Name: Stearic Acid
CAS No: Octadecanoic acid [57-11-4]
BP: Stearic Acid JP: Stearic Acid PhEur: Stearic Acid USP-NF: Stearic Acid
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.
Octadecanoic acid [57-11-4]
C18H36O2 284.47 (for pure material) The USP32–NF27 describes stearic acid as a mixture of stearic acid (C18H36O2) and palmitic acid (C16H32O2). In the USP32– NF27, the content of stearic acid is not less than 40.0% and the sum of the two acids is not less than 90.0%. The USP32–NF27 also contains a monograph for purified stearic acid; see Section 17. The PhEur 6.5 contains a single monograph for stearic acid but defines stearic acid 50, stearic acid 70, and stearic acid 95 as containing specific amounts of stearic acid (C18H36O2);
Stearic acid is widely used in oral and topical pharmaceutical formulations. It is mainly used in oral formulations as a tablet and capsule lubricant;(1–3) see Table I, although it may also be used as a binder(4) 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.(5) 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.(6,7) 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.
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.
See Table II.
Acid value 195–212 Boiling point 3838C Density (bulk) 0.537 g/cm3 Density (tapped) 0.571 g/cm3 Density (true) 0.980 g/cm3 Flash point 1138C (closed cup) Melting point 69–708C Moisture content Contains practically no water. NIR spectra see Figure 1. Partition coefficient Log (oil : water) = 8.2 Refractive index 1.43 at 808C Saponification value 200–220 Solubility Freely soluble in benzene, carbon tetrachloride, chloroform, and ether; soluble in ethanol (95%), hexane, and propylene glycol; practically insoluble in water.(8) Specific surface area 0.51–0.53 m2 /g
Stearic acid is a stable material; an antioxidant may also be added to it; see Section 13. The bulk material should be stored in a wellclosed container in a cool, dry place.
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,(9) 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.(10)
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 USP32–NF27 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.
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(11) LD50 (rat, IV): 21.5 mg/kg
Observe normal precautions appropriate to the circumstances and quantity of material handled. Stearic acid dust may be irritant to the skin, eyes, and mucous membranes. Eye protection, gloves, and a dust respirator are recommended. Stearic acid is combustible.
GRAS listed. Accepted as a food additive in Europe (fatty acids). Included in the FDA Inactive Ingredients Database (sublingual tablets; oral capsules, solutions, suspensions, and tablets; topical and vaginal preparations). Included in nonparenteral medicines licensed in the UK. Included in the Canadian List of Acceptable Non-medicinal Ingredients.
Calcium stearate; magnesium stearate; polyoxyethylene stearates; purified stearic acid; zinc stearate. Purified stearic acid Empirical formula C18H36O2 Molecular weight 284.47 CAS number [57-11-4] Synonyms Octadecanoic acid Acid value 195–200 Boiling point 3618C Density 0.847 g/cm3 at 708C Flash point 1968C Iodine number 41.5 Melting point 66–698C Refractive index nD 80 = 1.4299 Solubility Soluble 1 in 5 parts benzene, 1 in 6 parts carbon tetrachloride, 1 in 2 parts chloroform, 1 in 15 parts ethanol, 1 in 3 parts ether; practically insoluble in water. Vapor density (relative) 9.80 (air = 1) Comments The USP32–NF27 describes purified stearic acid as a mixture of stearic acid (C18H36O2) and palmitic acid (C16H32O2), which together constitute not less than 96.0% of the total content. The content of C18H36O2 is no less than 90.0% of the total.
Stearic acid is one of the materials that have been selected for harmonization by the Pharmacopeial Discussion Group. For further information see the General Information Chapter <1196> in the USP32–NF27, the General Chapter 5.8 in PhEur 6.0, along with the ‘State of Work’ document on the PhEur EDQM website, and also the General Information Chapter 8 in the JP XV. A wide range of different grades of stearic acid are commercially available that have varying chemical compositions and hence different physical and chemical properties; see Table III.(12) Stearic acid is highly soluble in structurally diverse solvents. Stearic acid/ solvent packing within a 24.8 ?3 cubic volume explains the stoichiometry of stearic acid solubility at multiple temperatures in multiple solvents.(13) In one study the release of an active drug in a formulation containing stearic acid was independent of compression pressure in the range 1–7 tons; the particle size of the stearic acid did have a significant influence on the drug release.(14) A potential application of stearic acid is in the preparation of ‘cushioning pellets’, composed of stearic acid : microcrystalline cellulose (4 : 1 w/w). The use of these pellets may avoid rupture of the coating of pellets during the compression step of manufacturing.(15) A specification for stearic acid is contained in the Food Chemicals Codex (FCC).(16) The EINECS number for stearic acid is 200-313-4.