Production of biodiesel using an alkali catalyst always produces some amount of soap. There will be more soap with recycled restaurant waste and animal fats and less with refined vegetable oils. Catalyst and soap tend to concentrate in the glycerol phase; however, some may be found in the biodiesel phase even after the washing process. Soap and Catalyst are evaluated to determine the presence of residual contaminants. The presence of soap is attributed to saponification of free fatty acids present in the feedstock. Residual catalyst may remain if the reaction is incomplete or may be attributable to the excessive use of initial caustic reagent material.
ASTM D6751 Requirement: None.