Electrochemical Reduction Behavior of Alizarin Red S at HMDE in Aqueous Solutions

Abstract

Abstract At hanging mercury-drop electrode (HMDE) Alizarin Red S (ARS) is electrochemically reduced in aqueous buffer solutions, giving rise to a prominent wave and a pre-wave. The prominent wave is diffusion-controlled and involves two electrons and two protons. The pre-wave is attributed to a strong reversible adsorption of the reduction product at the electrode. The reduction mechanism was proposed and discussed on the basis of the cyclic voltammetric and double potential-step chrono-amperometric & coulometric results. It was concluded that the reduction pathway is an ECEC mechanism in which the chemical reactions (C) are a dimerization of the formed radical anion and protonation of the dianion dimer, respectively. The rate-determining step is the protonation reaction. The adsorption process obeys a Langmuir isotherm with maximum surface excess of 1.4 × 10−10 mol cm−2 at a bulk concentration ≥4.6 × 10−5 mol dm−3, corresponding to 1.18 nm2/molecule. This indicates that ARS is horizontally oriented and resists reorientation upon increasing its bulk concentration.