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 C18H36O2is not less than 40.0percent,and the sum of the two is not less than 90.0percent.
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 á651ñ: not lower than 54.
Iodine value á401ñ: not more than 4.Proceed as directed in Method Iexcept to use 35mLof chloroform.
Residue on ignition á281ñ: not more than 4mg,determined on a 4-g portion (0.1%).
Mineral acid— Shake 5g of melted Stearic Acid with an equal volume of hot water for 2minutes,cool,and filter:the filtrate is not reddened by the addition of 1drop of methyl orange TS.
Neutral fat or paraffin— Add 1g of Stearic Acid to 30mLof anhydrous sodium carbonate solution (1in 60)in a flask,and boil the mixture:the resulting solution,while hot,shows not more than a faint opalescence.
Organic volatile impurities,Method Vá467ñ: meets the requirements.
Solvent— Use dimethyl sulfoxide.
Assay— Place about 100mg of Stearic Acid in a small conical flask fitted with a suitable reflux attachment.Place about 50mg of USP Stearic Acid RSand about 50mg of USP Palmitic Acid RSin a similar flask.Treat each flask as follows.Add 5.0mLof a solution prepared by dissolving 14g of boron trifluoride in methanol to make 100mL,swirl to mix,and reflux for 15minutes or until the solid is dissolved.Cool,transfer the reaction mixture with the aid of 10mLof chromatographic solvent hexane to a 60-mLseparator,and add 10mLof water and 10mLof saturated sodium chloride solution.Shake,allow to separate,then drain and discard the lower,aqueous layer.Pass the hexane layer through 6g 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-µLto 2-µLportion 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,1.5m in length and 3mm in inside diameter,and it is packed with 15%G4on 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 á621ñ),is not less than 2.0between 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 C18H36O2in the portion of Stearic Acid taken by the formula:
100(A/B),
in which Ais the area due to the methyl stearate peak,and Bis the sum of the areas of all of the fatty acid ester peaks in the chromatogram.Similarly,determine the percentage of C16H32O2.
Auxiliary Information— Staff Liaison:Justin Lane,B.S.,Scientific Associate
Expert Committee:(EMC)Excipients:Monograph Content
USP28–NF23Page 3092
Pharmacopeial Forum:Volume No.29(2)Page 480
Phone Number:1-301-816-8323