How Saponification Makes Soap

Soap and the Saponification Reaction

In the industrial manufacture of soap, tallow (fat from animals such as cattle and sheep) or vegetable fat is heated with sodium hydroxide. Once the saponification reaction is complete, sodium chloride is added to precipitate the soap. The water layer is drawn off the top of the mixture and the glycerol is recovered using vacuum distillation.

The crude soap obtained from the saponification reaction contains sodium chloride, sodium hydroxide, and glycerol. These impurities are removed by boiling the crude soap curds in water and re-precipitating the soap with salt. After the purification process is repeated several times, the soap may be used as an inexpensive industrial cleanser. Sand or pumice may be added to produce a scouring soap. Other treatments may result in laundry, cosmetic, liquid, and other soaps.

Types of Soaps

The saponification reaction may be tailored to produce different types of soaps:

Hard Soap: Hard soap is made using sodium hydroxide (NaOH) or lye. Hard soaps are especially good cleansers in hard water that contains magnesium, chloride, and calcium ions.

Soft Soap: Soft soap is made using potassium hydroxide (KOH) rather than sodium hydroxide. In addition to being softer, this type of soap has a lower melting point. Most early soaps were made using potassium hydroxide obtained from wood ash and animal fats. Modern soft soaps are made using vegetable oils and other polyunsaturated triglycerides. These soaps are characterized by weaker intermolecular forces between the salts. They readily dissolve, yet also tend not to last as long.

Lithium Soap: Moving down the periodic table in the alkali metals group, it should be obvious soap may be made using lithium hydroxide (LiOH) as easily as NaOH or KOH. Lithium soap is used as a lubricating grease. Sometimes complex soaps are made using lithium soap and also calcium soap.

Saponification of Oil Paintings

Sometimes the saponification reaction occurs unintentionally. Oil paint came into use because it withstood the test of time. Yet, over time the saponification reaction has led to damage of many (but not all) oil paintings made in the fifteenth through twentieth centuries.

The reaction occurs when heavy metal salts, such as those in red lead, zinc white, and lead white, react with the fatty acids in the oil. The metal soaps produced by the reaction tend to migrate toward the surface of the painting, causing the surface to deform and producing a chalky discoloration called "bloom" or "efflorescence." While a chemical analysis may be able to identify saponification before it becomes apparent, once the process starts, there is no cure. The only effective restoration method is retouching.

Saponification Number

The number of milligrams of potassium hydroxide needed to saponify one gram of fat is termed its saponification number, Koettstorfer number, or "sap." The saponification number reflects the average molecular weight of the fatty acids in a compound. Long chain fatty acids have a low saponification value because they contain fewer carboxylic acid functional groups per molecule than short chain fatty acids. The sap value is calculated for potassium hydroxide, so for soap made using sodium hydroxide, its value must be divided by 1.403, which is the ratio between the KOH and NaOH molecular weights.

Some oils, fats, and waxes are deemed to be unsaponifiable. These compounds fail to form soap when mixed with sodium hydroxide or potassium hydroxide. Examples of unsaponifiable materials include beeswax and mineral oil.

Sources

• Anionic and Related Lime Soap Dispersants, Raymond G. Bistline Jr., in Anionic Surfactants: Organic Chemistry, Helmut Stache, ed., Volume 56 of Surfactant Science Series, CRC Press, 1996, chapter 11, p. 632, ISBN 0-8247-9394-3.
• Cavitch, Susan Miller. The Natural Soap Book. Storey Publishing, 1994 ISBN 0-88266-888-9.
• Levey, Martin (1958). "Gypsum, salt and soda in ancient Mesopotamian chemical technology". Isis. 49 (3): 336–342 (341). doi:10.1086/348678
• Schumann, Klaus; Siekmann, Kurt (2000). "Soaps". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_247. ISBN 3-527-30673-0.
• Willcox, Michael (2000). "Soap". In Hilda Butler. Poucher's Perfumes, Cosmetics and Soaps (10th ed.). Dordrecht: Kluwer Academic Publishers. ISBN 0-7514-0479-9.