Quinone-hydroquinone redox couple

Kinetic Parameters of Quinone, Hydroquinone Redox Couple at a Platinum Interface0... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

D. Note that the oxidation of hydroquinone near 0.3 V is not observed until the second positive-going sweep is made. Also note the rather large separation in peak potentials of the quinone-hydroquinone redox couple. 

The quinone-hydroquinone redox couple built into complexes 124 and 125 fulfils the requirements for the design of a bistable electro-photoswitch both the oxidized and reduced forms are isolable and stable the reduced form 125 is luminescent, whereas the oxidized form 124 is quenched the electrochemical interconversion of the two species is reversible [8.256]. 

Thus, in analogy with the monomer quinone/hydroquinone redox couple, two electrons per monomer unit are assumed to be transferred finally. Examples for lithium metal negative electrodes in combination with PANI are reported in [508-510]. Details of the preparation of PANI positive electrodes are given in the patent literature (e.g., [358, 511-513]). 

A quinone/hydroquinone redox couple is used to test the functioning of platinum electrode. Thereductionreactionofthiscoupleisquinone + 2H+ + 2e = hydroquinone ° = 0.699V. Assume the ratio of the activities to be [quinone]/hydroquinone] = 1. The electrodes are tested in buffer solutions with pH of 7 or 4 containing the redox couple at 25°C. 

C.2 for further discussion of electron-mediated reductions) (Schwarzenbach, et al., 1990 Tratnyek and Macalady, 1989). Quinoid-type compounds are thought to be constituents of natural organic matter (Thurman, 1985 see Chapter l.B.3c). It has been hypothesized that some free radicals in humic substances are quinone-hydroquinone redox couples (Tollin et al., 1963 Steelink and Tollin, 1967). 

The electrochemical processes involved in the quinone-hydroquinone redox couple have recently been examined by Bagotzky et In acid solutions the rate determining step was found to be ... 

Among polymer coated electrodes which have been the object of active investigations during this last decade, particular attention has been paid to the conductive polypyrrole films, obtained by electrooxidation of pyrrole in acetonitrile. Such electrodes have been used to study the electrochemical behaviours of the quinone-hydroquinone redox couple " and tetrathiafulvalene , The controlled release of ferrocyanide from polypyrrole by reduction of the polymer has been demonstrated . As an application, electroinactive anions can be determined using a polypyrrole modified electrochemical detector in flow-injection analysis. Pyrrole can be polymerized from aqueous solutions. Enlarging the modification field, the polymerization step may be preceded by a chemical reaction between pyrrole and another substrate . 

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

The value of E for the quinone-hydroquinone couple is 0.699 V. Look at the list of electrode potentials given in Appendix 3 and decide which redox couples would be powerful enough to oxidize hydroquinone to quinone. 

Note that the anodic peak due to the oxidation of leucoadrenochrome to adrenochrome near 0 V is not seen until the second positive-going potential sweep is made. The voltage separation between the anodic and cathodic peaks for the oxidation of adrenaline (peak B, Fig. 21.4, bottom) and the reduction of adrenalinequinone (peak C) is large when compared to most chemically reversible redox couples. However, this behavior is typical of many quinone-hydroquinone systems on a carbon paste surface at intermediate values of pH. 

Substances undergoing redox reactions (such as quinone-hydroquinone, sulphide-disulphide, metal complexes, redox couples) may serve as electron carriers and allow the coupling of oxidation-reduction processes across membranes (see, for instance, [6.44-6.46]) to cation or anion transport. 

Quinones represent an important class of compound that undergo proton-coupled electron transfer reactions [35]. The order and kinetics of the two-electron/two-proton redox reactions of the quinone (Q)/hydroquinone (H2Q) couple continue to be active subjects of investigation. The interconversion of Q to H2Q can involve up to seven different intermediates depending on the pH of the solution and the solvent. However, in low-pH electrolytes electrochemically reversible behavior can be observed despite the significant changes that accompany redox switching. Beyond... 

