The measuring of glycerin content is considered one of the more important tests for biodiesel.  In the transesterification reaction that converts oils or fats to FAMEs, glycerin is the main by-product that is produced.  It is higher in density and more polar than the FAME products, and may be present in free form or bonded in mono-, di- and triglycerides.  Significant amounts of free glycerin are an indication of improper washing and removal of the glycerine by-product, while higher amounts of total glycerin are an indication of incomplete reaction of the feedstock to methyl esters.  Excessive amounts of glycerin in biodiesel can cause carbon deposits on fuel injection components, engine valves, valve seats, pistons and rings.  This leads to degraded engine performance and eventually engine failure.  Another potential engine problem is the formation of sludge in fuel storage tanks, resulting in plugged filters and engine starvation.

Significance

ASTM D6751 specifies only one test method for the determination of free and total glycerin in biodiesel, ASTM test D6584.  A biodiesel sample is analyzed by gas chromatography, after silyating with N-methyl-N-trimethylsilyltrifluoracetamide (MSTFA).  A calibration curve is developed using internal standards and standards for glycerin, monolein, diolein and triolein.  The amount of free and total glycerin (as a mass percentage) is then calculated using the sample chromatogram and information from the calibration curve.  ASTM D6751 allows for a free glycerin content of 0.02 (mass %) and a total glycerine content of 0.24 (mass %).