A new method is proposed to determine the acid number (AN) of ethanol fuel using coulometric titration with potentiometric detection. The method employs a potentiostat/galvanostat and an electrochemical cell containing a three-electrode system (Pt working electrode, Ag auxiliary electrode and Ag/AgCl reference electrode) and a glass pH electrode. It involves applying a constant current through the sample (formation of hydroxyl ions), which contains LiCl as supporting electrolyte, and acquiring the potential of the pH electrode. A preliminary evaluation of the method showed that carbon dioxide and metallic cations contribute significantly to the AN, whereas the addition of LiCl has two opposite effects: it increases the AN at low concentrations (unwanted reduction reactions) and decreases the AN due to the removal of some carbon dioxide (increase of polarity). The method was compared to a volumetric/potentiometric titration-based method to determine the AN of different ethanol samples, producing similar average results for the hydrated samples and lower standard deviations between replicates. (C) 2011 Elsevier Ltd. All rights reserved.