Several weighed portions of potassium hydroxide (KOH) ranging
from 1.0 to 3.0 g were carefully added to a specified volume
of solvent in a beaker. The mixture was strongly mixed until all
the KOH dissolved into the solvent solution and was ready for
solvent extraction. Then, the used oil sample was weighed (Woil)
and mixed with the solvent/KOH mixture in a beaker with solvent
to oil ratios of 1:1–3:1 for 15 min under continuous stirring
(stirring speed set at 350–400 rpm).