Soft Soap Batch at a Cooperative
A 12-member soap-making cooperative produces 20 kg batches from palm oil. Their standard recipe is 20 kg palm oil, 2.8 kg sodium hydroxide pellets, and 6 L water, mixed at 45 °C for 30 minutes and poured into moulds after 24 hours. The latest batch, made three days ago, is soft, greasy, and produces almost no lather. The cooperative's records show the palm oil has a free fatty acid content of 8% (expressed as oleic acid) and the sodium hydroxide pellets are 92% pure. Assume the saponification value of the oil is 200 mg KOH per gram and that the oil's fatty acid composition is close enough to oleic acid for the extra neutralisation calculation. The cooperative has orders to fill and wants to know whether to add more sodium hydroxide to the moulds or to rebatch.
1. Calculate the mass of 92% sodium hydroxide actually needed for a 20 kg batch, showing each step.
Read the worked answer
Step 1 - NaOH for the oil itself. Saponification value = 200 mg KOH per g oil. NaOH equivalent = 200 × (40/56.1) = 142.6 mg NaOH/g oil = 0.1426 kg NaOH/kg oil. For 20 kg oil: 20 × 0.1426 = 2.852 kg pure NaOH.
Step 2 - extra NaOH for the 8% free fatty acid. FFA = 8% of 20 kg = 1.6 kg, expressed as oleic acid (molar mass 282 g/mol). NaOH required = 1.6 × (40/282) = 0.227 kg pure NaOH.
Step 3 - total pure NaOH. 2.852 + 0.227 = 3.079 kg.
Step 4 - correct for 92% purity. 3.079 / 0.92 = 3.347 kg of pellets.
The recipe used 2.8 kg of pellets, which contains 2.8 × 0.92 = 2.576 kg pure NaOH. The batch was short by 3.079 − 2.576 = 0.503 kg pure NaOH. Even before allowing for purity, 2.8 kg is below the 2.852 kg needed for the oil alone.
2. Explain why the batch is greasy and lathers poorly, and why adding solid sodium hydroxide to the moulds is risky.
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The shortfall of about 0.5 kg NaOH means roughly 3.5 kg of the 20 kg oil remained unsaponified. Free oil makes the bar soft and greasy, reduces foam because oil suppresses lather, and leaves the soap feeling slippery. The pH of 10 does not prove excess alkali; it may come from uneven mixing, lye pockets, or the way the soap solution was tested. The greasy texture is the stronger evidence of under-neutralisation.
Adding solid NaOH to the moulds is risky because:
- solid pellets dissolve slowly and unevenly in cold, thick soap;
- you cannot stir the moulds properly, so caustic pockets can remain;
- a pocket can burn skin or eyes when the bar is used;
- the reaction may not reach all the free oil, so the batch remains inconsistent;
- adding a little more without calculation can overshoot and make a caustic, cracking bar.
A small test rebatch is safer than treating the whole batch blind.
3. Recommend a corrective procedure and two process controls for future batches.
Read the worked answer
Corrective procedure: Rebatch rather than dose the moulds. Grate or cut the soft soap into a pot, add about 1 L water, and warm gently to melt. Dissolve the calculated missing NaOH (about 0.5 kg pure, so 0.55 kg of 92% pellets) in 1 L water, cool it, and stir it in slowly. Stir until the mixture thickens to trace, then pour into moulds. Cure for four to six weeks in a dry, ventilated place. Test a small sample first: a pH strip on a freshly wetted surface should be around 9–10, and the bar should lather. If the rebatch still separates, use it as laundry soap rather than selling it as toilet soap.
Process controls for future batches:
- Measure FFA and NaOH purity for every incoming lot; adjust the recipe by calculation, not by habit.
- Use a written batch sheet with weights, temperature, and mixing time; check the zap test and pH before moulding.
- Keep the oil and lye solutions at a consistent temperature and stir to true trace.
Do not sell the current batch until it has been tested and cured.