Ionic Components of Charge in the Electrical Double Layer

Abstract

The rate of change with concentration of the capacity of the electrical double layer on mercury has been measured. From this there have been calculated Γ₊, the excess of cations in the double layer, and Γ_—, the excess of anions. The latter has been subdivided into η_—^d, the excess of anions in the diffuse double layer, and η_—ⁱ, the excess of anions in the inner part of the double layer. From these data it has also been possible to evaluate ψ⁰, the potential of the outer Helmholtz plane. It is found that fluoride ion remains unadsorbed on mercury at all potentials investigated, including potentials anodic to the e.c.max. Other anions are strongly adsorbed (chemisorbed) when the mercury is positively charged, and sometimes even when it is negatively charged. The ``hump'' in the capacity curves of salts of such anions as chloride, bromide, acetate, and nitrate, are found to be present in curves of C_—, the capacity attributable to anions alone. No chemisorption of monatomic cations could be detected. The kinetic theory of the diffuse double layer with constant dielectric constant is found to fit the experimental results within the expected accuracy. This fit extends to electrolytes of the 1:2 and 2:1 types also. A new method of evaluating salt adsorption at the potential of the electrocapillary maximum (e.c.max.) is described and results are given. Likewise a new method of determining C₊ at the e.c.max. is described and used.