Liquid soap with KOH: recipe and production guide
TL;DR: Liquid soap is made from potassium hydroxide (KOH, not NaOH) using the hot process method. KOH has a 1.403× higher SAP number than NaOH and is usually 90% pure (not 100%). The paste takes 3-5 hours to cook, then the paste is diluted with water to make the final liquid soap.
Liquid soap differs from solid soap in one basic way: sodium hydroxide (NaOH) is used instead of potassium hydroxide (KOH). Sodium soap salts are solid, potassium salts are soft and soluble in water - hence the liquid consistency.
But the production process is not just "the same, but with a different hydroxide“. Liquid soap goes through a thick paste phase, is diluted with water and requires a slower, controlled process. This page will show you the entire process step by step.
How does KOH differ from NaOH — and why does it matter?
KOH produces a liquid or soft soap, while NaOH produces a solid one. The SAP number for KOH is 1.403× higher and KOH is typically 90% purity (not 100%). These factors require recalculation and a special procedure.
| Result | Bar soap | Liquid soap or paste |
|---|---|---|
| SAP number | Lower (e.g. 0.134 for olive) | Higher (×1.403 versus NaOH) |
| Purity | Usually 100 % | Usually 90% (to be recalculated!) |
| Availability in the Czech Republic | Easily (drugstore as a pipe cleaner) | Worse - pharmacies, e-shops |
| Price | Lower | Higher |
Saponification numbers for KOH
The SAP values for KOH are approximately 1.403× higher than for NaOH. Reason: molecular weight of KOH (56.1 g/mol) vs. NaOH (40.0 g/mol). Ratio: 56,1 ÷ 40,0 = 1,403.
Examples:
- Olive oil: NaOH SAP 0.134 × 1.403 = KOH SAP 0,188
- Coconut oil: NaOH SAP 0.178 × 1.403 = KOH SAP 0,250
- Castor oil: NaOH SAP 0.128 × 1.403 = KOH SAP 0,180
KOH purity: critical factor
Commercial KOH is usually 90% pure (the remaining 10% is water and carbonates). Unlike NaOH, which is 100%, you must always convert the amount of KOH to actual purity:
If the calculator says 90 g of pure KOH and you have 90% KOH: 90 ÷ 0.90 = 100 g of real KOH you measure on the scale.
Always check for cleanliness on the packaging or in the supplier's technical data sheet.
How to make liquid soap — hot process with KOH
The production is done in three stages: cooking the paste (2-3 hours in a slow cooker), testing for doneness and dilution with water. The hot process requires patience, but the control is clear - the paste goes through distinct stages.
Liquid soap is made using the hot process method - we boil the paste until the saponification is complete, then dilute it with water.
Overview
| Paste yield | ~700 g |
|---|---|
| Yield of liquid soap (after dilution) | 1,5-2 l |
| Difficulty | ★★★☆☆ |
| Production time | 3-5 hours |
| Curing | 24-48 hours after dilution |
Ingredients (paste)
- 350 g olive oil (50 %)
- 210 g coconut oil (30%)
- 140 g castor oil (20 %)
- 96.4 g KOH (90% purity)
- 260 g distilled water
Calculation of KOH:
- Olive (SAP KOH 0,188): 350 × 0,188 = 65,8 g
- Coconut (SAP KOH 0,250): 210 × 0,250 = 52,5 g
- Castor (SAP KOH 0,180): 140 × 0,180 = 25,2 g
- Total pure KOH (3% SF): 143.5 × 0.97 = 139.2 g of pure KOH
- For 90% KOH: 139,2 ÷ 0,90 = 154,7 g KOH weigh in
(Always check the exact calculation in the calculator - set KOH mode and enter the purity.)
For diluting the liquid soap paste:
- 700 g finished paste
- 700-1 400 g distilled water (depending on desired density)
Tools
- Slow cooker or water bath pot
- Scales, stick blender, thermometer
- Safety goggles and gloves
- Sealable containers for liquid soap
Procedure — paste production
1. Preparation of KOH solution Wear protective equipment. KOH reacts even more exothermically with water than NaOH - be careful. Weigh the water, slowly add KOH while stirring. Allow to cool to 40-50 °C.
2. Preparation of oils Melt the coconut oil, add the olive oil and castor oil. Temper to 50-60°C - liquid soap is made at higher temperatures than solid CP soap.
3. Connections and first trace Pour KOH solution into the oils. Stir with a stick blender. Trace comes faster than with CP - in 2-5 minutes.
