Reduction/redox potentials

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Based on correlations between energy level positions and electrochemical redox potentials, it has been estabHshed that polymethine dyes with reduction potentials less than —1.0 V (vs SCE) can provide good spectral sensitization (95). On the other hand, dyes with oxidation potentials lower than +0.2 V ate strong desensitizets. 

Reduction Potentials—An Accounting Device for Free Energy Changes in Redox Reactions... 

FIGURE 21.2 Experimental apparatus used to measure the standard reduction potential of the indicated redox couples (a) the acetaldehyde/ethanol couple, (b) the fumarate/succi-nate couple, (c) the Fe /Fe" couple. 

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... 

The half-reactions and reduction potentials in Table 21.1 can be used to analyze energy changes in redox reactions. The oxidation of NADH to NAD can be coupled with the reduction of a-ketoglutarate to isocitrate ... 

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. 

Tlic power of these various concepts in codifying and rationalizing the redox chemistry of the clcineiUs is ilhislraled for Ihe case of nitrogen in tbe present section Standard reduction potentials and plots of volt equivalents against oxidation state fur odicr elements are presented in later chapters... 

In basic solutions a different set of redox equilibria obtain and a different set of reduction potentials must be used. I or example ... 

The aqueous solution chemistiy of nitrous acid and nitrites has been extensively studied. Some reduction potentials involving these species are given in Table 11.4 (p. 434) and these form a useful summaiy of their redox reactions. Nitrites are quantitatively oxidized to nitrate by permanganate and this reaction is used in titrimetric analysis. Nitrites (and HNO2) are readily reduced to NO and N2O with SO2, to H2N2O2 with Sn(II), and to NH3 with H2S. Hydrazinium salts yield azides (p. 432) which can then react with further HNO2 ... 

Oxidation-reduction potential Because of the interest in bacterial corrosion under anaerobic conditions, the oxidation-reduction situation in the soil was suggested as an indication of expected corrosion rates. The work of Starkey and Wight , McVey , and others led to the development and testing of the so-called redox probe. The probe with platinum electrodes and copper sulphate reference cells has been described as difficult to clean. Hence, results are difficult to reproduce. At the present time this procedure does not seem adapted to use in field tests. Of more importance is the fact that the data obtained by the redox method simply indicate anaerobic situations in the soil. Such data would be effective in predicting anaerobic corrosion by sulphate-reducing bacteria, but would fail to give any information regarding other types of corrosion. 

It is clear from what has already been stated that standard reduction potentials may be employed to determine whether redox reactions are sufficiently complete... 

A list of selected redox indicators, together with their colour changes and reduction potentials in an acidic medium, is given in Table 10.9. 

The calculation o E° for this cell illustrates an important feature of cell potentials. A standard cell potential is the difference between two standard reduction potentials. This difference does not change when one half-reaction is multiplied by 2 to cancel electrons in the overall redox reaction. 

Use tabulated standard reduction potentials to determine for the following redox reaction ... 

This is a quantitative calculation, so it is appropriate to use the seven-step problem-solving strategy. We are asked to determine an equilibrium constant from standard reduction potentials. Visualizing the problem involves breaking the redox reaction into its two half-reactions ... 

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