34 No. 2
||Information about new, current, and complete IUPAC projects and related initiatives.
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In the first two editions of the IUPAC Green Book, the transfer coefficient for an electroreduction reaction, denoted by αc, was defined as –(ν/n) (RT/F)∂lnIc/∂E, where Ic is the cathodic current at constant reactant concentration on the electrode surface, E is the applied potential, n is the number of electrons involved in the overall reaction, and ν is “the number of identical activated complexes formed and destroyed in the completion of the reaction.” While the dimensionless quantity (RT/F)∂lnIc/∂E is a directly measurable experimental quantity, the estimate of the two quantities n and ν requires some mechanistic considerations; this explains why this definition has been almost never used in the literature. In the third and most recent edition of the Green Book, the quantity ν was removed from the definition of the transfer coefficient, to comply with a definition adopted in many textbooks and dating back to the pioneering work of Butler and of Erdey-Gruz and Volmer. However, the deletion of the correlation between n and ν makes the significance of n ambiguous. This ambiguity has led to misinterpretations and erroneous mechanistic conclusions by many inexperienced researchers. A different school of thought, pioneered by Bockris and Gileadi, has identified the transfer coefficient with the directly measurable quantity -(RT/F)∂lnIc/∂E, independent of any mechanistic consideration. The present collaborative effort is aimed at critically evaluating the concept of transfer coefficient, proposing that its definition be based entirely on measurable quantities, and indicating correct procedures to exploit the experimental value of αc for the elucidation of electrode reaction mechanisms.
The partial charge transfer coefficient, denoted by λ, is commonly regarded as the positive or negative fraction of the electronic charge |e| that a molecule transfers to the electrode upon its adsorption on it. This is clearly an extrathermodynamic quantity, in that it cannot be estimated without having recourse to some modelistic assumption. In fact, the division of the bonding electrons into parts pertaining to the adsorbate and to the electrode is somewhat arbitrary. λ is often estimated by measuring the thermodynamic quantity l ≡ –(∂σM/∂Γ)E∂/F∂ called “electrosorption valency”, where ∂ σM is the charge density on the metal and Γ is the adsorbate surface excess; l includes contributions other than λ that must be estimated and subtracted from l in order to estimate λ. The partial charge transfer coefficient can also be estimated by measuring the dipole moment of the metal-adsorbate bond (e.g., the Me-S bond for thiol adsorption on a metal Me), since λ is expected to increase with the covalent nature of this bond, and hence with a decrease in its polarity. A further goal of this project is the definition of the partial charge transfer coefficient and a critical evaluation of the extrathermodynamic procedures adopted for its estimate.
last modified 5 March 2012.
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