Biodiesel needs a base catalyst, and a clean oil takes 3.5 g of sodium hydroxide (NaOH) per litre, or 4.9 g of potassium hydroxide (KOH) for the same effect, because KOH weighs 1.4 times as much for the same number of molecules. The lye must be dry solid, kept sealed, and dissolved in methanol before use; a lye solution in water, such as ordinary wood-ash lye, brings in water that turns the oil to soap.
Part of Biodiesel Production
The lye catalyst, sodium or potassium hydroxide, speeds the oil-methanol reaction; this covers which to use, how much, where to get it and how to handle it.
NaOH And KOH Do The Same Job; The Weight Differs
The parent gives 3.5 g NaOH per litre of clean oil and 7 g of KOH. Equal catalytic effect needs equal moles: 3.5 g of NaOH is 3.5 ÷ 40.00 = 0.0875 mol, and 0.0875 mol of KOH weighs 0.0875 × 56.11 = 4.9 g. The 7 g figure is 1.4 times that, and the balance on Triglyceride + Methanol → FAME + Glycerin cannot account for it; this page uses 4.9 g and flags the 7 g.
| NaOH | KOH | |
|---|---|---|
| Per litre of clean oil | 3.5 g | 4.9 g (equal in moles) |
| Glycerin layer (parent) | tends to set solid | stays liquid |
| Dissolving in methanol (parent) | slower, 5-10 minutes | faster |
When the oil is acidic, the titration adds base: total NaOH = 3.5 g + the titre in mL, and for KOH multiply that total by 1.4 (Oil Preparation & Titration).
What You Need
- Solid sodium or potassium hydroxide: drain cleaner that is 100% NaOH or KOH, no additives (the parent’s check); read the label, because many contain aluminium, which reacts with lye to release hydrogen gas
- A scale accurate to about 0.1 g, or a way to weigh many small batches by making a larger measured batch
- A dry, tightly lidded container, because lye takes up moisture from the air
- Protection: goggles, gloves, apron (Protective Gear); a container made for alkalis (Container Selection, written for acids and alkalis)
Where Lye Comes From After The Shops Are Empty
The parent suggests wood-ash lye. That is a water solution of potassium salts (Wood Ash Lye), unknown in strength, and water is exactly what the reaction cannot tolerate: it is fine for soap and unsuitable for this. Making a dry solid hydroxide from ash means evaporating a solution to dryness at the risk of carbonate contamination and caustic burns; the corpus has no sourced procedure, so treat biodiesel as dependent on a stock of purchased lye until a dry solid source is established. Sodium hydroxide from another route is described in Caustic Soda; check its dryness before use.
Method: Handling
Step 1: Weigh the lye quickly into a dry container with a lid; do not leave the stock open.
Step 2: Add the lye to the methanol, not the other way, in a closed container with a vent opened periodically, outdoors and with eye protection, stirring until it dissolves. The mixture warms.
Step 3: Use the methoxide the same day; it picks up water and loses strength.
Common Mistakes
- Using aluminium or galvanised vessels. Lye attacks aluminium and releases hydrogen.
- Treating a lye solution as a pellet. The water in it ruins the batch.
- Adding the parent’s 7 g of KOH without question. It is 1.4 times the NaOH-equivalent amount.
Lye burns, and methoxide blinds twice over
Lye causes severe, often painless, burns to skin and eyes (Spill Response: flush with water for a long time). Dissolved in methanol it makes methoxide, which is caustic, toxic through skin and vapour, and flammable. Never mix in an enclosed room or near flame, and never seal the vessel tight while it warms.
What’s Next
- Learn next: Free Fatty Acid Problem, which sets how much extra lye a dirty oil needs
- Combine with: Mixing & Reaction, where the lye goes to work