CONDUCTANCE OF THE ALKALI HALIDES. I. POTASSIUM CHLORIDE IX DIOXAKE-WATER MIXTURES'" BY JOHX E. LIR'D,J R . ,AND ~ RAYMOXD M. Fuoss Contrzhxtton No. I648 f r o m the Sterlznq Chemzstry Laboratory of Yale linzuerszty, N e w Haven, Conn. Receaaed December 1 6 , 1960
The conductance of potassium chloride was measured a t 25" in dioxane-water mixtures which cover the range 78.54-12.74 in dielectric constant. The data conform to the equation A = ho - S(cr)'/z Ecr log CY J c r J ~ ( c Y )~ 'K~ACY f A ithin a precision of 0.01-0.057,, thus establishing the validity of this functional form of the conductance equation for a typical inorganic salt. Association to ion pairs begins to be increasingly visible below D = 40: a t D = 12.74, the association constant is li00. The contact distance a was found to varv from 2.5 to 6.4, depending on the method used to compute it; this variation suggests that the model and the theory need further refinements for the case of small ions in hydrogen bonding solvents.
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The electrostatic theory of conductance4 ascribes the decrease of equivalent conductance with increasing concentration to the effects of the ionic charges on mobility and on relative concentration of free ions, using as a model charged spheres to represent the ions and a structureless continuum to represent the solvent. This model and the theory based on it has been quite successful in accounting for the observed behavior of quaternary ammonium salts in a variety of solvents5; in these cases, the ions are large compared to the solvent molecules. In particular, the contact distance U K between ion pairs obtained from the dependence of the association constant on dielectric constant agrees closely with the sum U A of the radii obtained from the limiting conductances by applying an electrostatic correction to Stokes law. Also, the value of this parameter aJ obtained from the curvature of the phoreogram is in fair agreement with the other two values. I n order to test the theory and model further, we have started a systematic study of the alkali halides. These salts have small ions; since association depends on the Bjerrum parameters b = e2/aDkT, we would expect association to become significant a t higher dielectric constants than for the quaternary salts. Also, deficiencies in the present theory might be expected to be magnified for the case of small ions. Water-dioxane was chosen as the first solvent system for investigation, due to the wide range of dielectric constant which can be covered. Since water is a hydrogen bonding solvent, and since the water molecule has a fairly large dipole moment in a small volume, ioniolvent interactions should be readily obseravable. In this paper, we present a discussion of the ronductance of potassium chloride in dioxanewater mixtures which cover the range 12.74 D 78.54. The system conforms to the general theory, with some marked deviations in details:
