Formal Electrode Potentials Aprotic Solvents

Although there is a general view that the electrochemical reduction of O2 in aprotic media is independent of media and electrode materials, the cyclic voltammograms of Fig, 2-6 and the electrochemical data of Table 2-5 provide clear evidence that both have a significant effect on the reversibility. Although the peak separation (AEp) varies with solvent and with electrode material, the median potentials (Ep c + Ep,a)/2 are essentially independent of electrode material and provide a reasonable measure for the formal reduction potential (E° ) for the 02/02-- couple. The average values for E° are summarized in Table 2-5 together with the value for an aqueous solvent at pH 1. 

The ease of reduction of Q may be quantified by the reduction potential of the couple Q/Q , usually denoted by E and expressed in volts relative to the Normal Hydrogen Electrode (NHE). Only if both oxidant Q and reductant Q are in their thermodynamic standard states will the potential E equate to the standard reduction potential of the couple and symbolized as or E . In the present context, interest is focused on effective potentials under physiological conditions, i.e. in water at pH values close to 7. Reduction potentials such as polarographic half-wave potentials obtained in non-aqueous media, especially aprotic solvents such as dimethylformamide or acetonitrile and relative to a standard calomel electrode (for example), will be numerically quite different. should not be confused with a formal potential Eq, which is usually defined as the potential when the ratio of the total concentrations of oxidized and reduced... 

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