Stearic acid

Category

Emulsifying Agents; Solubilizing Agents; Tablet and Capsule Lubricant

Molecular Formula

C18H36O2

Molecular Weight

284.47 (for pure material)

Grade

Pharmceutical Excipients

Description

Stearic acid is a hard, white or faintly yellow-colored, somewhat glossy, crystalline solid or a white or yellowish white powder. It has a slight odor (with an odor threshold of 20 ppm) and taste suggesting tallow.

UNII

4ELV7Z65AP

Chemical Name

Octadecanoic acid

Synonyms

Acidum stearicum; cetylacetic acid; Crodacid; Cristal G; Cristal S; Dervacid; E570; Edenor; Emersol; Extra AS; Extra P; Extra S; Extra ST; 1-heptadecanecarboxylic acid; Hystrene; Industrene; Kortacid 1895; Pearl Steric; Pristerene; stereophanic acid; Tegostearic

Administration route

Sublingual; oral; topical and vaginal

Dosage Form

Sublingual tablets; oral capsules, solutions, suspensions, and tablets; topical and vaginal preparations

Stability and Storage Conditions

Stearic acid is a stable material; an antioxidant may also be added to it. The bulk material should be stored in a wellclosed container in a cool, dry place.

Source and Preparation

Stearic acid is manufactured by hydrolysis of fat by continuous exposure to a countercurrent stream of high-temperature water and fat in a high-pressure chamber. The resultant mixture is purified by vacuum steam distillation and the distillates are then separated using selective solvents. Stearic acid may also be manufactured by the hydrogenation of cottonseed and other vegetable oils; by the hydrogenation and subsequent saponification of olein followed by recrystallization from alcohol; and from edible fats and oils by boiling with sodium hydroxide, separating any glycerin, and decomposing the resulting soap with sulfuric or hydrochloric acid. The stearic acid is then subsequently separated from any oleic acid by cold expression. Stearic acid is derived from edible fat sources unless it is intended for external use, in which case nonedible fat sources may be used. The USP states that stearic acid labeled solely for external use is exempt from the requirement that it be prepared from edible sources. Stearic acid may contain a suitable antioxidant such as 0.005% w/w butylated hydroxytoluene.

Applications

Stearic acid is widely used in oral and topical pharmaceutical formulations. It is mainly used in oral formulations as a tablet and capsule lubricant, although it may also be used as a binder or in combination with shellac as a tablet coating. It has also been suggested that stearic acid may be used in enteric tablet coatings and as a sustained-release drug carrier. In topical formulations, stearic acid is used as an emulsifying and solubilizing agent. When partially neutralized with alkalis or triethanolamine, stearic acid is used in the preparation of creams. The partially neutralized stearic acid forms a creamy base when mixed with 5-15.times its own weight of aqueous liquid, the appearance and plasticity of the cream being determined by the proportion of alkali used. Stearic acid is used as the hardening agent in glycerin suppositories. Stearic acid is also widely used in cosmetics and food products.

Safety

Stearic acid is widely used in oral and topical pharmaceutical formulations; it is also used in cosmetics and food products. Stearic acid is generally regarded as a nontoxic and nonirritant material.However, consumption of excessive amounts may be harmful. LD50 (mouse, IV): 23 mg/kg LD50 (rat, IV): 21.5 mg/kg

Incompatibilities

Stearic acid is incompatible with most metal hydroxides and may be incompatible with bases, reducing agents, and oxidizing agents. Ointment bases made with stearic acid may show evidence of drying out or lumpiness due to such a reaction when compounded with zinc or calcium salts. A number of differential scanning calorimetry studies have investigated the compatibility of stearic acid with drugs. Although such laboratory studies have suggested incompatibilities, e.g. with naproxen, they may not necessarily be applicable to formulated products. Stearic acid has been reported to cause pitting in the film coating of tablets applied using an aqueous film-coating technique; the pitting was found to be a function of the melting point of the stearic acid.