STRUCTURE OF THE ELECTRICAL DOUBLE LAYERAT
A
4113
MERCURY ELECTRODE
Structure of the Electrical Double Layer at a Mercury Electrode in the Presence of Adsorbed Nitrate Ions
by Richard Payne Air Force Cambridge Research Laboratories, L . G. Hanswm Field, Bedford, Massachusetts (Recsived June 4, 1966)
The adsorption of the nitrate ion on a mercury electrode from mixed solutions of ammonium nitrate and ammonium fluoride has been studied by measuring the double-layer capacity as a function of solution composition at 25’. The adsorption can be described by Frumkin’s modification of the Langmuir isotherm with a small repulsive lateral interaction coefficient, a nonlinear charge dependence of the standard free energy of adsorption, and a low saturation coverage. The large apparent area occupied by the adsorbed ion is tentatively interpreted in terms of two-dimensional hydration. The amounts adsorbed are calculated and the capacity is resolved into its component parts with the aid of the isotherm and diffuse-layer theory. The (integral) capacity measured at constant amount adsorbed decreases linearly with the amount adsorbed, and the capacity measured a t constant charge is a function of the charge. These observations are critically discussed in terms of the GrahameStern model of the double layer, and an alternative interpretation based on a model of mixed adsorption of the anion and the solvent is suggested.
Introduction The specific adsorption of the nitrate ion at the mercury-solution interface is more complex than the adsorption of the structurally simpler halide ions which have been studied in some Grahame and Soderberg4 first recognized this anomalous behavior which they attributed to the operation of weaker adsorption forces of the van der Waals type in the case of the nitrate ion. However, the nitrate ion appears to be at least as extensively adsorbed as the chloride ion although the adsorption increases more rapidly with the potential in the former case. For this reason, the characteristic capacity “hump” which is largely obscured by the adsorption in chloride solutions is well developed in nitrate solutions. The interpretation of the hump is of considerable theoretical interest in view of its close connection with the structure of the solvent dielectric in the inner region of the double layer.6 Considerable insight into the structure of the inner region is possible from a study of specific adsorption of the anion.112 Because the adsorption of the nitrate ion is not so strongly potential dependent as that of previously
studied anions, it is difficult to obtain accurate measurements of the amount adsorbed especially a t the lower concentrations because of uncertainties in the diffuse layer concentration of anions.s For this reason, measurements have been made in mixed solutions of ammonium nitrate and ammonium fluoride of constant ionic strength. This method possesses the double advantage of eliminating the diffuse-layer correction to the amount adsorbed and of minimizing the effect of the diffuse-layer capacity in the subsequent analysis.
Experimental Section The double-layer capacity at a dropping mercury electrode was measured for eight solutions of 2 M NH4NOs (1 - 2) M NH4F in water for the range 0 2 6 1 at 25’ using the a.c. bridge method de-
