A spot test for iodine value determination in biodiesel based on digital images exploiting a smartphone

A spot test was developed for indirect determination of iodine value in biodiesel by using digital images taken by a smartphone. The procedure is based on the iodine consumption in the halogenation reaction of unsaturated compounds in biodiesel. The remaining reagent was determined after complex formation with starch, thus circumventing the background absorption inherent to biodiesel samples. The images were captured using the cell phone camera in a paper support and converted to RGB values using free application software (PhotoMetrix (R) 1.1.1). The reflectance values from the red channel were inversely proportional to the color intensity because of the complementary color of the iodine-starch complex. Hexane was used for diluting a biodiesel reference sample, whose iodine value was predetermined by the Friedman method for constructing the calibration curves. A linear response was observed in the range 10-106 g I-2/100 g of biodiesel, as described by the equation S = 155 + 0.722C (g I-2/100 g), r = 0.990. Coefficient of variation and the detection limit were estimated as 4.9% (n = 10) and 8 g I-2/100 g, respectively. The spot test consumed only 49 mu g of I-2 and generated 64 NI of effluent per determination. Analytical responses for biodiesel derived from different sources were in agreement, thus demonstrating the absence of matrix effects. These results agreed with those of the iodometric reference procedure at the 95% confidence level. The proposed procedure, which is the first application of a spot test for biodiesel analysis, is practical, inexpensive, and robust Moreover, it uses less toxic chemicals and minimizes reagent amount and waste generation in relation to the usual methods for iodine value determination. (C) 2017 Elsevier B.V. All rights reserved.