Desenvolvimento e validação de metodologias de preparo de amostras para determinação de Ca, Mg e K em Biodiesel por técnicas de Espectrometria Atômica.

Abstract

Inorganic contaminants commonly found in biodiesel can be incorporated during their production process, mainly due to the use of catalysts. The monitoring of these contaminants, especially in metals form in biodiesel samples is very important, because its presence can affect the engine performance and also increase the problem of environmental contamination. Thus, in this work were developed two sample preparation methods for the determination of Ca, Mg and K in biodiesel samples obtained from different sources: the use of emulsion and acid digestion in an open system. The quantification of metals was performed by Flame Atomic Absorption Spectrometry (F AAS) for Ca and Mg and Flame Atomic Emission Spectrometry (F AES) for K. The first methodology evaluated for the sample preparation was the use of emulsions of oil in water (o/w) obtained by mixing of biodiesel, formic acid (HCOOH), Triton X-100 and anti-foam. Calibration was performed using inorganic standards. The emulsions formed were stable for at least 2 hours. The limits of detection in the sample, in mg kg-1 were 2,30, 0.20 and 0.70 for Ca, Mg and K, respectively. The second methodology evaluated was the acid digestion in an open system. For this methodology, the biodiesel samples were digested with concentrated HNO3 at 220 °C for 4h. The sample digestion procedure was performed in a block digester with the cold-finger system for water circulation. Calibration was performed with aqueous inorganic standards. The limits of detections in the sample, in mg kg-1 were 0,12, 0.36 and 1.96 for Ca, Mg and K, respectively. Comparing the two developed methodologies for sample preparation of biodiesel, it was observed advantages in relation to the official method for the determination of metals in biodiesel which use dilution with organic solvent (xylene), such as the stability of the analytes in the middle emulsified the possibility of calibration using aqueous inorganic standards, better limits od detection, no use of organic solvents that are toxic and pose risks to the analyst and not using organometallic standards for calibration (unstable and expensive). Different samples of biodiesel were analyzed and the concentration of analytes Ca, Mg and K determined. For comparison, the samples were also analyzed by the official method and by analyzing a standard reference material for biodiesel. The results were in agreement, demonstrating the effectiveness of the methods developed as an alternative for the determination of metals in samples of biodiesel.