Single-phase determination of calcium and magnesium in biodiesel using smartphone-based digital images

Abstract

A microanalytical procedure using smartphone-based digital image colorimetry is proposed for determining calcium and magnesium concentrations in biodiesel without analyte extraction. The analytical method relies on the discoloration of an alkaline Eriochrome Black T (EBT) solution because of complex formation with the analytes. Ethanol was used as a mediator solvent for dissolving both biodiesel and EBT. The analytical response was based on the measurement of reflected radiation from digital images captured using a smartphone camera. Photometric measurements were based on the RGB color system, taken R channel values as the analytical signal because of complementarity with the color of the EBT solution. The coefficient of variation (n = 10) and limit of detection were 1.0% and 3 µmol L–1, respectively. A linear response was observed in the 10–75 µmol L–1 range, described by the equation R = 0.612C + 93.1 (r = 0.999). The reagent amounts consumed per determination were as little as 25 µg of EBT and 120 µg of NaOH, with generation of only 935 µL of waste. The procedure was selective for calcium and magnesium, without interference of metal ions (Na+, K+, Zn2+, Cu2+, Ni2+, Fe2+, and Fe3+) as well as other sample components (glycerol, methanol, and water) in concentrations higher than those expected in biodiesel. Matrix effects for biodiesel obtained from different raw materials were negligible (recoveries from 90 to 104%), making feasible external standard calibration with aqueous/ethanolic reference solutions. The results agreed with those obtained by ICP OES after sample preparation by emulsion breaking, demonstrating the reliability of the proposed approach.