Linear sweep voltammetry of adsorbed neutral red

Abstract

The strong adsorption of neutral red on mercury causes its linear sweep voltammetry to be adsorption controlled over a wide range of conditions. At pH 4.5 or below, the surface coverage of DH + increases monotonically from 1 monlayer in 3×10 −7 M solution to ca. 3 monolayers in 3×10 −4 M solution, and to a good approximation the monomeric, dimeric and tetrameric forms of the dye are adsorbed equally strongly. The morphology of the adsorption peak indicates that interactions between adsorbed molecules are substantial and that the nature of these interactions changes from moderately repulsive below pH 4 to strongly attractive above pH 6. Over this pH range the voltammograms of 2×10 −4 M dye narrow progressively and ultimately split into two extremely sharp peaks. The first of these corresponds to the single voltammetric peak observed for dilute dye solutions, wheres the second represents the reduction of an electrochemically distinguishable phase of adsorbed dye. Above pH 9 the isotherm for adsorption of D exhibits an abrupt increase when the bulk concentration exceeds the equilibrium solubility.