Making Soap with Caustic Soda: Saponification Values Explained

Soap is made by reacting fats or oils with an alkali. For solid bar soap, that alkali is caustic soda (sodium hydroxide). The amount you need depends on which oils you use, and getting it wrong gives soap that is greasy or harsh. This guide explains saponification values and how to use them.

What saponification means

Oils and fats are triglycerides. When they react with sodium hydroxide, they split into glycerine and sodium salts of fatty acids, which is soap. The reaction is called saponification.

The saponification value

Each oil needs a specific amount of alkali to turn completely into soap. This is its saponification (SAP) value, often given as grams of NaOH per gram of oil. Commonly used approximate values are:

Values vary slightly between sources and batches. Commercial soap makers test their oils; small makers use published values with a safety margin.

Working out the caustic soda

1. Multiply each oil's weight by its SAP value and add the results.
2. Reduce the total by a superfat margin, commonly around 5%, so a little oil stays unreacted and the soap is mild.
3. Correct for the purity of your caustic. For 99% flakes, divide by 0.99.

Example: 700 g olive oil and 300 g coconut oil. 700 × 0.135 = 94.5 g, plus 300 × 0.183 = 54.9 g, total 149.4 g. With 5% superfat: 141.9 g. Corrected for 99% purity: about 143 g of caustic soda flakes.

Safety when making soap

• Wear goggles and gloves throughout. Fresh soap batter is still caustic.
• Add caustic to water, never water to caustic, in a stainless steel or heat-resistant plastic container. It gets very hot.
• Never use aluminium pots or utensils.
• Let the soap cure for several weeks before use so the reaction completes.

Quality of the caustic

Use a consistent, high-purity caustic soda so your calculation holds batch after batch. Soap makers in Oman can find caustic soda flakes 99% with a published daily price at https://muscatchemical.com/caustic-soda.