To run the reaction, dissolve the measured lye in the methanol to make methoxide, pour it into oil held at 55-60°C, and stir vigorously for an hour, because the methoxide and oil do not mix on their own and the reaction happens where they touch. Keep below methanol’s 64.7°C boiling point, work outdoors with eye protection and gloves, and keep every spark and flame away from the vapour.
Part of Biodiesel Production
The reaction stage: preparing sodium methoxide, combining it with warmed oil and stirring until the oil has converted to biodiesel and glycerin.
Mixing Is The Reaction Rate
Methoxide (methanol with lye) and oil sit in two layers. The reaction proceeds at the surface between them, so the parent’s rule is vigorous stirring for an hour, steady and thorough. It must also be warm: the parent heats oil to 55-60°C, “not higher”. The margin is small: methanol boils at 64.7°C, only 4.7°C above 60°C. A hot spot from a flame under the vessel boils methanol off as toxic, flammable vapour and leaves a batch short of alcohol.
What You Need
- Oil, dry and titrated, in a vessel that can be closed (60+ litres for a 20-litre batch, per the parent)
- A thermometer reading 0-120°C
- Methanol and lye measured in advance (Production Process gives the quantities)
- A closed container for the methoxide, made of a material alkali and methanol do not attack (not aluminium) (Container Selection)
- A stirrer: a paddle or a paint mixer on a drill. Read the warning below before using the drill
- Heating by water bath or a firebox outside the vapour area
- Goggles, gloves, apron
Method
Step 1: Warm the oil to 55-60°C in a water bath or with gentle indirect heat, checking the thermometer. Do not heat the oil over a naked flame.
Step 2: Make the methoxide. Outdoors, with eye protection and gloves, pour the measured methanol into the closed container through a funnel, add the lye, close the lid, and swirl or stir until it dissolves completely (5-10 minutes for NaOH, faster for KOH). It warms. Loosen the lid briefly if the vessel feels hot or pressurised.
Step 3: Combine. Pour the methoxide steadily into the warm oil and close the reaction vessel with a lid that is not airtight.
Step 4: Stir hard for an hour, keeping the temperature at 55-60°C. Check the thermometer every few minutes.
Step 5: Stop and let it settle (Settling & Separation).
Worked Example: Strength Of The Methoxide
For a 20-litre batch with a titre of 2 mL, the methoxide is 4.0 litres of methanol (4.0 × 0.792 = 3.17 kg) plus 110 g of NaOH, so about 3.5% lye by weight (110 ÷ 3,170 = 3.5%). At 20 L the oil weighs about 18.4 kg, so the methoxide is roughly 15% of the mix by weight (3.28 ÷ 21.7).
Common Mistakes
- Going above 60°C. Methanol flashes off; the margin to boiling is under 5°C.
- Stirring too gently or stopping early. The batch stays two layers and converts partly.
- Using the methoxide a day later. It absorbs water and loses strength.
- Mixing in aluminium, which lye attacks.
- Sealing the vessel tight while warm. Vapour pressure builds.
A drill is a spark source in methanol vapour
The parent suggests a paint mixer on a drill. The brush motor of a drill makes sparks, and methanol vapour is flammable at roughly 6-36% in air (recalled) and slightly heavier than air. Keep the motor outside the vapour space with the shaft passing through a lid, or stir by hand with a long paddle, outdoors, with the vessel covered. Methanol is toxic through skin and vapour and blinds permanently; see Methanol Hazards. Never stir with bare hands or without goggles.
What’s Next
- Learn next: Settling & Separation, the stage that follows
- Combine with: Lye Catalyst, for the dissolving step