Eigen-Fuoss equation

In all cases the key to assigning mechanism is the ability to detect and measure the equilibrium constant K. The equilibrium constant Kos can be estimated through the Fuoss-Eigen equation,10 as shown in equation 1.22. Usually, Kos is ignored in the case of L = solvent. 

The equilibrium constant X, 2 for formation of the ion-pair (sometimes referred to, less specifically, as Kq ) can be derived from the Fuoss—Eigen equation [59, 60]... 

In fact, given the distance dependencies of A0 and V, electron transfer is expected to be dominated by reactants in close contact. In that limit the experimentally observed rate constant is related to ktt and the association constant between reactants, KA, as in equation (32). KA can be estimated for spherical reactants, using the Eigen-Fuoss result in equation (33). The electrostatic term, wR, was defined in equations (19) and (20). 

The value of the equilibrium constant for an encounter is certainly of prime importance in the discussion of interchange pathways of complex formation. This was first suggested, in fact by Werner [4] as early as in 1912. Most of the work on ligand substitution in complexes is based on the assumption that encounter equilibria could be calculated from the ion-pairing equation of Fuoss [5] which was derived in turn from a consideration of diffusion-controlled reactions by Eigen [6]. At zero ionic strength, the encounter equilibrium constant, Kp is given as... 

The parameter Kp can be estimated from the Eigen Fuoss model, which was originally derived to estimate the ion pair formation constant (see section 3.10). One of the ions is assumed to have a radius a which is equal to the distance between the reactants at the reaction site. Other ions of the redox couple which are within or on a sphere of radius a can react. The estimate of Kp also takes into consideration any electrostatic work done to bring the reactants to the reaction site. The equation for Kp in units of M is... 

For ion pairs, Fuoss and Eigen developed an equation to estimate AT, based on extended Debye-Huckel theory and a hard-sphere model for the ions. It is given by... 

---