Open AccessConductimetric determination of thermodynamic pairing constants for symmetrical electrolytes
Author(s) -
Raymond M. Fuoss
Publication year - 1980
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.77.1.34
Subject(s) - chemistry , electrolyte , conductance , thermodynamics , ion , strong electrolyte , activity coefficient , stoichiometry , arrhenius equation , inorganic chemistry , aqueous solution , activation energy , organic chemistry , physics , electrode , mathematics , combinatorics
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 constantKa equalsap /(a ±)2 , and ΔG , the difference in free energy between paired ions (activity =ap ) and free ions (activity =a ± ), equals (-RT lnKa ). Converting to the molarity scale,Ka = (1000 ρ/M )[1 - γ)/cy 2 (y± )2 ]. Here ρ is the density of the solvent of molecular weightM ,c is stoichiometric concentration of electrolyte (mol/liter), γ is the fraction of solute present as unpaired ions, andy ± is their activity coefficient. The corresponding conductance function Λ = Λ(c ;Λ0 ,R ,△G )involves three parameters: Λ0 , the limiting equivalent conductance;R , the sum of the radii of the cospheres of the ions; and ΔG . 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 forR and ΔG and properties characteristic of salt and solvent are then discussed.