Redox system reduction

Generally the oxidant is compounded in one part of the adhesive, and the reductant in the other. Redox initiation and cure occur when the two sides of the adhesive are mixed. There also exist the one-part aerobic adhesives, which use atmospheric oxygen as the oxidant. The chemistry of the specific redox systems commonly used in adhesives will be discussed later. The rates of initiation and propagation are given by the following equations ([9] p. 221). 

The initial radicals formed from the Ce(IV) ion redox system can initiate a monomer to polymerize and form an end group of the resulting polymer. When the reductant exhibits a carbonyl group, the amide group can be conveniently detected by the FT-IR spectrum of the polymer, such as polyacrylonitrile (PAN). The FT-... 

The next step is the insertion of a lattice oxygen into the allylic species. This creates oxide-deficient sites on the catalyst surface accompanied hy a reduction of the metal. The reduced catalyst is then reoxidized hy adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system. ... 

Oxidation-reduction (redox) Inert metal (normally Pt but certain other metals can act in a similar manner) in a solution containing two species that give rise to a redox system. E depends on of the system and the relative activities of the oxidised and reduced forms. Quinone-hydroquinone QH4O2 -1- 2H+ -1- 2e-CjH4(OH)2, which is thus pH dependent Fe - -/Fe + Mn04-/Mn +... 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

Formation of the ethano-dimer of a-tocopherol (12) by reduction of spiro dimer (9) proceeds readily almost independently of the reductant used. This reduction step can also be performed by tocopheroxyl radicals as occurring upon treatment of tocopherol with high concentrations of radical initiators (see Fig. 6.10). The ready reduction can be explained by the energy gain upon rearomatization of the cyclohexadienone system. Since the reverse process, oxidation from 12 to 9 by various oxidants, proceeds also quantitatively, spiro dimer 9 and ethano-dimer 12 can be regarded as a reversible redox system (Fig. 6.22). 

The redox system consists of pyrene or 9,10-phenanthrene quinone as oxidant and an alkyl ester of 3,3, 3"-nitrilopropionic acid as reductant.121 This system deactivates oxidation by bisimidazole when irradiated at 380-550nm, since the quinone is reduced to hydroquinone and thus stabilizing the previously generated dye image.122,123... 

This method K4 yields a reduction or cathodic additional current owing to the pulse increase with a current maximum around E analogously, K3 yields an oxidation or anodic additional current owing to the pulse decrease with a current minimum around E. Both additional currents are depicted in Fig. 3.52 for a completely reversible redox system the cathodic maximum and the anodic... 

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

The formal potential of a reduction-oxidation electrode is defined as the equilibrium potential at the unit concentration ratio of the oxidized and reduced forms of the given redox system (the actual concentrations of these two forms should not be too low). If, in addition to the concentrations of the reduced and oxidized forms, the Nernst equation also contains the concentration of some other species, then this concentration must equal unity. This is mostly the concentration of hydrogen ions. If the concentration of some species appearing in the Nernst equation is not equal to unity, then it must be precisely specified and the term apparent formal potential is then employed to designate the potential of this electrode. 

Figure 12.21 Relationships between reduction potential and pH for various redox systems and dye classes [214]...

The essence of the technique is as follows [239]. The cathode reduces the mediator which then reduces the dye. This mediator (the regenerable redox system) must continually produce a consistent reduction potential in the dye liquor, so that no reducing agent has to be added. The prevailing potential is defined by the Nemst Equation (12.2) ... 

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

Figure 2.87 Schematic of the cyclic voltammogram expected from a reversible electrochemical redox system 0 + e + R having a standard reduction potential °. E is the potential of the working electrode, and I the current.

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