Electrical double layer at alumina/electrolyte interface: II. Adsorption of supporting electrolyte ions

Abstract

Adsorption densities of supporting electrolyte ions onto γ-Al 2O 3 were measured using a radioisotopic method. The measurements were performed in NaCl, CsCl, and NaI solutions of concentrations ranging from 0.0001 to 1.0 M. The isotopes 22Na, 137Cs, 36Cl, and 131I were used. The surface complexation constants of anion and cation were estimated: p ∗K Na + int = 8.6, p ∗K Cl int = 7.5 , and Δp K complex = 1.1. The potential distribution within the electrical double layer and C 2 capacity were also evaluated using potentiometric titration and electrokinetic data. It was found that the complete results are inconsistent if Δp K a = 6.2. It was presumed that hitherto existing methods of determination of surface ionization constants can be the reason for this inconsistency. The new concept of p K a1,a2 int determination from adsorption measurements is presented in detail. It was estimated that p K a1 int ⩾ 6.0 ± 0.2, p K a2 int ⩽ 10.2 ± 0.2, and Δp K a ⩽ 4.2 ± 0.4. Using Δp K a = 4.2 the surface potential and ionic components of surface charge were determined vs pH. The capacitances C 1 and C 2 were also estimated for differing ionic strengths as a function of pH.