Nova abordagem para a determinação do glicerol livre em biodiesel baseada na oxidação com o periodato

Abstract

The quality standards for biodiesel commercialized in Brazil are set by the National Agency of Petroleum, Natural Gas and Biofuel (ANP), through Brazilian Resolution No. 45 of 2014 (RANP 45/2014). RANP 45/2014 sets the maximum free glycerol content in biodiesel added to diesel oil at 0.02% w/w. The higher free glycerol content could to lead troubles in fuel storage tanks, formation of deposits and clogging in the engine injection system and emission of toxic aldehydes. In this work, a new approach to determine free glycerol in biodiesel from soybean, palmist, macauba (pulp) and used frying oils was developed employing the glycerol oxidation by the periodate and, after, masking of the excess periodate by the molybdate. This provided a decrease in the oxidation time probably due to excess periodate employed. The iodate formed in oxidation of glycerol was quantified by the addition of iodide, forming the tri-iodide anion. The iodometric determination was performed in three days with spectrophotometric and titrimetric techniques. In the spectrophotometric method, the glycerol was extracted by partition in ethanol-water/n-heptane and the tri-iodide was determined in the presence of starch (λ = 582 nm). Two analytical curves were constructed after analysis of glycerol standards in a dual and single beam spectrophotometer. The latter had lower precision in the measurements, and its analytical curve presented low limits of detection (1.9 mg L-1) and quantification (5.7 mg L-1), and good coefficient of determination (R2 = 0.9972). The repeatability of spectrophotometric method was satisfactory (CV < 12%), however the intermediate precision was unfavorable (CV < 39%), according to ANOVA free glycerol contents obtained between the days. The analysis of glycerol standards in water by titrimetric method was done by the titration of tri-iodide with Na2S2O3 standard solution, in the presence of starch, and presented excellent accuracy, with relative errors below 3.0%. Detection and quantification limits were 1.2 mg L-1 and 3.5 mg L-1, respectively. Biodiesels were analyzed by titrimetric method after glycerol extraction by partition in water/n-heptane. The precision of this method was good in repeatability (CV < 14%) and intermediate precision (CV < 8%), evaluated by ANOVA. The accuracy of the titrimetric method was evaluated by recovery tests at three fortification levels: 250, 500 and 1000 mg L-1, reaching 91% to 105% of recovery. The average global levels of free glycerol levels in the biodiesels obtained by both methods did not differ significantly at the 95% confidence level, according to the Student's t-test. However, the titrimetric method presented smaller variations between days, lower cost and greater simplicity compared to the spectrophotometric method. In addition, the analyses were quick (10 min) when compared to the other techniques described in the literature and lower detection limit compared to other titrimetric methods.