Adsorption of Benzoic Acid on a Mercury Electrode

Abstract

Abstract The differential capacity and the surface tension at the electrode-electrolyte interface of a dropping-mercury electrode were measured in 0.5 M H2SO4 and 0.5 M Na2SO4 solutions for various concentrations of benzoic acid. On the basis of the Frumkin-Damaskin theory, the adsorption of benzoic acid on mercury was studied in the acid solution, and the parameters involved in the F-D theory were determined: A=1.6 μJ cm−2 (Γm=6.5×10−10 mol cm−2), B0=597 l mol−1, a=−0.35–2.5 E/V, and C′=5.95 μF cm−2. The experimental values of the capacity and the surface tension are in good agreement with the theoretical values calculated by using the parameters obtained. The absence of an anodic maximum in the capacity-potential curve in the presence of benzoic acid was determined to be associated with a strong repulsive force between the adsorbed molecules. The value obtained for the maximum surface excess of benzoic acid is explained in terms of adsorption of the acid molecules for a vertical orientation on the electrode surface. This conclusion was supported by comparing the B0 value for benzoic acid with those for phenol and aniline. The adsorption of benzoic acid in neutral solutions is also discussed.