Saponification Definition and Reaction

Saponification is the chemical reaction that makes soap.
Saponification is the chemical reaction that makes soap. Zara Ronchi / Getty Images

Definition of Saponification

In saponification, a fat reacts with a base to form glycerol and soap.
In saponification, a fat reacts with a base to form glycerol and soap. Todd Helmenstine

Saponification Definition

Usually, saponification is a process by which triglycerides are reacted with sodium or potassium hydroxide (lye) to produce glycerol and a fatty acid salt, called 'soap'. The triglycerides are most often animal fats or vegetable oils. When sodium hydroxide is used, a hard soap is produced. Using potassium hydroxide results in a soft soap.

Lipids that contain fatty acid ester linkages can undergo hydrolysis. This reaction is catalyzed by a strong acid or base. Saponification is the alkaline hydrolysis of the fatty acid esters. The mechanism of saponification is:

  1. Nucleophilic attack by the hydroxide
  2. Leaving group removal
  3. Deprotonation

Saponification Example

The chemical reaction between any fat and sodium hydroxide is a saponification reaction.

triglyceride + sodium hydroxide (or potassium hydroxide) → glycerol + 3 soap molecules

One Step Versus Two Step Process

Saponification is the chemical reaction that makes soap.
Saponification is the chemical reaction that makes soap. Zara Ronchi / Getty Images

While most often the one-step triglyceride reaction with lye is considered, there is also a two-step saponification reaction. In the two-step reaction, steam hydrolysis of the triglyceride yields carboxylic acid (rather than its salt) and glycerol. In the second step of the process, alkali neutralizes the fatty acid to produce soap.

The two-step process is slower, but the advantage of the process is that it allows for purification of the fatty acids and thus a higher quality soap.

Applications of the Saponification Reaction

Saponification sometimes occurs in old oil paintings.
Saponification sometimes occurs in old oil paintings. Lonely Planet / Getty Images

Saponification may result in both desirable and undesirable effects.

The reactions sometimes damages oil paintings when heavy metals used in pigments react with free fatty acids (the "oil" in oil paint), forming soap. The process was described in 1912 in works from the 12th through 15th century. The reaction starts in the deep layers of a painting and works toward the surface. At present, there is no way to stop the process or identify what causes it to occur. The only effective restoration method is retouching.

Wet chemical fire extinguishers use saponification to convert burning oils and fats into non-combustible soap. The chemical reaction further inhibits the fire because it is endothermic, absorbing heat from the surroundings and lowering the temperature of the flames.

While sodium hydroxide hard soap and potassium hydroxide soft soap are used for everyday cleaning, there are soaps made using other metal hydroxides. Lithium soaps are used as lubricating greases. There are also "complex soaps" consisting of a mixture of metallic soaps. An example is a lithium and calcium soap.

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Helmenstine, Anne Marie, Ph.D. "Saponification Definition and Reaction." ThoughtCo, Oct. 3, 2017, thoughtco.com/definition-of-saponification-605959. Helmenstine, Anne Marie, Ph.D. (2017, October 3). Saponification Definition and Reaction. Retrieved from https://www.thoughtco.com/definition-of-saponification-605959 Helmenstine, Anne Marie, Ph.D. "Saponification Definition and Reaction." ThoughtCo. https://www.thoughtco.com/definition-of-saponification-605959 (accessed November 18, 2017).