Redox coupling reactions

The applicability of the foregoing procednre has been tested by modeling simple reaction under semi-infinite diffusion conditions (reaction 1.1) and EC mechanism coupled to adsorption of the redox couple (reaction (2.177)) [2]. The solutions derived by the original and modified step-function method have been compared in order to evaluate the error involved by the proposed modification. As expected, the precision of the modified step-function method depends solely on the value of p, i.e., the number of time subintervals. For instance, for the complex EC mechanism, the error was less than 2% for p>20. This slight modification of the mathematical procedure has opened the gate toward modeling of very complex electrode mechanisms such as those coupled to adsorption equilibria and regenerative catalytic reactions [2] and various mechanisms in thin-film voltammetry [5-7]. 

Tp NbI(CO)(PhC=CMe)(RC=N) (Scheme 47).621 The assigned formal electron counts on the alkyne and nitrile ligands are compatible with detailed 13C NMR data. Electrochemical studies indicated that an equilibrium existed in solution between the f72(3e)-alkyne/ 72(3e)-nitrile and 72(4e)-alkyne/771(2e)-nitrile complexes.50 PhCN is displaced by PMe2Ph or PhC=CMe to provide Tp Nb(CO)(PhC=CMe)(PMe2Ph) and Tp Nb(CO)(PhC=CMe)2, respectively. Protonation of these species can induce intramolecular redox coupling reactions to produce Nbv metallocycles (e.g., Scheme 47). 

All redox reactions involve an oxidizing agent (a substance that accepts or takes electrons) and a reducing agent (a substance that gives or donates electrons). For example, to go forward, the redox couple reaction... 

The water stability boundaries and the locus of measured Eh and pH measurements in natural waters, as reported by Baas-Becking et al. (I960), are shown in Fig. 11.3 (see also Fig. 11.4). It has been observed that frequently the Eh values measured with a Pt electrode differ significantly from values computed from Gibbs free energies or standard potentials and solution concentrations. When they exist, there are two important reasons for such differences. These include (1) misbehavior of the Pt or other indicator electrode (2) the irreversibility or slow kinetics of most redox couple reactions and resultant di.sequilibrium between and among different redox couples in the same water and (3) the common existence of mixed potentials in natural waters (see below). 

The technique has been described in detail elsewhere. [26] In short, a pulse of high energy electrons induces a series of chemical reactions that can be monitored, e.g., using time resolved UV-vis spectroscopy. The reaction of interest is usually induced by a reaction between a radical formed from radiolysis of the solvent (usually water) and a solute molecule. The primary radiolysis products in aqueous solution are HO, e q", H, HjOj, H2 and The major radical species, HO and e q, are formed in equimolar concentrations, 0.28 ol/J each, on electron or y-irradiation.[27] As can be seen in reaction 2, the hydroxyl radical does not yield a benzene radical cation instantly upon reaction with a substituted benzene. For this reason, secondary oxidants, such as S04, Brj and N3, are usually used to generate benzene radical cations. To determine one-electron reduction potentials of radical cations, the redox equilibrium between the radical cation of interest and a redox couple with a known one-electron reduction potential is studied. The equilibrium constant can be derived from the rate constants of the electron-transfer reaction and the back reaction and/or the equilibrium concentrations of the two redox couples (reaction 6).[28]... 

Redox couple Reaction equation when redox couple is reduced at 25 C/V... 

Several types of redox reactions can produce pseudocapacitance, for example, surface underpotential deposition, lithium intercalation, bulk redox couple reactions, and electrically conducting polymers. In the following sections, we will discuss these reactions and their corresponding pseudocapacitances. 

