Free fatty acid (FFA) in used or rancid oil reacts with the lye catalyst to make soap instead of biodiesel, and each millilitre of titration (on the parent’s 0.1% lye test) means 0.77% FFA in the oil and about 7.6 g of soap per litre. Oil at the parent’s 5-15% FFA would turn 5-16% of its mass to soap, which is why heavily used oil is blended, refined or sent to soap-making instead of being made into fuel.
Part of Biodiesel Production
Free fatty acids are fatty-acid chains released from the glycerol backbone by heat, water or age; they consume the lye catalyst and make soap.
FFA Eats The Catalyst And Makes An Emulsifier
An oil molecule that has lost its glycerol (a free fatty acid) is an acid. The lye neutralises it at once: FFA + NaOH → soap + water. That does three harms: it uses up catalyst, so the reaction stalls; it makes soap, which is an emulsifier that holds oil, glycerin and water in a stable milky mix; and it makes water, which can break more oil into FFA. Frying, heat, light and age all create FFA, so the parent’s “worse the oil smells, the more lye” is a rough pointer, not a measurement.
What One Millilitre Of Titration Means
The parent’s titration uses 1 mL of oil and a 0.1% (1 g per litre) NaOH solution, so each mL used equals 1 mg of NaOH per mL of oil, which is 1 g of NaOH per litre of oil.
- Moles of NaOH: 1 g ÷ 40.00 = 0.025 mol.
- FFA neutralised, as oleic acid (282.46 g/mol): 0.025 × 282.46 = 7.06 g. In a litre of oil weighing about 920 g (assumed) that is 7.06 ÷ 920 = 0.77% FFA.
- Soap made, as sodium oleate (304.44 g/mol): 0.025 × 304.44 = 7.6 g per litre of oil.
| Titre (mL) | FFA in oil | Total NaOH (g/L oil) | Soap made (g/L) | Soap as % of oil mass |
|---|---|---|---|---|
| 1 | 0.77% | 4.5 | 7.6 | 0.8% |
| 2 | 1.5% | 5.5 | 15.2 | 1.7% |
| 5 | 3.8% | 8.5 | 38 | 4.1% |
| 8 | 6.1% | 11.5 | 61 | 6.6% |
| 10 | 7.7% | 13.5 | 76 | 8.3% |
| 19.5 | 15.0% | 23.0 | 148 | 16.1% |
The first row matches the parent’s worked example: a titre of 2 mL gives 3.5 + 2 = 5.5 g of NaOH per litre. The parent’s “5-15% FFA” corresponds to titres of 5 ÷ 0.7676 = 6.5 mL up to 15 ÷ 0.7676 = 19.5 mL, which requires 10 to 23 g of NaOH per litre and makes soap equal to roughly 5-16% of the oil’s mass. That is not a working batch by the one-step method; the soap would set the batch to gel. (For KOH multiply the total by 1.4.)
The corpus has no sourced titre above which the one-step method fails; reports I recall put the limit at a few per cent FFA, but that is not sourced here, so find your own limit with small test batches.
What To Do With High-FFA Oil
- Blend with a low-FFA oil to bring the mixture’s titre down. The blend titre is the volume-weighted average: 1 L at titre 8 plus 3 L at titre 0.5 gives (8 + 1.5) ÷ 4 = 2.4 mL.
- Refine first: neutralise the FFA with lye and remove the soap, then react the oil. The soap you remove is the FFA you started with, so you lose that fraction of the oil.
- Acid pre-treatment (acid and methanol first) is a known industrial route but uses concentrated acid; the corpus has no sourced village procedure and this page does not write one.
- Make it into soap. High-FFA rancid fat is poor fuel and a fair soap feedstock (Fat Selection, Soap Making).
Common Mistakes
- Treating the extra lye as free. Every mL of titre adds 1 g of NaOH per litre and makes soap that carries oil with it; the titration sets the dose, it does not remove the loss.
- Trusting smell. A bad smell is unquantified; the titration is the measurement.
- Skipping a drying step. Water makes FFA; dry the oil first (Waste Cooking Oil).
Titration reagents and the batch itself
The lye solution is caustic and isopropyl alcohol is flammable; work with eye protection and gloves away from flame. A batch gone to soap still contains methanol and lye, so treat the mixture as hazardous.
What’s Next
- Learn next: Oil Preparation & Titration, the test itself
- Combine with: Lye Catalyst, which sets the base dose