The Molar Activity Coefficients of Electrolytes Predicted from Conductivity Data

Abstract

The molar activity coefficients of uni-univalent electrolytes may be predicted on the basis of conductivity data with an accuracy of at least 1 percent up to concentrations of the order of 0.01 M in solvents of dielectric constant as low as 10. The method is based on Bjerrum's theory of ion association,1 with the ion pair dissociation constant evaluated from conductivity data.4–6 The success of the method depends on the use of expressions for y′, the quasi-activity coefficient of the dissociated fraction, and for Λ, the equivalent conductivity, which go beyond the limiting law approximations of the interionic attraction theory. Equations (6), (8), (10), (12), and (13) have been developed for this purpose. In solvents of dielectric constant 10–40 the prediction involves no adjustable parameters characteristic of the electrolyte. The equations have been tested by comparing the conductivity-predicted y values for hydrochloric acid with those determined from emf measurements. To insure the significance of this comparison, new emf data for the cell Pt–H2(g)/HCl AgCl–Ag have been obtained for 82.00 wt percent dioxane (D=10.56) in the concentration range 0.0001–0.01 M.