## Abstract

The transfer coefficient *α* is a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3^{rd} edition of the IUPAC Green Book, the cathodic transfer coefficient *α*_{c} is defined as –(*RT*/*nF*)(*d*ln*k*_{c}/d*E*), where *k*_{c} is the electroreduction rate constant, *E* is the applied potential, and *R, T*, and *F* have their usual significance. This definition is equivalent to the other, -(*RT*/*nF*)(*d*ln|*j*_{c}|/d*E*), where *j*_{c} is the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficient *α*_{a} is defined similarly, by simply replacing *j*_{c} with the anodic current density *j*_{a} and the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake of *n* electrons from the electrode in the case of *α*_{c}, or their release to the electrode in the case of *α*_{a}. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions of *α*_{c} and *α*_{a} has led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity is *d*ln|*j*|/d*E*, which can be made dimensionless upon multiplication by *RT*/*F*, yielding (*RT*/*F*)(*d*ln|*j*|/d*E*). One common source of misinterpretation consists in setting this experimental quantity equal to *αn*, according to the above definition of the transfer coefficient, and in trying to estimate *n* from *αn*, upon ascribing an arbitrary value to *α*, often close to 0.5. The resulting *n* value is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantities *α*_{c }= –(*RT*/*F*)(*d*ln|*j*_{c}|/d*E*) and *α*_{a }= (*RT*/*F*)(*d*ln*j*_{a}/d*E*), which are independent of any mechanistic consideration.

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