A new method for determining the acid number of ethanol fuel using coulometric titration

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.