The ester content is an important parameter to be monitored in biodiesel for evaluation of the transesterification reaction yield and for assessing the purity of the final product. This is also a relevant quality parameter in diesel/ biodiesel blends to avoid frauds, because legislation establishes a minimum amount of biodiesel to be added to diesel. The official method EN14103 requires the addition of an alternative internal standard (methyl nonadecanoate) for analysis of biodiesel from bovine tallow because the methyl heptadecanoate is found in high amounts in this product. In this work, it is proposed a fast, simple, practical, and environmental friendly flow-based spectrophotometric procedure, which exploits the formation of the violet complex between Fe(III) and the hydroxamate generated by the reactions of the alkyl esters with hydroxylamine. All involved steps are carried out inside the syringe pump of a sequential injection analyzer (lab-in-syringe approach). A single phase is attained by using ethanol as mediator solvent between the organic sample and aqueous soluble reagents. Linear responses for biodiesel samples and diesel/biodiesel blends were obtained from 4-99%(v/v) to 2.0-40%(v/v) methyl esters, described by the equations: A = 0.342 + 0.00305C (r = 0.997) and A = 0.174 + 0.00503C (r = 0.999), respectively. The analytical curve can be obtained by in-line dilution of a methyl linoleate stock solution. For biodiesel samples, the coefficient of variation (n = 10), limit of detection (99.7% confidence level), and sampling rate were estimated at 0.8%, 0.36%(v/v), and 151 h(-1), respectively, whereas the corresponding values for the blend samples were 0.20%, 0.03%(v/v), and 12 h(-1), respectively. The procedure consumes only 860 mu g of hydroxylamine, 366 mu g of Fe-2(SO4)(3)center dot H2O, and 2.0 mL ethanol and generates ca. 3.0 mL of residue per determination. The results agreed with those obtained by official methods EN14103/2011 e EN14078, at the 95% confidence level. (AU) |