The mixture in the reaction flask was heated and well-stirred at
a constant stirring speed of 600 rpm[3,21] for all test runs. The
crude roselle oil, methanol, and potassium hydroxide were used
in amounts established for each experiment, with the oil preheated
to the desired temperature in the reaction flask. In order to maintain its catalytic activity, the solution of potassium hydroxide in
methanol was freshly prepared for each run, since prolonged contact with air would diminish the effectiveness of the catalyst
through interaction with moisture and carbon dioxide [2,3]. The
potassium hydroxide-methanol solution was then added to the
preheated oil in the reaction flask, at which point the time measurement began. Heating and stirring were continued at different
reaction times at atmospheric pressure. After the reaction, the mixture was allowed to settle by gravity for 24 h in the separatory funnel. Two layers were formed: the upper layer consisted of methyl
ester, methanol traces, residual catalyst, and other impurities,
whereas the lower layer consisted of glycerin, excess methanol,
catalyst, and other impurities. The glycerin layer was then drawn
off and the methyl ester layer was purified by washing with hot
distilled water at 60C until the washing water had a pH value
similar to that of distilled water. The hot distilled water-to-crude
methyl ester ratio was 1:1. The washing step was done gently to
prevent the possibility of loss of methyl ester due to emulsion formation. Next, the methyl ester layer was dried under reduced pressure at 70C by rotary evaporator. Finally, the products from the
reactions were analyzed by using the standard test method, EN
14103, to determine methyl ester content.