Further studies on the dynamic interactions of polyphenols with physiological compoimds endowed with antioxidant activity showed that the polyphenols may be more intricately involved with physiologically relevant antioxidant mechanisms. Using continuous-flow EPR measurement, Laranjinha and Cadenas (1999) have demonstrated that the caffeic acid-derived o-semiquinone radical formed upon regeneration of a-TOH ifom a-tocopheroxyl radical may be reduced back to caffeic by ascorbate. Therefore, a sequence of redox-coupled reactions can be envisage whereby the radical character is sequentially transferred from lipid phases to the aqueous medium through the one-electron reduction of tocopheroxyl radical by caffeic acid and, in turn, of the caffeic acid radical by ascorbate. This sequence amplifies the antioxidant effects of individual compounds in lipid structures such as LDL (Laranjinha Cadenas, 1999). 

When redox couples are employed the rate of free radical generation is increased compared to that provided by thermal generation at the same temperature. Therefore, when using Redox couples reaction temperatures as low as 30°C are not uncommon. 

VFB electrode materials currently adapted can be divided into two types (1) metal and (2) carbon materials. A range of metal electrode materials such as Pb, Au, Pt, platinised titanium (Pt-Ti) and iridium oxide dimensionally stable electrodes (DSAs) have been evaluated for their suitability as positive electrodes in VFBs [41]. It was foimd that the electrochemical reversibility for the VO /V02 redox couple was not sufficient on the Au electrode. The Pb and Ti electrodes were easily passivated in the potential range, where the /V02 redox couple reactions occur. The passivation film formed on the surface would increase the electric resistance. The growth of the passivation... 

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

The ladder diagram for this system is shown in Figure 11.24a. Initially the potential of the working electrode remains nearly constant at a level near the standard-state potential for the Fe UFe redox couple. As the concentration of Fe + decreases, however, the potential of the working electrode shifts toward more positive values until another oxidation reaction can provide the necessary current. Thus, in this case the potential eventually increases to a level at which the oxidation of H2O occurs. 

Electrochemical Reversibility and Determination of m In deriving a relationship between 1/2 and the standard-state potential for a redox couple (11.41), we noted that the redox reaction must be reversible. How can we tell if a redox reaction is reversible from its voltammogram For a reversible reaction, equation 11.40 describes the voltammogram. 

Oxidation Reactions. Potassium permanganate is a versatile oxidizing agent characterized by a high standard electrode potential that can be used under a wide range of reaction conditions (100,133—141). The permanganate ion can participate in a reaction in any of three distinct redox couples. 

A closer analysis of die equilibrium products of the 1 1 mixture of methane and steam shows the presence of hydrocarbons as minor constituents. Experimental results for die coupling reaction show that the yield of hydrocarbons is dependent on the redox properties of the oxide catalyst, and the oxygen potential of the gas phase, as well as die temperamre and total pressure. In any substantial oxygen mole fraction in the gas, the predominant reaction is the formation of CO and the coupling reaction is a minor one. 

An electron transfer reaction may be separated into two half-reactions or redox couples so that the free energy, AG°, can be separated into AGa and AGg the free energies of reduction of the donor (D) and the acceptor (A), respectively, by... 

The cure reaction of structural acrylic adhesives can be started by any of a great number of redox reactions. One commonly used redox couple is the reaction of benzoyl peroxide (BPO) with tertiary aromatic amines. Pure BPO is hazardous when dry [39]. It is susceptible to explosion from shock, friction or heat, and has an autoignition temperature of 79°C. Water is a very effective stabilizer for BPO, and so the initiator is often available as a paste or a moist solid [40], The... 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

An oxidation half-reaction is a conceptual way of reporting an oxidation the electrons are never actually free. In an equation for an oxidation ha If-reaction, the electrons released always appear on the right of the arrow. Their state is not given, because they are in transit and do not have a definite physical state. The reduced and oxidized species in a half-reaction jointly form a redox couple. In this example, the redox couple consists of Zn2+ and Zn, and is denoted Zn2+/Zn. A redox couple has the form Ox/Red, where Ox is the oxidized form of the species and Red is the reduced form. 

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