3. Results and discussion
Waste glycerol from biodiesel preparation varies widely in color and
physical properties. For base-catalyzed reactions, the glycerol layer generated
tends to be dark brown in color and the samples range from a viscous
liquid to a gelatinous solid. Waste glycerol from acid-catalyzed
reactions using the same feedstock oil tends to be much lighter in
color than base-catalyzed preparations, and in our hands has always
produced a somewhat viscous liquid. In both cases, excess methanol
(required to force the equilibrium reaction to completion) must first
be removed by rotary evaporation. This also allows the methanol to be
re-used in subsequent biodiesel preparations. Not surprisingly, the pH
of the glycerol layers after methanol removal from the two different
methods of catalysis is very different. In general, the base-catalyzed
glycerol layer has a pH around 10, while the acid-catalyzed glycerol
layer has a pH around 1.