The presented ion chromatographic (IC) method is applicable to all biodiesel types and blends. Before chromatographic separation,
free glycerol and bound glycerol are isolated by a straightforward extraction and saponification-extraction technique. Pulsed
amperometric detection (PAD) following chromatographic separation achieves an outstanding method detection limit (MDL) of
0.5 ppm by mass for glycerol and therefore easily fulfills ASTM and EN performance specifications. The described method fully
complies with ASTM D 7591.
The four primary driving forces behind the biofuel boom are the world's increasing thirst for petroleum (80 Mbarrels/day),
the diminishing supply of fossil fuels, global warming, and the intention to reduce the dependence on fuel imports. In addition,
most biofuels are produced by straightforward manufacturing processes, are readily biodegradable and non-toxic, have low emission
profiles, and can be used as is or blended with conventional fuels.
Biodiesel is produced by transesterifying the triglycerides in the parent oil or fat with an alcohol, usually methanol, in
the presence of a catalyst (base, acid, or enzyme) to yield fatty acid methyl esters (FAME) and free glycerol as coproduct
(Figure 1). As reaction rates under acid or enzyme catalysis are relatively slow, most producers use the rapid alkali-catalyzed
Figure 1: Base-catalyzed transesterification of a triglyceride with methanol.
An incomplete reaction leads to the formation of residual glycerol intermediates such as mono-, di-, and triacylglycerides
(bonded glycerols). In contrast, complete conversion results in the formation of highly water-soluble glycerol (free glycerol).
The latter is separated from the final product at the end of the production process. However, traces of glycerol are frequently
found in the ester phase. Both free and bonded glycerols (= total glycerol) lead to severe operational problems such as injector
and valve deposits or filter clogging. Accordingly, the US ASTM D 6751 (1) specifies a maximum total glycerol content of 2400
ppm (0.24%), while the European EN 14214 (2) stipulates 2500 ppm (0.25%). In both standards, the free glycerol content is
limited to 200 ppm (0.02%).
Based on the analysis of biodiesel blends made from coconut oil, this article demonstrates sensitive analysis of the free
and total glycerol content via simple and innovative ion chromatography (IC) followed by pulsed amperometric detection (PAD)
according to ASTM D 7591 (3).
The chromatography system consisted of the 850 Professional IC with Amperometric Detector and the 858 Professional Sample
Processor (all Metrohm AG, Figure 2). For all separations, a Metrosep Carb 1 - 150/4.0 anion-exchange column was used with
a flow rate of 1 mL/min. The injection valve was fitted with a 20 μL injection loop and separation was achieved by isocratic
elution employing a 100 mmol/L NaOH eluent.
Figure 2: 850 Professional IC with IC Amperometric Detector and 858 Professional Sample Processor.
The amperometric detector consists of a gold working electrode in combination with a solid-phase reference electrode and a
stainless-steel auxiliary electrode. A triple-step potential waveform was applied.
Instrument control, data acquisition, and processing were performed using MagIC Net software (Metrohm).