**Figure 8.**

*Glycerol reactions generating a colored compound analyzable by spectrophotometry. Adapted from Muniz et al. [36].*

To determine total glycerin, it is necessary to use solid-phase extraction in an aminopropyl cartridge to separate the biodiesel (FAMEs) from the fraction containing the free glycerin (FG), monoacylglycerols (MAGs), diacylglycerols (DAGs), triacylglycerols (TAGs), and combined glycerin, as shown in **Figure 9a** [36]. To determine free glycerin, the biodiesel (FAME) is separated and isolated from the fraction containing the free glycerin (FG) by solid-phase extraction in a silica cartridge (see **Figure 9b**).
