J. Gandhi,¹ A. Wille² and A. Steinbach,^2
1 Metrohm USA Inc., Riverview, Florida, USA,
² Metrohm International Headquarters, Herisau, Switzerland.

The free and total content of detrimental glycerol in vegetable oil methyl esters (biodiesel) is of paramount importance for the quality of biodiesel and is, therefore, limited by the US ASTM D 6751 and the European EN 14214 standards. Both regulations currently stipulate gas chromatographic (GC) analysis of free and total glycerol. However, the GC method, apart from being expensive, requires tedious derivatizations and fails for glycerol determinations in coconut or palm kernel oil methyl esters.

In contrast, the presented ion chromatographic method is applicable to all types of vegetable oil methyl esters. Prior to chromatographic separation, free glycerol and total glycerol are isolated by a straightforward extraction and saponification-extraction technique, respectively. Pulsed amperometric detection following chromatographic separation achieves an outstanding method detection limit (MDL) of 0.7 ppm by mass for glycerol and thus easily fulfils ASTM and EN performance specifications.

The four primary driving forces behind biofuels 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. Additionally, most biofuels are produced by straightforward manufacturing processes, are readily biodegradable and nontoxic, have low emission profiles and can be used as is or blended with conventional fuels.¹

Figure 1

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 6751² and the European EN 14214³ specify a maximum total glycerol content of 2400 ppm (0.24%) and 2500 ppm (0.25%), respectively. The maximum free glycerol content is limited to 200 ppm (0.02%) in both standards, which stipulate gas chromatographic (GC) analysis involving time- and reagent-consuming derivatizations. In addition, GC methods still encounter coelution problems with biodiesel samples produced from or containing lauric oils, such as coconut and palm kernel oil. Moreover, the GC method is not applicable to biodiesel blends.

Based on the analysis of biodiesel blends made from coconut oil, this paper demonstrates sensitive analysis of the free and total glycerol content via simple and innovative ion chromatography followed by pulsed amperometric detection (PAD).