Redox reactions identification

For many redox reactions, such as those involving oxygen or other highly electronegative elements, the substances being oxidized and reduced can be identified easily by inspection. For other redox reactions, identification is more difficult. For example, consider the redox reaction between carbon and sulfur. 

Linschitz and Rennert (80) showed that the chlorophyll photobleaching reaction was not restricted to liquid systems. They observed a similar rapid reversible photobleaching of chlorophyll a in a solid solution of ether iso pentane alcohol at liquid nitrogen temperature. A mechanism involving either electron or hydrogen transfer was postulated as the initial step. Although further identification of the intermediates is necessary, it appears likely that a one-electron redox reaction is at least involved in the primary process. 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

On the reducing side of PS II, the primary acceptor (Qa had been considered the primary acceptor until pheophytin was discovered to precede it) is reduced in less than 400 ps by Pheo . The reduction of Qa is conveniently monitored by the increase of PS II fluorescence from an initial value, Fq, to a maximal level, indicative of the steady-state level of Q /Qa- K reoxidation of Qa is prevented by the specific inhibitor DCMU (or other herbicides having the same effect), the fluorescence yield of PS II increases sharply, because Qa becomes fully reduced. The reduced form is an anion semiquinone (see the review by Cramer and Crofts [36]), and the absorption spectrum of this compound with a maximum at 326 nm serves for its identification [19] and offers an alternative method for kinetic studies of Qa redox reactions (see Ref. 37 for review). 

The oxidation number is a hypothetical charge assigned to atoms in molecules and ions using a set of specific rules. Since redox reactions involve transfers of electrons, identification of the atoms which change oxidation number will show the atoms, ions or molecules which are specifically involved in the redox process. 

DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species, but differ in their identification of the nature of the adsorbed reactive ammonia (protonated ammonia vs. molecularly coordinated ammonia), of the active sites (Br0nsted vs. Lewis sites) and of the associated reaction intermediates [16,17]. Concerning the mechanism of SO2 oxidation over DeNOxing catalysts, few systematic studies have been reported up to now. Svachula et al. [18] have proposed a redox reaction mechanism based on the assumption of surface vanadyl sulfates as the active sites, in line with the consolidated picture of active sites in commercial sulfuric acid catalysts [19]. Such a mechanism can explain the observed effects of operating conditions, feed composition, and catalyst design parameters on the SO2 SO3 reaction over metal-oxide-based SCR catalysts. 

Recently, electron spin resonance (ESR) and other spectroscopic techniques have been applied to the study of redox reactions of Fe and Mn oxides (Kung and McBride, 1988 McBride, 1989a,b). Identification and quantification of adsorbed reactants, intermediates, and products by spectroscopic techniques could substantially improve our understanding of surface chemical reaction mechanisms. 

Along this line, the combination of electrochemistry and IR spectroscopy has been extremely successful in the identification and unraveling of molecular mechanisms of biological electron transfer and of catalysis coupled to redox reactions. 

The occurrence of a redox reaction in the oil/water system is primarily established analytically by identification of the reaction products, often contained in different phases. Other indirect data, however, are also used, such as changes in the interfacial tension at the interface between immiscible hquids [22] or in the Volta potential [22-29]. A change in the transmembrane potential may also be an indication of a redox reaction at the membrane/electrolyte interface [6, 7, 26-28]. 

Actually, a redox reaction takes place. It is due to the reducing properties of para and ortho diphenols, stmctures that were masked in a-tocopherol. Hence, the driving force of this particular reaction is the formation of a quinone. It is sufficiently strong to induce the break of the ether-oxide bond of pyran. This is remarkable. The identification reaction of a-tocopherol is called Emmeri-Engel s reaction. 

In this chapter, the reactions of metal ions in a high oxidation state with inorganic and organic substrates are discussed. Such investigations have provided much mechanistic data and an increasingly important aspect is the evidence for inner-sphere reactions with the formation of metal-ion complex intermediates. The question of replacement as a prerequisite to redox reactions has been discussed and the role of the intermediate defined. It is of interest to note that identification of an intermediate does not necessarily infer its involvement in the rate-determining process, in that, for the reactions... 

Perhaps the simplest of these techniques are the potentiostatic photocurrent transients (79) that were shown to be sensitive to the semiconductor electrodes down to 1 ns. (80) and below (81). Often the time resolution is limited by the RC of the system and the technique is most valuable in the longer time scales for identification of intermediates and products of photo redox reactions (79). The interpretation of the data follows the routine in some of the methods that we have explored to interpret impedance data, i.e., assume an equivalent circuit and analyze the decay as a superposition of exponential decays where the time constants are correlated with the elements of the equivalent circuit (79)(80)(82). The time constant that was associated with the space charge layer was in reasonable agreement with the Mott-Schottky data (79)(80). The time-scale of the predicted response (83) is much faster than the one observed by the authors of Ref. 79, but the much faster resolution reported in Ref. 81 was in agreement with the time-dependent version of Gartner s model. Etching wasfoundtohavealargeeffecton the amplitude and decay time of the transients (82). This method was also applied to the study of dye sensitization and the role of a super sensitizers in these systems (84). 

Organic sulfur compounds are present in gasoline and diesel. With the increased emphasis on the requirement for more environmentally friendly transportation fuels [1], oxidative desulfurization, using H202 and redox-molecular sieves [2,5,6,7], has been studied and shown to significantly reduce the sulfur content of gasoline and diesel. The reaction of thiophene and its derivatives were successfully converted to oxidized compounds, but the identification of oxidized compounds was not simple because the concentrations of individual sulfur compounds were low. Most of the previous literature has reported sulfone formation. 

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

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