saponification

Saponification is the base-catalyzed hydrolysis of esters, most commonly the ester bonds in triglycerides found in fats and oils. Under alkaline conditions, triglycerides are converted into glycerol and the salts of fatty acids, collectively known as soap.

In the typical reaction, a triglyceride reacts with a strong base such as sodium hydroxide (NaOH) to yield glycerol and three fatty acid sodium salts (soap). The general stoichiometry can be summarized as triglyceride + 3 NaOH → glycerol + 3 RCOONa. The underlying mechanism involves hydroxide ions attacking the ester carbonyls, leading to breakdown of the triglyceride and formation of alkoxide intermediates that are protonated to give glycerol and fatty acid salts.

Common bases used in soap making include NaOH for harder, bar soaps and KOH for softer or liquid soaps. The fatty acid chains vary with the starting fats or oils, producing a range of soap properties such as hardness, lather, and melting point. Some non-saponifiable matter, such as certain sterols and hydrocarbons, remains unused in the reaction.

Processing methods include cold process, where mixtures are allowed to saponify at ambient temperatures over time, often followed by curing; and hot process, which uses applied heat to speed saponification and shorten production. The quality of the resulting soap depends on the fatty acid composition, presence of impurities, and the degree of saponification, which is often expressed as a saponification value (the milligrams of KOH required to saponify one gram of fat).

Saponification has applications in traditional soap making, industrial detergent production, and qualitative chemistry demonstrations. It is typically followed by washing and purification steps to remove excess alkali and glycerol.