What Is Saponification?

Saponification is a chemical reaction between a strong base and a triglyceride that results in the formation of a salt. This process involves hydrolysis, where water molecules cleave into hydroxide anions and hydrogen cations. The resulting salt can create an oil and water emulsion for cleaning and is better known to laypeople as soap. This process also results in the formation of glycerin, a chemical compound with a variety of uses.

There are two ways chemists can accomplish this reaction. In both cases, they start with a material rich in triglyceride fats, like animal or vegetable oil. One technique involves treating the fat with lye or another very powerful base to saponify it, while the other approach requires two steps, one to steam the fat and another to treat it with alkali. This process tends to result in a soap of purer quality, and one that may also be less harsh on the skin.

The saponification process is one of the oldest chemical reactions known to humans and has been widely used in the production of soap for centuries. Soap makers could use rendered animal fat in their work to make crude basic soaps, or vegetable oils like olive oil for higher quality products. They also learned how to add fillers to soaps to make them less harsh, add scents, or create an exfoliating quality in the finished product. Some regions, like France, are particularly famous for their soap making.

Beyond being useful for soap production, this reaction can be observed and studied in the laboratory. Researchers can explore a variety of materials that are known to saponify in the right conditions to learn more about how, why, and when this process occurs in nature. They can also determine the saponification number, which indirectly references the length of the fatty acid chains in the triglycerides. The shorter the number, the longer the chains.

Two unexpected places where it is possible to observe this process in the wild are in forensics and art conservation. For reasons unclear to researchers, some dead bodies saponify, usually when they are kept in moist conditions. All or part of the body may develop a waxy deposit known as adipocere, or grave wax, that is the result of this reaction. Art historians have also observed the process in some old oil paintings, the result of a chemical reaction with the oils used in the paints.