4. Cooking the paste (2-3 hours) Move the pot to a slow cooker (LOW setting) or to a water bath (80-90 °C). Stir every 30 minutes.
The paste goes through phases:
- Applesauce phase: thick, uneven - like applesauce
- Taffy phase: glossy, stiff, difficult to stir - like caramel
- Vaseline phase: transparent or translucent, smooth, gel - done
Doneness test: Dissolve a small sample of the paste (½ teaspoon) in 30 ml of water. If the solution is clear or slightly cloudy without greasy bits - the paste is ready. If the solution is cloudy with oil droplets - keep cooking.
Phenolphthalein test: A drop of phenolphthalein solution on a small sample of paste should remain colorless. Red = free KOH = keep cooking.
5. Cooling of the paste Let the finished paste cool. Store the paste in an airtight container - it will last for 1 year or more.
Procedure — paste dilution
- Weigh the paste into the pot. Add an equal amount of hot distilled water (80-90 °C).
- Stir slowly until the paste is completely dissolved - 15-30 minutes. Do not hurry, do not boil.
- Let stand for 24 hours - the liquid will clear and settle any impurities.
- Pass through a fine sieve.
- Adjust the density: add more water for a thinner consistency.
- Add essential oils or perfume (0.5-1% by weight of liquid soap).
Consistency and thickening
Paste liquid soap is naturally thinner than commercial liquid soap. For a thicker consistency:
Salt solution: Dissolve 1 teaspoon of table salt in 50 ml of water and add to the liquid soap while stirring. Test the density - too much salt will cause thinning.
Hydroxyethylcellulose (HEC): A natural thickener, available in cosmetics stores. 0.5-1% creates a gel-like consistency.
Xanthan gum: 0.3-0.5% for thick, thick liquid soap.
Preservation of liquid soap
Liquid soap has a higher pH (9-10) and resists bacteria. Yet, when water and botanical additives are added, the microbial risk increases.
No added botanical ingredients: Preservative is not necessary. High pH protects naturally.
With botanical ingredients (herbal extracts, aloe vera, hydrolates): Recommend a preservative. Suitable for home use: Optiphen (0.75-1.5%) or Liquid Germall Plus (0.1-0.5%) - both available in cosmetics e-shops.
Shelf life: Without preservative and botanist 6-12 months in an airtight container. With preservative 12-18 months.
Frequently asked questions
What is the difference between 90% and 100% KOH and how does it affect the soap? KOH in the store is usually 90% pure, the remaining 10% is water and carbonates. If the calculator says 100 grams of pure KOH, measure 111 grams of commercial 90% KOH on the scale. Without recalculation, the soap will be undersaponified (cloudy).
How do I know if the paste is ready? A small sample of the paste is dissolved in 30 ml of cold water to form a clear solution without greasy bits. Phenolphthalein test: a drop of indicator on the paste should remain colourless (not red - this would indicate free KOH).
Pasta still cloudy in water after cooking - what to do? Turbidity means unsaponified oils. Keep cooking. After 4+ hours, check KOH calculation - may be too little KOH or miscalculated purity.
Liquid soap is too thin - how to thicken it? Add a thickener (salt solution, HEC or xanthan gum). Or use less water when diluting the paste.
What are the stages of cooking the paste and how do I know which is which? Applesauce (uneven, loose), Taffy (glossy, solidifies), Vaseline (translucent, gel - done). Each stage takes 30-60 minutes, progressing in succession.
How much water should I add to the paste? The ratio is 1:1 to paste weight for a standard fine consistency. More water = thinner soap, less water = thicker. Test gradually.
Do I need to add a preservative to liquid soap? Without botanical additives, no preservative is necessary - the high pH protects naturally. With herbs (aloe, hydrolates) add Optiphen (0.75-1.5%).
See also:
- Soap calculator - KOH mode and cleanliness settings
- Chemistry of saponification - why KOH gives liquid and NaOH gives solid soap
- EU regulations - liquid soap is subject to the same rules as solid
⚠️ Recipe disclaimer
This recipe has been created or revised with the help of artificial intelligence tools and has undergone a recalculation of the KOH grammages. We still recommend checking the lye grammage in an independent calculator before production (e.g. SoapCalc). Working with potassium hydroxide requires protective equipment - see Lye safety. Information is for educational purposes; the manufacturer is not liable for damages resulting from their use.
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