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Saponification of a lipid with potassium hydroxide.

Simplified diagram of the saponification process

Saponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of a carboxylic acid. Saponification is commonly used to refer to the reaction of a metallic alkali (base) with a fat or oil to form soap. Saponifiable substances are those that can be converted into soap.

CH₂-OOC-R - CH-OOC-R - CH₂-OOC-R (fat) + 3 NaOH ( or KOH)

both heated →

CH₂-OH -CH-OH - CH₂-OH (glycerol) + 3 R-CO₂-Na (soap)

where R=(CH₂)₁₄CH₃ in the example (right)

Sodium hydroxide (NaOH) is a caustic base. If NaOH is used a hard soap is formed, whereas when potassium hydroxide (KOH) is used, a soft soap is formed. Vegetable oils and animal fats are fatty esters in the form of triglycerides. The alkali breaks the ester bond and releases the fatty acid and glycerol. If necessary, soaps may be precipitated by salting it out with saturated sodium chloride.

Saponification can also refer to the other soft tissue in a conversion of the fat of a corpse into adipocere, often called "grave wax." This process is more common where the amount of fatty tissue is high, the agents of decomposition are absent or only minutely present, and the burial ground is particularly alkaline.

Fires involving cooking fats and oils (classified as Class K) burn hotter than other typical combustible liquids, rendering a standard class B extinguisher ineffective. Such fires should be extinguished with a wet chemical extinguisher. Extinguishers of this type are designed to extinguish cooking fats and oils through saponification. The extinguishing agent rapidly converts the burning substance to a non-combustible soap. This process is endothermic, meaning it absorbs energy (in this case, thermal energy) from its surroundings, decreasing the temperature and eliminating the fire.

• Saponification value
• Soapmaking
• Unsaponifiable
• Incorruptibility

• Soapmaking at Bellaonline - Soapmaking articles, forum and supplier links.
• Soap Naturally Web and Mailing List - Resources for natural handmade soapmakers.
• Soap Recipe Corner - Soapmaking explained.
• About Candle and Soap Making - Soap making at About.com
• Glossary for the Modern Soap Maker - A collection of terms, definitions and acronyms for today's soap maker.
• Adipocere - A collection of resources on soap mummies and adipocere formation.
• The Handbook of Soap Manufacture - A book from 1908.