Hydroxyl reduction potential

Identification, isolation, and removal of (polyhydroxy)benzenes from the environment have received increased attention throughout the 1980s and 1990s. The biochemical activity of the benzenepolyols is at least in part based on thek oxidation—reduction potential. Many biochemical studies of these compounds have been made, eg, of enzymic glycoside formation, enzymic hydroxylation and oxidation, biological interactions with biochemically important compounds such as the catecholamines, and humic acid formation. The range of biochemical function of these compounds and thek derivatives is not yet fully understood. 

Of course, superoxide may reduce ferric to ferrous ions and by this again catalyze hydroxyl radical formation. Thus, the oxidation of ferrous ions could be just a futile cycle, leading to the same Fenton reaction. However, the competition between the reduction of ferric ions by superoxide and the oxidation of ferrous ions by dioxygen depends on the one-electron reduction potential of the [Fe3+/Fe2+] pair, which varied from +0.6 to —0.4 V in biological systems [173] and which is difficult to predict.)... 

Cleavage of C—O bonds by direct electron transfer from a cathode is usually difficult because of the negative reduction potential of the bond. Therefore, the reduction of aliphatic alcohols (R-OH) to the corresponding hydrocarbons (R-H) is often carried out by the transformation of hydroxyl groups to good leaving groups such as halides (X = Br, I), methanesulfonates (OMs), and... 

Peroxynitrous acid is a powerful oxidizing agent with estimated one- and two-electron reduction potentials of ° (ONOOH, H+/"N02, HjO) = 1.6-1.7 V and ° (ONOOH, H /N02 , H2O) = 1.3-1.4 V, respectively . In addition, it was reported that, upon protonation, ONOO can undergo decomposition via homolytic 0—0 cleavage to generate nitrogen dioxide radical ("NO2) and hydroxyl radical ( OH) in approximately 30% yields... 

The hydroxyl radical is a powerful oxidant, having a reduction potential of 2.7 V in acidic solution. In neutral solution, where the free energy of neutralization of OH by H3O is not available, the reduction potential decreases to 1.9 V (Table 2). Several inorganic anions and low-valency transition metal ions readily undergo one-electron oxidation by reaction with OH, which is often represented as a simple electron transfer ... 

The catalytic properties of copper during polyols conversion in aqueous phase may be drastically modified by some additives. Metals having a standard oxido-reduction potential higher than that of copper (Ir, Rh, Ru, Pd, Pt, Au) can be deposited on it by oxido-reduction reaction. The first atoms of second metal deposited exchange with hydroxylated... 

Nitro compounds may add to carbon-centered radicals and thus also with the majority of the DNA radicals (Chap. 6.3 only the very strongly reducing radicals such as eaq and C02 reduce the nitro sensitizers to (unstable) hydroxyl-amines McClelland et al. 1984). Originally, the nitro compounds and 02 have just been taken as oxidants irrespective of their mode of action, especially as the efficiency of the sensitizers correlates with their reduction potential (Adams and Cooke 1969 Tallentire et al. 1972 Simic and Powers 1974 Adams et al. 1976a, 1981). This concept is expressed in relationship (88), where C is the sensitizer concentration required to achieve a constant sensitizing response (e.g. an enhancement ratio of 1.6) and E7 the one-electron reduction potential of the sensitizer at pH 7. 

Antioxidant, regulation of intracellular oxidation-reduction potentials, hydroxylation reactions that require copper or iron... 

Chloride ions will be discharged at a platinum, graphite or magnetite anode from saturated neutral solutions of sodium or potassium chloride rather than hydroxyl ions although at equilibrium conditions it should be the very opposite, as the reversible deposition potential (reduction potential) of oxygen in neutral solution is much lower ) (7Eoh- i o2. Pt = 0.815 V at 25 °C) than the standard... 

Fig. 2. Oxidation state diagram of oxygen at pH 7 at otherwise standard conditions (lmolal concentrations, 1 atm for gases). The x-axis gives the oxidation state, the y-axis the product of reduction potential and oxidation state. As such the slope represents the reduction potential. Adapted from ref. [4]. A compound that lies above a line joining its neighbours is unstable with respect to disproportionation, as is the case for superoxide and hydrogen peroxide. The line from hydrogen peroxide to the middle of the water-hydroxyl line represents the one-electron reduction potential of the couple H202/ 0H, H2O.

The reduction potential for the hydroxyl/water couple was not precisely known until recently [106,107]. Based on older values for the hydroxyl radical/water couple one will find higher values of 0.8 V [108] or 0.46 V [34] in earlier papers by the present author. The implication of the value of 0.32 V for the reduction... 

Reaction between phenol and hydroxyl yields the dihydroxybenzenes, which can then undergo further oxidation (hydroquinone to benzoquinone, further hydroxylated to hydroxybenzoquinone, catechol and resorcinol to trihydroxybenzenes [79,100]). The condensation products, phenoxyphenols and dihydroxybiphenyls, most likely originate from the reaction between phenol and the phenoxyl radical [101]. Their presence indicates that some phenoxyl forms in the system, due to the reaction of phenol with OH or NO2. The possibility for NO2 to oxidise phenol to phenoxyl has been the object of a literature debate [102,103] in the context of nitration processes. The problem can be tackled upon consideration of the reduction potentials of the various species. The reduction potential of phenoxyl to undissociated phenol is E = 1.34 V - 0.059 pH [104], while for the reduction of nitrogen dioxide to nitrite it is E = 0.90 V [105]. Accordingly oxidation of phenol to phenoxyl would be possible above pH 7.5, and of course in the presence of phenolate (pH > 10 [106]). 

These equations refer to one-electron reductions versus the standard hydrogen electrode. Substrates M with more positive reduction potentials for the couple M/ M are stronger oxidants than substrates with lower or negative E. Therefore, in this case, M is easier to reduce. Eor example, the couple Cl /Cr has a reduction potential E of 2.200 to 2.600 V, and therefore chloride ions can theoretically be oxidized in water to chlorine atoms by hydroxyl radicals with E( OH, H / H20) = 2.730 V, according to Eq. 6-3 ... 

Thus, carbonate and bicarbonate ions are efficient scavengers of hydroxyl radicals in water due to their relatively low reduction potentials. 

Throughout the literature, many authors argue for the high reduction potential of hydroxyl radicals being responsible for the oxidative reactions observed in AOPs. However, simple electron transfer reactions such as those of Eq. 6-21 seem to be unlikely because of the large solvent reorganization energy involved in the formation of the hydrated hydroxide ion (Buxton et al., 1988). Instead, in the case of halide ions X or pseudo-halide ions, the formation of intermediate adducts with hydroxyl radicals is observed (Eq. 6-24). 

In particular, O2 adsorbed to Ti02 can be reduced by e, generating in a thermodynamically feasible but rather slow electron transfer reaction (Hoffmann et al., 1995). Values of (02/02 )= —0.3 V and (02/H02) = —0.05 V have been reported for homogeneous solutions the reduction potentials onto the Ti02 surface are probably less negative. As the following set of equations indicates, this cathodic pathway is an additional source of hydroxyl radicals ... 

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