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An experimental procedure is presented that was developed by fifth-year chemical engineering and industrial chemistry undergraduates at the Federal University of Rio de Janeiro doing the discipline on Experimental Organic Technology. The aim of this study was to apply the solvatochromic effect of the dye Nile Blue chloride to the characterization of biodiesel/diesel blends with different biodiesel content, using an alternative image processing analysis method involving low-cost, simple, and rapid assays. The effect of the solvatochromic dye Nile Blue chloride was monitored using the software ImageJ, which is being used currently, yielding rapid and effective responses. As an outcome, the students proposed a procedure that had potential application to colorimetric determination of biodiesel content in diesel oil. This gave students the opportunity to put the knowledge acquired in the course into practice, by dealing with issues concerning the fuels industry, making the learning process more dynamic, engaging, and effective.

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Biodiesel content on biodiesel/diesel blends is obtained by determining the band intensity of CO bond in the fatty acid methyl esters (FAME) of the biodiesel by mid-infrared spectroscopy (reference method: EN 14078, 2014). The potential for biodiesel/diesel blends to be adulterated with vegetable oils constitutes a limitation of the reference method’s capacity to accurately quantify the biodiesel content in these blends since vegetable oils, composed primarily of triacylglycerols, also contain the CO bond. This study employed normal-phase high-performance liquid chromatography with a refractive index detector (NP-HPLC-RI) to quantify the biodiesel in biodiesel/diesel blends and detect potential adulterations of these blends with vegetable oils. Two calibration curves (4 to 12% vol and 5 to 30% vol) were plotted for the biodiesel quantification based on which 12 verification samples were analyzed (samples prepared at different concentrations from the calibration curves), as well as 20 samples of commercial diesel, acquired at gas stations in the southeast region of Brazil. The NP-HPLC-RI method presented good analytical performance in terms of linearity, limit of detection (LOD), limit of quantification (LOQ), precision (repeatability), accuracy (recovery), and robustness. Linearity was determined by the coefficient of determination (R 2) for concentrations of biodiesel and vegetable oil in diesel varying from 4 to 12% vol (R 2 = 0.9924 and R 2 = 0.9950, respectively) and from 5 to 30% vol (R 2 = 0.9968 and R 2 = 0.9962, respectively). The LOD and LOQ for the quantification of the biodiesel were 0.08 and 0.23% vol, while for the quantification of soybean oil, these values were 0.07 and 0.21% vol, respectively. The recovery values varied from 97.7 ± 1.8% to 107.1 ± 4.1%, indicating good accuracy, and the method proved robust when the temperature was changed from 40 to 35 °C. The paired sample t-test showed the nonexistence of significant differences between the proposed and reference methods (with 95% confidence), indicating the capacity of NP-HPLC-RI to detect and quantify biodiesel and vegetable oil adulterants in samples of diesel both rapidly and effectively, thereby demonstrating its importance for the quality control of this fuel since the current methodology (EN 14078) used in several European Union countries, as well as in Brazil and Argentina, cannot identify this kind of adulteration and cannot accurately analyze the biodiesel content in biodiesel/petrodiesel blends.

This work presents the results of the physical characterization of palygorskite and its adsorptive behaviour for three solvatochromic dyes (Nile blue chloride (NBC), methylene blue (MTB) and dithizone (DTZ)). Adsorption isotherms were used to determine the maximum adsorption of the solvatochromic dyes on the palygorskite. The characterization of palygorskite was carried out via mineralogical and chemical analysis with X-ray diffraction, X-ray fluorescence, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy, surface-charge measurement (ζ-potential), thermogravimetric analysis, textural analysis and cation-exchange capacity analysis. The material consists of palygorskite and quartz and its chemistry is dominated by SiO2, MgO and Fe2O3. The specific surface area and cation-exchange capacity of the palygorskite are 142 m2 g–1 and 41 cmol(+) kg–1, respectively. The SEM and TEM analyses showed a fibrous structure with fibres 20–100 nm long. The thermogravimetric analysis showed three endothermic events at 57.3°C, 171.8°C and 439.6°C. The adsorption capacities of the palygorskite for NBC (basic pH), MTB (basic pH) and DTZ (neutral pH) were 0.082, 0.013 and 0.102 g g–1, respectively. The adsorptions of NBC and MTB were fitted with the Langmuir isotherm model and the adsorption of DTZ was fitted with the Sips model.