Before performing the transesterification reaction, the soya oil must also be free of any residual water. To accomplish this, sodium sulfate will be added to the oil; this mixture will be stirred and allowed to sit for a few minutes before vacuum filtration through a silicon-impregnated filter paper. Your professor and lab technician will dry the oil on a large scale before the experiment begins. If waste oil is to be used, it must be filtered to remove any solid non-oil materials and also dried to remove residual water. These steps will be completed for you on a large scale before the experiment begins.
In this synthesis, the oil will be mixed with methanolic KOH solution, and this mixture will be refluxed. Approximately 50 g of oil will be used. Calculate the amount of methanol needed in order that there is a 10 : 1 molar ratio of methanol : oil. To do this you must first determine the molar mass of the oil using the triglyceride calculator found on the University of Idaho Web site. The amount of methanol should be about 20 mL. Show your calculation to the professor.
The amount of base catalyst (KOH) needed is 1% by weight of the oil used. Calculate the amount of KOH required (check with your professor) and then weigh this amount into a dried 50-mL Erlenmeyer flask. Quickly, add the calculated volume of methanol to the 50 mL Erlenmeyer flask containing the KOH using a dried graduated cylinder. Cover the Erlenmeyer with a small watch glass to keep out water vapour and carbon dioxide, as well as to reduce evaporation of the solvent. Swirl the mixture to dissolve the KOH. If necessary, stir or heat the mixture on a hot plate to speed the process.
While the KOH is dissolving, set up the reflux apparatus. Then, weigh out approximately 50 g of oil directly into a dried 100-mL round-bottom flask. Add the methanolic KOH solution to the oil. Swirl to mix the contents well. Do they mix? If not, why? Add a magnetic stir bar and bring the oil mixture to a boil. Gently reflux for 1.5 hours.
After the reflux period, remove the reflux apparatus from the heat source and let it cool to a manageable temperature. Once cool, pour the reaction mixture into a separatory funnel. Two layers should appear in the separatory funnel: a lighter layer (top), which contains the biodiesel methyl esters, and a darker layer (bottom), which contains glycerol and unreacted oil. Carefully drain the bottom layer from the separatory funnel. Note the mass of the glycerol layer.
To purify the biodiesel methyl esters, first add concentrated ethanoic acid (aka. glacial acetic acid) dropwise to the methyl esters with swirling until the biodiesel is at a pH of 5-6. Then, wash the methyl esters by adding 20 mL of hot water and swirling, after letting it settle into two layers, remove the aqueous layer. Repeat the wash process twice more using a total of 60 mL of water for the 3 washings. Transfer the biodiesel methyl esters to a beaker and heat the sample to remove excess water. To avoid splattering, and possibly an explosion, place a boiling stick in the biodiesel sample while heating and stir constantly. Be careful to avoid superheating of the sample. Once cooled, note the final weight of the product.