To find how much lye an oil needs, dissolve 1 g of NaOH in 1 litre of water, mix 1 mL of oil into 10 mL of isopropyl alcohol, and add the lye solution a few drops at a time until the mixture turns pink on phenolphthalein or reaches pH 8-9 and holds; the mL used plus 3.5 is the grams of NaOH per litre of oil, and the mL times 0.77 is the free fatty acid per cent. For KOH multiply the total by 1.4.
Part of Biodiesel Production
Titration measures the acidity of an oil by neutralising a sample with a standard lye solution, giving the extra catalyst needed.
The Method Measures Acid By Counting Lye
The test neutralises the oil’s free fatty acid with a known weak lye. A 0.1% solution is 1 g of NaOH per litre, so 1 mL contains 1 mg. The sample is 1 mL of oil, so each mL of solution used is 1 mg of NaOH per mL of oil, which is 1 g of NaOH per litre of oil. That is why the titre in mL equals the extra grams per litre of oil, with the base 3.5 g added (Free Fatty Acid Problem derives the 0.77% per mL).
What You Need
- NaOH, a scale and 1 litre of clean water, and a bottle for the solution. Weighing 1 g is the weakest point; to reduce the error, dissolve 10 g in 10 litres or weigh the largest batch your scale handles accurately
- Isopropyl alcohol (rubbing alcohol; the parent says to use it)
- Phenolphthalein indicator or pH strips reading 7-10
- A syringe, dropper or small spoon marked in millilitres, and a clean glass
- A calibrated dropper: count how many drops fill 1 mL, so drops convert to millilitres
Method
Step 1: Make the 0.1% solution. 1 g NaOH in 1 litre of water, labelled; it keeps in a closed bottle.
Step 2: Prepare the sample. Warm the oil gently so it is liquid and mixed. Dissolve 1 mL of oil in 10 mL of isopropyl alcohol and swirl until clear. If a pH strip held in the alcohol alone does not read neutral, note that reading and subtract it.
Step 3: Titrate. Add the lye solution a few drops at a time, swirling after each, counting the drops. The end point is the first pink that stays for 30 seconds with phenolphthalein, or a stable pH of 8-9 on a strip (the parent’s range). A red-cabbage extract (Red Cabbage) estimates pH only to about one unit, which is too coarse to find this end point reliably.
Step 4: Calculate. Titre (mL) = t. NaOH needed = t + 3.5 grams per litre of oil. For a 20-litre batch, 20 × (t + 3.5) grams. FFA per cent ≈ t × 0.77. For KOH, multiply the total by 1.4.
Step 5: Repeat three times and average. Results differing by more than a millilitre mean the technique needs work.
Worked Example
The parent’s example: a titre of 2 mL gives 2 + 3.5 = 5.5 g NaOH per litre, which is 110 g for 20 litres. FFA ≈ 2 × 0.77 = 1.5%. For KOH: 5.5 × 1.4 = 7.7 g per litre.
Fresh unused oil is the parent’s exception: skip the titration and use the base amount.
Common Mistakes
- Using a stale or wet lye solution. It slowly absorbs carbon dioxide from the air and weakens; make it fresh.
- Reading the end point too early from a pink that fades.
- Titrating a hot, wet oil. Water in the oil muddies the colour change.
- Carrying on at a very high titre. The parent’s 5% FFA is a titre of 6.5 mL, and soap losses rise with every millilitre (Free Fatty Acid Problem).
Small quantities, real hazards
Isopropyl alcohol is flammable and the lye solution can burn the eyes. Work away from flame, with eye protection, and flush spills with water (Spill Response). Do not pour titrated samples down a drain or onto soil you grow food in.
What’s Next
- Learn next: Mixing & Reaction, where the lye amount is used
- Combine with: Lye Catalyst, for NaOH versus KOH