Extrapolation of high quality adsorption data to very low concentrations by means of Nernstian equilibrium expressions is explored as a method to evaluate the effect of adsorption from very diluted solutions on the electrochemical response of actual systems The competitive adsorption of chloride and hydroxide anions on Pt(1 1 1) is discussed in order to ascertain the process responsible of the butterfly feature observed on perchloric acid solutions The results in concordance with previous kinetic studies points to the chaotropic effect of trace levels of chloride promoting hydroxyl adsorption Instead of alternative interpretations of this characteristic voltammetric feature as the outcome of chloride adsorption even at such low concentrations A dependence of the formal partial charge number with the amount of surface excess is detected for sulfate adsorption but not for chloride a result consistent with previously reported studies of OH- adsorption onto Pt(1 1 1) in the presence of trace levels of chloride and sulfate (C) 2010 Elsevier B V All rights reserved