Stearic Acid

Octadecanoic acid.
Stearic acid [57-11-4].
» Stearic Acid is manufactured from fats and oils derived from edible sources and is a mixture of Stearic Acid (C18H36O2) and palmitic acid (C16H32O2). The content of C18H36O2 is not less than 40.0 percent, and the sum of the two is not less than 90.0 percent.
note—Stearic Acid labeled solely for external use is exempt from the requirement that it be prepared from edible sources.
Packaging and storage— Preserve in well-closed containers.
Labeling— If it is for external use only, the labeling so indicates.
Congealing temperature 651: not lower than 54.
Iodine value 401: not more than 4. Proceed as directed in Method I except to use 35 mL of chloroform.
Residue on ignition 281: not more than 4 mg, determined on a 4-g portion (0.1%).
Mineral acid— Shake 5 g of melted Stearic Acid with an equal volume of hot water for 2 minutes, cool, and filter: the filtrate is not reddened by the addition of 1 drop of methyl orange TS.
Neutral fat or paraffin— Add 1 g of Stearic Acid to 30 mL of anhydrous sodium carbonate solution (1 in 60) in a flask, and boil the mixture: the resulting solution, while hot, shows not more than a faint opalescence.
Assay— Place about 100 mg of Stearic Acid in a small conical flask fitted with a suitable reflux attachment. Place about 50 mg of USP Stearic Acid RS and about 50 mg of USP Palmitic Acid RS in a similar flask. Treat each flask as follows. Add 5.0 mL of a solution prepared by dissolving 14 g of boron trifluoride in methanol to make 100 mL, swirl to mix, and reflux for 15 minutes or until the solid is dissolved. Cool, transfer the reaction mixture with the aid of 10 mL of chromatographic solvent hexane to a 60-mL separator, and add 10 mL of water and 10 mL of saturated sodium chloride solution. Shake, allow to separate, then drain and discard the lower, aqueous layer. Pass the hexane layer through 6 g of anhydrous sodium sulfate (previously washed with chromatographic solvent hexane) into a suitable flask. Using a syringe fitted with a suitable needle, introduce a 1-µL to 2-µL portion of the assay preparation (which contains the Stearic Acid) into a suitable gas chromatograph equipped with a flame-ionization detector. The column preferably is of glass, 1.5 m in length and 3 mm in inside diameter, and it is packed with 15% G4 on support S1A. The carrier gas is helium, passed through a bed of molecular sieve for drying, if necessary. The temperatures of the port and the detector are maintained at 210, and the column temperature is maintained at 165.
System suitability— In a suitable chromatogram, the resolution factor, R (see Chromatography 621), is not less than 2.0 between the peaks from methyl palmitate and methyl stearate (located by comparison with the chromatogram of the standard preparation), and five replicate injections of a single sample show a coefficient of variation of not more than 1.5% in the percentage of methyl stearate and methyl palmitate, respectively. Measure the peak areas of the fatty acid esters in the chromatogram, and determine the percentage of C18H36O2 in the portion of Stearic Acid taken by the formula:
100(A / B)
in which A is the area due to the methyl stearate peak, and B is the sum of the areas of all of the fatty acid ester peaks in the chromatogram. Similarly, determine the percentage of C16H32O2.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Kevin T. Moore, Ph.D.
(EM105) Excipient Monographs 1
Reference Standards Lili Wang, Technical Services Scientist
USP32–NF27 Page 1360
Pharmacopeial Forum: Volume No. 29(2) Page 480
Chromatographic Column—
Chromatographic columns text is not derived from, and not part of, USP 32 or NF 27.