Adsorption of Inorganic Anions on a Mercury Electrode from Solutions in Formamide. I. Adsorption of Iodide Ions

Abstract

The adsorption of the iodide ion on a mercury electrode from solutions of potassium iodide in formamide has been studied by measuring the double-layer capacity and the interfacial tension as a function of potential and concentration at 25°C. The adsorption can be described by a modified Langmuir (Frumkin) isotherm with a large lateral repulsive interaction energy, but examination of the inner-layer capacity suggests that the adsorbed layer behaves more like a two-dimensional imperfect gas than an array of particles on localized sites implied by the Langmuir model. The amounts adsorbed are calculated and the capacity resolved into its component parts with the aid of the isotherm and diffuse-layer theory. The capacity of the inner region of the double layer measured at constant amount adsorbed is almost independent of the amount adsorbed and approximates to the inner-layer capacity for a nonadsorbed electrolyte, i.e., potassium fluoride. The relative distances from the interface of the inner and outer Helmholtz planes are calculated from the components of the capacity and compared with similar values for aqueous potassium iodide. Methods devised by Grahame and Parsons for determining the potential at an anion site and the Stern specific adsorption potential Φ are compared and discussed in relation to the form of the isotherm, and values of Φ are calculated. [The symbol Φ is also used elsewhere to denote surface pressure because of established usage. It is obvious from the context which parameter Φ refers to wherever it occurs.] The results are shown to be consistent with both the ideal (Henry's law) and Langmuir forms of the chemical part of the electrochemical free energy of the adsorbed ions. The discreteness of charge effect and the Esin and Markov coefficient are compared for the formamide and aqueous solutions. The absence of any appreciable specific adsorption of potassium ions is demonstrated by comparison of the cation surface excesses with the predictions of diffuse layer theory.