Monitoring the short-chain carboxylic acids produced during the storage of different fatty composition biodiesels and their binary blends using ion chromatography

Abstract

Short-chain carboxylic acids are final products of lipid oxidation (formic acid, acetic acid). In this work, they were determined using ion chromatography as carboxylate anions, which formed during storage of methyl biodiesels prepared from soybean oil (MBS), macauba kernel oil (MBMK), palm kernel oil (MBPK), MBS + MBPK (44:56 m/m) binary blend, and MBS + MBMK (50:50 m/m) binary blend. For MBS, analytical monitoring showed that the sum of formate and acetate concentrations reached 30 mg kg−1 (concentration above that which biodiesel can present corrosive activity) in 335 days of storage. The sum of formate and acetate concentrations did not exceed 20.11 mg kg−1 for MBS + MBPK and 14.25 mg kg−1 for MBS + MBMK after as many as 497 days of storage. For MBMK and MBPK samples, only formate was determined, and its concentration did not exceed 6 mg kg−1 after up to 640 days of storage for MBPK and 880 days for MBMK. Considering slopes of concentration versus time curves for the formic acid formation (determined as formate) in each case, the following decreasing order of oxidative stability was assigned to the studied biodiesels and blends: MBMK > MBPK > MBS + MBMK > MBS + MBPK ≫ MBS. This sequence indicates that more stable biodiesels present higher saturated and lower unsaturated contents, in this case, mainly linoleic (C18:2) and linolenic (C18:3) fatty acids. Also, the average curve was traced to the formic acid (determined as formate) formation rate as function of the saturated/unsaturated ratio of the samples (R2 = 0.9788).