Rapid and Reliable Determination of Free and Total Glycerol in Biodiesel by Iodometric Titration

Biodiesel is a renewable fuel produced by the transesterification of vegetable oils or animal fats. Regulatory agencies set strict limits on free and total glycerol levels in biodiesel intended for commercialization, as glycerol can impair combustion efficiency, promote engine deposits, and generate acroleina highly toxic aldehydeduring combustion. Current regulations typically require gas chromatography with flame ionization detection (GC-FID) for glycerol determination, which involves expensive standards and instrumentation. The iodometric titrimetric methods ABNT NBR 15771 and ABNT NBR 15344, previously accepted by the National Agency of Petroleum, Natural Gas and Biofuels (ANP, Brazil), were discontinued due to operational complexity, notably the multiple extraction steps with toxic solvents and the interference of excess periodate, which often required reanalysis due to excessive titrant consumption in blank titrations. This work proposes the masking of excess periodate with molybdate and the elimination of the extraction step, overcoming the disadvantages associated with the original ABNT methods. The proposed methods exhibited linear response ranges of 107-1911 mg L^-1 for total glycerol and 10.4-312 mg L^-1 for free glycerol. Detection and quantification limits were 1.42 and 4.32 mg L^-1 for free glycerol, and 5.35 and 16.2 mg L^-1 for total glycerol, respectively. Repeatability RSD ranged from 2.4 to 8.8% for free glycerol and 3.3 to 7.2% for total glycerol. Interday reproducibility did not exceed 8.0%. Recovery assays for glycerol in biodiesel samplescommercial, soybean, and palm kernelranged from 90.9 to 106.0%. A comparison with the ASTM D6584 reference method across nine biodiesel samples demonstrated statistically equivalent results for both free and total glycerol, with values in accordance with international regulatory limits. The proposed methods provide a rapid, reliable, and low-cost approach for glycerol determination in biodiesel, offering valuable applicability for screening, educational purposes, and preliminary quality assessments.