Spot test exploiting smartphone-based digital images for determination of biodiesel in diesel blends

Abstract

A simple, inexpensive, and fast colorimetric spot test is proposed for quantification of biodiesel in diesel blends. The procedure was based on formation of a violet complex between Fe(III) and the hydroxamate ion generated by the reaction of the alkyl esters characteristic of biodiesel with hydroxylamine. The analytical measurements were based on digital images acquired using a smartphone camera and converted to RGB values using a free application software (PhotoMetrix® 1.6). Values of the G channel corresponding to the intensities of the reflected radiation were taken as analytical parameter because of the complementarity with the absorption spectrum of the Fe(III)/hydroxamate complex. The effects of the reagents concentrations were evaluated by a Box-Bhenken experimental design and the optimized condition was 0.20 mol L−1 hydroxylamine, 0.30 mol L−1 NaOH, and 5.0 mmol L−1 Fe2(SO4)3. A linear response range was achieved from 3.0 to 30.0% (v/v) of biodiesel, described by equation G = 59.4 + 1.72 C (r = 0.999). The coefficient of variation and the limit of detection (99.7% confidence level) were estimated at 0.9% (n = 10) and 1.0% (v/v) of biodiesel, respectively. The proposed procedure consumed only 10 µL of sample, 830 μg NH2OH.HCl, and 110 μg Fe(III) and generated ca. 1.0 mL of residue per determination. The results for determination of biodiesel in diesel blends agreed with those obtained by the official method EN14078 at the 95% confidence level. The proposed procedure also allows monitoring formation of fatty acid methyl esters during the transesterification reaction.