Application of the Fuoss-Onsager theory to conductance data for sodium and potassium iodide solutions in various solvents

The 1957 Fuoss-Onsager conductance equation has been used to analyze data for sodium iodide and potassium iodide solutions in 13 one-component solvents, ranging from water (dielectric constant 78) to pyridine (dielectric constant 12). Values of the unknowns Λ 0 , å J , an ion size parameter, and, where appropriate, K a , the ion association constant, are calculated. In solvents of high dielectric constant it is found that the salts are completely dissociated (K a = 0) and the mean value of å J is 4.5 Å for each salt. However, the ion size parameters from the conductance equation do not seem to have any physical significance in solvents of lower dielectric constant, where there is evidence of ionic association. An analysis of these data is considered in which the Fuoss-Onsager equation reduces to a one-parameter equation, with K a as the unknown. The conductance data for sodium and potassium iodide can then be understood if the closest distance of approach of the ions is 4.5 Å in all 13 solvents.