Standard potential: of redox

Table 5 Standard potentials of redox couples Nu /Nu of interest in inverted spin trapping under oxidizing conditions."...

Table 5.2. Standard Potential of Redox Couples of Alkanes in HF (at H0 22.1) and Acidity Levels of Oxidation (pHR in HF).79...

The driving force for an ET reaction at the ITIES consists of two components, i.e., the difference of standard potentials of redox mediators and the interfacial potential drop [Eq. (14)]. The dependence of kf on AE° was studied for reactions between ZnPor+ in benzene and the series of similar cyanide complexes in water (26) ... 

Tetraalkylborate anion is oxidized into tetraalkylborate radical at 0.60 V (Scheme 12), which is then transformed into Bu that can react with 1,3,5-trinitrobenzene. The resulting radical species eliminate a proton, thus giving the corresponding nitroaromatic radical-anion. The latter is oxidized by the cyclohexadienyl radical (according to the standard potentials of redox pairs) [38, 66]. This reaction is somewhat similar to the termination step in the SrnI aromatic substitution reactions [64,67,68]. When the electrolysis is carried out at 1.06 V, the o -complexes are oxidized (path A, Scheme 12), as well as tetraalkylborate anions (path B, Scheme 12). The generation of Bu allows to improve yields of the Sn products, but Bu can also attack the Sn product to form dialkyl trinitrobenzene. This process appears to be more important at the final stage of the reaction, when... 

Fig. 2.5 Schematic representation of a cell permitting the determination of standard potentials of redox couples...

Standard Potentials of Redox Systems and Metal/Metal Ion Electrodes... 

It is evident that the abrupt change of the potential in the neighbourhood of the equivalence point is dependent upon the standard potentials of the two oxidation-reduction systems that are involved, and therefore upon the equilibrium constant of the reaction it is independent of the concentrations unless these are extremely small. The change in redox potential for a number of typical oxidation-reduction systems is exhibited graphically in Fig. 10.15. For the MnO, Mn2+ system and others which are dependent upon the pH of the... 

In redox couple notation, E°(HJ"/H2) = 0 at all temperatures. A hydrogen electrode in its standard state, with hydrogen gas at 1 bar and the hydrogen ions present at 1 mol-L 1 (strictly, unit activity), is called a standard hydrogen electrode (SHE). The standard hydrogen electrode is then used to define the standard potentials of all other electrodes ... 

Calculate the standard potential of a redox couple from two others relating to different oxidation states (Example 12.6). 

Figure 4 shows what is known to date about the standard potential of the silver microelectrode as a function of the agglomeration number n f For n = 1, i.e. the redox system... 

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

The standard potentials of some redox systems are listed in Table 3.5. 

Standard redox potentials can be determined approximately from the titration curves for suitably selected pairs of redox systems. However, these curves always yield only the difference between the standard potentials and a term containing the activity coefficients, i.e. the formal potential. The large values of the terms containing the activity coefficients lead to a considerable difference between the formal potential and the standard potential (of the order of tens of millivolts). 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

SCHEME 2.13. P/Q, redox catalyst couple A, substrate B, C, intermediates generated from the substrate D, product EPajb, standard potential of the substrate redox couple. 

Back electron transfer is at the diffusion limit because the homogeneous electron transfer reaction is uphill, owing to the fact that the standard potential of the redox catalyst is necessarily chosen as positive of the reduction potential of the substrate. 

The method consists of plotting the forward electron transfer rate constant against the standard potential of a series of redox catalysts as illustrated by Figure 2.29. Three regions appear on the resulting Bronsted plot, which correspond to the following reaction scheme (Scheme 2.14). The... 

Using, for example, cyclic voltammetry, the cathodic peak current (normalized to its value in the absence of RX) is a function of the competition parameter, pc = ke2/(ke2 + kin), as detailed in Section 2.2.6 under the heading Deactivation of the Mediator. The competition parameter can be varied using a series of more and more reducing redox catalysts so as eventually to reach the bimolecular diffusion limit. km is about constant in a series of aromatic anion radicals and lower than the bimolecular diffusion limit. Plotting the ratio pc = keij k,n + km) as a function of the standard potential of the catalysts yields a polarogram of the radical whose half-wave potential provides the potential where ke2 = kin, and therefore the value of... 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

The most elementary follow-up reaction is the homogeneous ET to another solution species, which may be identical with the donor molecule itself [52]. The equilibria of homogeneous ET reactions are governed by the standard potentials of the involved redox couples and are easily calculated with given data according to Eqs. (6-9) ... 

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