Test soap for free alkali, hardness and lather before anyone washes with it: cabbage-juice indicator should read blue-green at pH 8-10, deep green or yellow means pH 12 or higher and too much lye, and the bar should be cured for 4-6 weeks. The best quality control happens before the pot is lit, by calculating the lye the fat needs from the reaction’s molar masses and weighing it. A soap that fails the pH test is not thrown away; it is cooked again with more fat.
Part of Organic Chemistry
Checking pH, hardness and lather on finished soap, and calculating the lye charge beforehand so the tests confirm rather than rescue.
Tests Confirm; The Calculation Prevents
The carrier gives two quality tests and a cure time.
- The taste-zap test. The carrier says to touch a small amount to the tongue, carefully: it should taste soapy, not burning, and if it “zaps like a battery” there is excess alkali, so add more fat and re-cook. Treat this as a test for cured soap only; fresh soap can still hold free lye that burns.
- The pH indicator test. The carrier says good soap reads pH 8-10 (blue-green) on cabbage juice, and pH 12 or more (deep green to yellow) is too alkaline. The pH Testing page gives the same indicator logic and adds that fresh, uncured soap tests 12-13, so read the indicator on cured soap.
- Cure for 4-6 weeks before use, from the carrier’s mistakes table.
Add three simple bench tests that need no corpus figure: press a thumbnail into a cured bar (it should dent slightly, not crumble or smear), shake a small shaving in a jar of soft water (thick, lasting lather), and repeat in hard water (curds and scum are normal for any fat soap in hard water, and are not a sign of poor soap).
What You Need
- A kitchen or bench scale accurate to 1 g or better.
- Cabbage-juice indicator and a clear glass.
- Soft water (rainwater), and a sample of your hard water for comparison.
- Eye protection and gloves for any raw soap.
Method: A Pre-Pour Lye Check
Step 1: Weigh the fat for the batch.
Step 2: Calculate the lye for pure sodium hydroxide, as in the worked example.
Step 3: Reduce it by the superfat you want, so a little fat is left unreacted. Corpus pages on Saponification give 5-10% more fat than the lye can react with.
Step 4: After cure, test with indicator and by thumbnail, then lather.
Worked Example: Pure Sodium Hydroxide Needed For 1 kg Of Tallow
Treat tallow as tristearin, C57H110O6, a stand-in for a fat; real tallow is a mixture. Use rounded atomic masses C = 12, H = 1, O = 16, Na = 23.
- Molar mass of the fat: 57 × 12 + 110 × 1 + 6 × 16 = 684 + 110 + 96 = 890 g per mole.
- Each fat molecule reacts with 3 NaOH (one per acid chain). NaOH is 23 + 16 + 1 = 40 g per mole, so 3 × 40 = 120 g.
- Lye per gram of fat: 120 ÷ 890 = 0.135.
- For 1 kg of fat (1,000 g): 1,000 × 0.135 = 135 g of pure NaOH.
- Superfat by 5%: 135 × 0.95 ≈ 128 g.
Potassium hydroxide (KOH, 56 g per mole) needs 3 × 56 = 168 g per 890 g of fat, which is 168 ÷ 890 = 0.189, or 189 g per kilogram.
The carrier’s ratio. The carrier states approximately 1 part alkali to 2.5 parts fat by weight, which is 0.40 kg of alkali per kilogram of fat. That is about three times the 135 g for pure sodium hydroxide, so it cannot describe dry lye. It fits if the “alkali” is a solution of roughly one third strength by weight: 135 ÷ 0.33 ≈ 410 g of solution per kilogram of fat, a ratio of 1 to 2.4. That reading is an inference, not something the carrier says, so read the carrier’s ratio as a solution and confirm the strength by test. Wood-ash lye is much weaker still, which is why the corpus’s Saponification page gives its quantities in litres of strong lye per kilogram of fat.
Common Mistakes
- Tasting uncured soap. A tongue test on fresh soap can burn the mouth. Cure first, test a tiny amount from a cured bar.
- Judging the soap by how strongly it lathers in hard water. Hard-water curds are a water property.
- Compensating for a failed pH test with water. Dilution does not remove free lye; re-cook with more fat as the carrier says.
- Using a recalled ratio without weighing. Always weigh.
Safety
Raw soap is a caustic
Fresh soap can still hold free lye at pH 12-13, which burns skin and eyes. Wear gloves and eye protection when cutting and testing, keep it away from children, and let it cure. A bar that fails the pH test is cooked again, not given away. See Protective Gear and Spill Response.
What’s Next
- Learn next: Antiseptic Compounds — a different use of the same hygiene chemistry
- Combine with: Soap Varieties — what different fats and alkalis change