A single-phase spectrophotometric procedure for in situ analysis of free glycerol in biodiesel

Biodiesel can be obtained from renewable sources by catalytic transesterification of vegetable oils or animal fats, yielding alkyl esters of fatty acids and glycerol as a by-product. Glycerol needs to be removed before commercialization of the biofuel because high concentrations can damage motors and produce highly toxic gases during combustion. The aim of this work was to develop a procedure for in situ analysis of free glycerol in biodiesel samples exploiting a single-phase system. The time-consuming analyte extraction and phase separation, usually employed in previously proposed procedures, were both avoided. Anhydrous ethanol was used to simultaneously dissolve the biodiesel and the chromogenic reagents and the final solution should contain at least 85% ethanol to ensure the formation of a single phase before the spectrophotometric measurements. A linear response was observed from 20.0 to 400.0 mg kg(-1) glycerol, described by the equation: A = 0.0018 C + 0.0204, r = 0.999, in which C is the concentration of glycerol in mg kg(-1). The detection limit was estimated at 2.0 mg kg(-1) (99.7% confidence level) and the coefficient of variation was 2.1% (n = 10). The proposed procedure was successfully applied to biodiesel samples from different fat sources and the results agreed with the reference procedure at the 95% confidence level. (c) 2012 Elsevier B.V. All rights reserved.