Acidum Stearicum. Stearic Acid.
Related entry: Oleic acid
Stearic acid, sometimes incorrectly called "stearin," is a mixture of solid fatty acids, chiefly stearic and palmitic, obtained by the hydrolysis of various fats with subsequent removal of the liquid acids by cooling and filtration; it may, however, contain from 20 to 30 per cent. of oleic acid. Stearic acid occurs in white, hard masses, showing signs of crystallisation, almost odourless and tasteless. Melting-point, 56° to 65°. Acid number, 200 to 210; ester number, 0 to 10; saponification number, 200 to 220. If stearic acid be warmed with a solution of an alkali carbonate, it combines with the alkali, and carbon dioxide is evolved. It should leave no residue on ignition. If 5 grammes be heated with 1 gramme of sodium hydroxide and 30 to 40 mils of alcohol until combined, the solution evaporated to dryness with 20 grammes of sand that has been washed with hydrochloric acid, and the perfectly dry residue then extracted with petroleum ether, the latter should leave only the slightest residue on evaporation (absence of paraffin). Stearic acid may be powdered by sprinkling it with alcohol during trituration or by melting 2 parts of the acid with 1 part of absolute alcohol and rubbing in a mortar till cool. Pure stearic acid, C17H35COOH (C18H36O2 = 284.288), is obtained by recrystallising the commercial substance from hot alcohol. It occurs in white, shining, flaky crystals, or as a hard, somewhat glossy solid. Melting-point, 69.2°. It is official in the U.S.P. Stearic acid should be kept in a cool place, and not exposed to light.
Insoluble in water; soluble in alcohol (1 in 50), readily soluble in ether or chloroform.
Use.—Combined with sodium carbonate, stearic acid is used as a basis for glycerin suppositories. It is also used for the preparation of stearates of some of the metals, such as copper and zinc, and of alkaloidal stearates, the latter being prepared by adding the free alkaloids to alcoholic solutions of the acid.