Earlier theories of electrolytic conductance are reviewed; all of these, with the exception of the Arrhenius-Ostwald theory, are based on physical models. Their theory failed to describe the conductance of strong electrolytes because it did not include the effects (then unsuspected) of long-range forces on mobility. Thermodynamic derivations are independent of model; applied to the postulated equilibrium A + + B - ⇄ A + B - between free ions and nonconducting paired ions, the thermodynamic pairing constant K a equals a p /( a ±) 2 , and Δ G , the difference in free energy between paired ions (activity = a p ) and free ions (activity = a ± ), equals (- RT ln K a ). Converting to the molarity scale, K a = (1000 ρ/ M )[1 - γ)/ cy 2 (y ± ) 2 ]. Here ρ is the density of the solvent of molecular weight M , c is stoichiometric concentration of electrolyte (mol/liter), γ is the fraction of solute present as unpaired ions, and y ± is their activity coefficient. The corresponding conductance function Λ = Λ( c ;Λ 0 , R ,△ G )involves three parameters: Λ 0 , the limiting equivalent conductance; R , the sum of the radii of the cospheres of the ions; and Δ G . Conductance data for cesium bromide and for lithium chloride in water/dioxane mixtures and for the alkali halides in water are analyzed to determine these parameters. Correlations between the values found for R and Δ G and properties characteristic of salt and solvent are then discussed.

Fuoss, R. M. (1980). Conductimetric determination of thermodynamic pairing constants for symmetrical electrolytes. Proceedings of the National Academy of Sciences, 77(1), 34–38.