Sequential spectrofluorimetric determination of free and total glycerol in biodiesel in a multicommuted flow system

Abstract

A new procedure for spectrofluorimetric determination of free and total glycerol in biodiesel samples is presented. It is based on the oxidation of glycerol by periodate, forming formaldehyde, which reacts with acetylacetone, producing the luminescent 3,5-diacetyl-1,4-dihydrolutidine. A flow system with solenoid micro-pumps is proposed for solution handling. Free glycerol was extracted off-line from biodiesel samples with water, and total glycerol was converted to free glycerol by saponification with sodium ethylate under sonication. For free glycerol, a linear response was observed from 5 to 70 mg L(-1) with a detection limit of 0.5 mg L(-1), which corresponds to 2 mg kg(-1) in biodiesel. The coefficient of variation was 0.9% (20 mg L(-1), n = 10). For total glycerol, samples were diluted on-line, and the linear response range was 25 to 300 mg L(-1). The detection limit was 1.4 mg L(-1) (2.8 mg kg(-1) in biodiesel) with a coefficient of variation of 1.4% (200 mg L(-1), n = 10). The sampling rate was ca. 35 samples h(-1) and the procedure was applied to determination of free and total glycerol in biodiesel samples from soybean, cottonseed, and castor beans.

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

“…Resistive heating devices and portable lowcost photometers can be used for in situ analysis. [6] LLE with hexane/ethanol/water (20 min) 25-150 --MCFA-SP [7] LLE with water (30 min) 5-50 1.0 1.5 MCFA-FL [8] LLE with water (30 min) 5-75 0.5 1.0 Flow-batch-FL [9] On line analyte extraction with water; Isooctane for system washing (4 min) 0.1-5.0 0.04 1.5 GC [4] Derivatization with MTSFA and dissolution in n-heptane (15 min) 1.0-6.0 -1.3 CE [10] LLE with chloroform/water (25 min) 12-82 4.3 1.1 Voltametry [11] LLE with water, SPE with C18-bonded silica and solvent evaporation (ca. 30 min) 15-150 2.3 0.7 Flow-injection amperometry [12] LLE with water (15 min …”

“…Anhydrous ethanol (3.5 mL) was added at the end of this process. In all experiments, pH was maintained at 5.5 in agreement with previous studies [6][7][8]. Spectrophotometric measurements were then carried out at 410 nm.…”

“…230 times) in relation to formaldehyde. Large excess of the ketone is generally used to increase the rate of formation of the chromogenic product [6][7][8][9], which involves a slow chemical reaction. However, the blank signal increased almost linearly with the reagent concentration due to formation of a side product between acetylacetone and ethanol which absorbs radiation in the same spectral range.…”

“…In addition, the chromogenic reaction requires heating at 70°C and a strict time control. Other alternatives exploited the same reaction in flow-based systems with Microchemical Journal 106 (2013) 23-26 spectrophotometric [7] and fluorimetric [8] detection. Despite the increasing in sample throughput and simplification of analyte extraction, this step was yet off-line performed, which hinders the in situ analysis.…”

Exploitation of pulsed flows for on-line dispersive liquid–liquid microextraction: Spectrophotometric determination of formaldehyde in milk

“…This corresponds to at least an 80-fold molar excess of acetylacetone, which may be justified by the effect of the reagent on the reaction rate. As a slow reaction and a short residence time are involved, an excess of acetylacetone favored product formation -a large reagent excess was also adopted when the Hantzsch reaction was exploited for the determination of glycerol in biodiesel [25]. From 10 reagent pulses the absorbance diminishes linearly because of sample dilution.…”