5.5 Ionic Activity

It has been shown that chemical reactions are described by their equilibrium constants (q.v. Section 5.1), which are correlated to the concentrations of the reactants. The concentration of a certain reactant can differ in one area compared to other areas[196,197], for which the before discussed double layer is an example. Many different effects can cause an excess ion concentration. Here, only concentration changes caused by electrostatic potentials will be treated. In areas containing varying concentrations it would be incorrect to describe chemical reactions by their bulk concentration values. Instead, one must use the local concentration values of the corresponding reactants at the location of the short-range chemical reactions. The activity of ions based on their bulk values and related to their local electrostatic potential with respect to the bulk are:

$$\begin{array}{ccc} \left[a^{+}\right]&=&\,a_{0}e^{-\frac{q \p... ...]&=&\,a_{0}e^{\frac{q \psi}{k_{\text{B}} T}} \quad. \end{array}$$ (5.15)

(5.15) is exploited, when the site-binding model by Yates et al. [198] is derived (Section 5.6).