Liquid-liquid microextraction in sequential injection analysis for the direct spectrophotometric determination of acid number in biodiesel

Sequential injection analysis (SIA) is used here for acid number determination in biodiesel exploiting liquid-liquid microextraction and spectrophotometric detection. Free fatty acids (FFA) from biodiesel are partially extracted to an aqueous bromocresol green (BCG) aliquot placed in between two aliquots of biodiesel without any pretreatment. Microextraction is favored by using holding coils and transmission lines made of Teflon® due to the formation of biodiesel film in the tube walls. Detection was performed in a glass tube coupled to a fiber-optic spectrophotometer and analytical signals were related to the discoloration of the BCG after FFA microextraction. A linear response was observed up to 0.6 mg KOH g− 1, with detection limit, coefficient of variation, and sampling rate estimated at 0.0074 mg KOH g− 1, 2.9 % (n = 10) and 12 h− 1, respectively. The procedure is a fast, reliable, and mechanized alternative for acid number determination, which requires only 250 μL of sample, 10 μg BCG and 500 μL of ethanol. A reference procedure based on visual titration was used for comparison and results agreed at the 95% confidence level.