This chapter intends to discuss the fundamental role played by carbons, taking particularly into account their nanotexture and surface functionality. The general properties of supercapacitors are reviewed, and the correlation between the double-layer capacitance and the nanoporous texture of carbons is shown. The contribution of pseudocapacitance through pseudofaradaic charge transfer reactions is introduced and developed for carbons with heteroatoms involved in functionalities able to participate to redox couples, e.g., the quinone/hydroquinone pair. Especially, we present carbons obtained by direct carbonization (without any further activation) of appropriate polymeric precursors containing a high amount of heteroatoms. 

Redox systems other than the H2/H+ couple can be used to monitor the potential of the parent metal particles. For example, the quinone-hydroquinone system can be used to keep the electrochemical potential between 0.5 and 0.0 V/NHE by varying the pH from 0 to 7 for solutions of equal concentrations of quinone and of hydroquinonc [57], UPD clearly opens up a vast... 

Extended comparative measurements of the current efficiencies for the photo-oxidation of a large variety of compounds, in competition with the oxygen evolution, have essentially confirmed the same general tendency for the decrease of the oxidation rate with increasing the standard potential of the redox couple -The highest current efficiency for the competitive photo-oxidation at Ti02 in add solutions has been reported for hydroquinone - which (i.e., the quinone/hydroquinone couple) has the standard redox potential slightly more positive than that of the I2/I couple. 

Brunmark and Cadenas (27A15) reviewed the major mechanisms that are involved in quinone-induced cytotoxicity in 1989. The redox chemistry of quinoid compounds was surveyed in terms of (1) reactions involving only electron transfers, such as those accomplished during the enzymatic reduction of quinones and nonenzymatic interaction with redox couples generating semiquinones, and (2) nucleophilic addition reactions. In their explanation of the mechanisms involved, quinone is reduced to the hydroquinone or semiqui-none radical by cellular reductase. The semiquinone radical then undergoes rapid autooxidation with the generation of the parent quinone and concomitant formation of superoxide. The hydroquinone reacts rapidly with superoxide to form H2O2 and the semiquinone. 

The use of carbon as a catalyst was reviewed. It was shown that interesting activity correlations could be obtained by studying the catalytic performance of a series of carbon materials prepared from the same precursor with similar tex-mral properties and different amounts of surface functional groups. The redox couple quinone-hydroquinone was found to be involved in the oxidative dehydrogenation of hydrocarbons, while carboxylic acid groups are the active sites for the dehydration of alcohols. In both cases, thermal treatments at different temperatures were used to identify the nature of the active sites, aud correlations... 

These results may indicate the kinetic dependence of induction time wherein the time is lowered with an increase in reaction temperature. Further, it has been earlier envisaged by Germain et al. [106] that addition of catechol or hydroquinone removes the induction period for faujasitc KAU 2,5 (Si/AI = 2.5) and this was attributed to initial oxidation of the additive in generating an autocatalytic quinonic redox couple. However, when we added catechol (20 1 mole ratio of phcnohcatechol) to the initial reaction mixture (in an anticipation to sec whether it has any influence on the induction time), no reduction in the induction time was... 

In an alternative approach to mimic tyrosinase activity a copper(I)-copper(n) redox couple and a hydroquinone-quinone redox couple were incorporated in one complex (scheme 17). The hydroquinone moiety should act as an electron shunt between an external reducing agent, i.e. ascorbic acid, zinc or electrochemical reduction, and the copper ions. Catalytic oxygenation by monooxygenases is usually accompanied by the formation of water, with the aid of an external electron and proton source.35 46 Activation of O2 by dinuclear copper(I) complex 58 results in superoxo- or p-peroxo-dicopper(II) complex 59, which oxygenates an external substrate molecule. Internal electron transfer to quinone dicopper(II) complex 60 is followed by quinone to hydroquinone reduction. The electron transfer system shown here is reminiscent of the quinone based systems found in the primary photochemical step of bacterial photosynthesis, and in (metallo)porph3nin-quinone electron transfer systems.In contrast to expectation, the hydroquinone dinuclear copper(II) complex 60 (L = (2-pyridylethyl)formidoyl, scheme 17), designed to mimic step c in this cycle, is a stable system in which the hydroquinone moiety is not oxidized to a quinone structure 61. 

Accompanying the oxidation of carbon support, the double layer capacitance of the catalyst layer increases gradually, and a new redox process may appear at ca. 0.6 V from quinone/hydroquinone couple. 

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