Determination of Free Glycerol in Biodiesel Using UV-Visible Spectroscopy: a Validation Study

High glycerol content in biodiesel impairs engine performance. Brazilian legislation, (ANP Resolution 42/2004), limits the maximum of 0.02% w/w for free glycerol in biodiesel. The ASTM D6584 standard method for glycerol determination is gas chromatography, a time consuming and expensive technique. As alternative, free glycerol in biodiesel can be determined, after derivatization, by absorption spectroscopy in the visible region (410 nm). In this work, we have used a validation protocol for the spectroscopy method. Results indicate that the proposed method is linear (r2 = 0.993) for the studied concentration range and has acceptable accuracy and precision. The method presented relative standard deviations values of intermediate repeatability and accuracy below 5.0%. To calculate the limits of detection and quantification, three different calculation methods were used. Two of them resulted in appropriate quantification limits for glycerol. However, for the INMETRO calculation method, we found a higher value of quantification limit. Considering that the method is based on a chemical derivatization reaction, to produce a color product, the robustness of the method was analyzed for small changes in reaction temperature, wavelength and type of cuvette used in measurements. The results have shown that the method is robust for small variations in the reaction temperature, but not for small changes in wavelength and type of cuvette used (glass or plastic cuvette).