Determinação eletroquímica de glicerol em biodiesel por amperometria de múltiplos pulsos com eletrodo de ouro

Abstract

Biodiesel is the fuel obtained from vegetable oils or animal fats that can partially or totally replace diesel oil. Besides being obtained from renewable sources, biodiesel is less polluting than diesel and biodegradable. One disadvantage of biodiesel is the byproduct of transesterification reaction of raw material, glycerol, which can cause damages to combustion engines and therefore is limited to 0.25 % (m/m) as total glycerol and 0.02 % (m/m) as free glycerol according to technical regulations as ANP 7, ASTM D 6751 and EN 14214. This paper proposes an amperometric method for the determination of free glycerol in biodiesel at gold electrode in basic medium, coupled with flow injection analysis system (FIA). Voltammetric characterization was made from -0.8 to 0.5 V, verifying an electrochemical profile consistent with literature. To glycerol determination in the FIA system, multiple pulse amperometry was used with application of three sequential potentials: -0.2, 0.25 and 0.7 V, each one applied for 100 ms. Free glycerol was extracted to the analyzed aqueous phase by vortex stirring (5 min) followed by centrifugation (5 min). FIA system was evaluated in order to obtain the highest current signal to glycerol detection. Best conditions were sampling loop of 150 μL and flow of 4.0 mL min-1. To validate some parameters as repeatability, linearity, recovery, detection and quantification limits were determined. The repeatability had low standard deviation (4.8% with 14 injections), high analytical frequency (100 injections h-1), recovery values from 96% to 117 %, linear range from 49.10 to 2288 μmol L-1, detection limit of 14.73 μmol L-1 e quantification limit of 49.10 μmol L-1. Therefore, the determination of free glycerol in biodiesel based on aqueous extraction and gold electrode detection in basic medium under flow can be used to determine concentrations less than 15.0 μmol L-1 glycerol, while the regulated upper limit is 200 ppm, which corresponds to 2.17 mmol L-1.