Concentration dependence of electrolyte conductance. Part 1.—Comparison of the Fuoss-Onsager and Pitts treatments

Abstract

The theory of electrolyte conductivity developed by Fuoss and Onsager (F) is compared with that of Pitts (P). There are five main differences. (i) In (F) the force on an ion i in the presence of another j includes explicitly a contribution from the ionic atmosphere around i. In (P) this is regarded as implicit in the definition of electrostatic field about the j ion. (ii) In (P) the ionic radius is not the same as the closest distance of approach, whereas the (F) theory requires that they are identical, in contradiction to their physical meaning. (iii) In (F) the net force on a volume element of solvent arises from electrical effects and also a hypothetical effect of ionic Brownian motion. In (P) only the electrical part is used in the electrophoretic calculation. (iv) An approximate electrostatic boundary condition is used in (P) together with hydrodynamic conditions consistent with the model. In (F) an exact electrostatic condition is used, but the remaining conditions refer to relative ionic velocities at closest approach; these are not correctly calculated from the hydrodynamic equations. (v) The final expression for conductivity in (F) is assembled by a method which is not simply related to the calculated forces acting on the ions. In addition, there are mathematical details in (F) which are questionable. A small correction is made to (P) and the theories are compared numerically for a typical 1–1 salt in water. Their divergence is such that accurate experiments should enable a decision to be made between the theories.