Multilayer Membrane Electrodes. III. Activity of Alkaline Earth Salts in Mixed Electrolytes

Multilayer membranes composed of the alkaline earth salts of stearic and hexadecylsulfuric acids act as reversible electrodes to these cationic species only in mixed electrolytic solutions with even large excesses of alkali metal cations when the membrane is maintained at nearly constant volume by the imposition of a high pressure. Mixed electrolytic solutions of calcium or barium chloride with sodium or potassium chloride were studied at ionic strengths from 0.0003 to 1.5; these obeyed Harned's rule with slopes of zero at ionic strengths < 0.1, rising to -- 0.08 at ionic strength 1.5. Thus, thermodynamic activity coefficients of 2--1 electrolytes in mixed solutions with 1-- 1 salts were measured directly.