Charge transfer coefficient definition

Parsons, R., Electrode reaction orders, charge transfer coefficients and rate constants. Extension of definitions and recommendations for publication of parameters. Pure Appl. Chem., 52, 233 (1979). 

So, for very fast reactions, the theory predicts a variation of a with potential. There is some evidence that this occurs, but given the multistep nature of any electrode reaction no definitive conclusions can be taken, and mechanisms can be elaborated which have constant charge transfer coefficients. Indeed the fact that the enthalpic and entropic parts of the coefficients have different temperature dependences leads to the question as to what is the real significance of the charge transfer coefficient, a topic currently under discussion9. 

One problem with the electrochemical hterature is that the definition of the charge transfer coefficient is not universal. Some groups prefer to use ct for the cathodic reaction and 1 — c anodic reaction. Other symbols (preferentially / ) can be found. 

Here X is the partial charge-transfer coefficient, namely, the fraction of electron charge transferred from a sulfur atom to the mercury upon adsorption, and a is the degree of dissociation of the sulfhydryl group. Since partial electron transfer from the sulfhydryl groups and their deprotonation are strictly correlated events, a and X are expected to assume close values. The Hg notation is merely used to denote chemisorption of the RSH thiol on Hg, and has no stoichiometric implications. As a matter of fact, under conditions of total charge transfer (a = 2. = 1), formation of mercurous [3, 69] or both mercurous and mercuric [77, 79] thiolate monolayers has been postulated. However, no definite... 

From the previous section, it was seen that the activation losses depend primarily on the exchange current density, jo, and the charge transfer coefficient a. For a reaction with larger exchange current density, the required overpotential is smaller and hence there are lower electrode losses. Thus, increasing jo enhances the electrode kinetic performance. In order to understand how jo can be increased, we look at the definition of exchange current density. It is given by... 

Charge Transfer under Temperature Gradient and Seebeck Coefficient Selected Definitions... 

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