are still often referenced to the NHE, converted from other reference electrodes. For aqueous solvents the SCE (Hg/Hg2Cl2 (KCl)) and the silver/silver chloride (Ag/AgCl) electrode are now commonly used as reference electrodes. To convert from the SCE to the NHE, E (vs. NHE) = E (vs. SCE) + 0.24 V. For nonaqueous solvents the silver/silver nitrate (Ag/AgNOs) reference electrode is often used. A pseudo-reference electrode can also serve as a reference point for aqueous or nonaqueous solutions. A silver or platinum wire can be used as a... 

In the following section some pulse radiolysis investigations are described in which the radiolysis of nonaqueous solvents has been used to generate one-electron redox reagents and, in some cases, to characterize their involvement in electron-transfer reactions. 

We can write equilibrium constants for many types of chemical processes. Some of these equilibria are listed in Table 6.1. The equilibria may represent dissociation (acid/base, solubility), formation of products (complexes), reactions (redox), a distribution between two phases (water and nonaqueous solvent—solvent extraction adsorption from water onto a surface, as in chromatography, etc.). We will describe some of these equilibria below and in later chapters. 

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