A novel spot test based on digital images for determination of methanol in biodiesel

Methanol is used in transesterification reaction for biodiesel production, being an important quality parameter to evaluate the purity of the final product. Methanol concentrations higher than 0.20% (m/m) reduce the biofuel efficiency. The Brazilian, United States, and European standards recommend the reference method based on gas chromatography with flame ionization detection, which involves a time-consuming procedure and requires high-cost equipment. In this work, it is proposed a simple and low cost spot test based on digital images acquired with a smartphone camera for the determination of methanol in biodiesel. The procedure was based on the oxidation of methanol to formaldehyde and subsequent reaction with Schiff reagent, resulting in a violet product. Analytical signals were based on the reflected radiation measured on the G channel (RGB system), which refers to the complementary color of the reaction product. Linear responses were obtained within 20-500 mg kg^-1 and 500-2000 mg kg^-1 methanol for 600 and 150 µL of sample aqueous extracts, respectively. The coefficient of variation (n = 10) and the limit of detection (99.7% confidence level) were estimated as 4.8% and 10 mg kg^-1 (600 µL of sample), respectively. The procedure requires only 75 µg of potassium permanganate, 6 mg of oxalic acid, and 1.5 mg of p-rosaniline and generates ca. 1.2 mL of residue per determination. The results agreed with those obtained by the reference procedure at the 95% confidence level, demonstrating that the proposed method is an alternative for routine analysis of the biofuel.