Sodium Stearate
(soe' dee um steer' ate).

Octadecanoic acid, sodium salt.
Sodium stearate [822-16-2].
» Sodium Stearate is a mixture of sodium stearate (C18H35NaO2) and sodium palmitate (C16H31NaO2), which together constitute not less than 90.0 percent of the total content. The content of C18H35NaO2 is not less than 40.0 percent of the total. Sodium stearate contains small amounts of the sodium salts of other fatty acids.
Packaging and storage— Preserve in well-closed, light-resistant containers.
USP Reference standards 11
USP Palmitic Acid RS Click to View Structure
USP Stearic Acid RS Click to View Structure
A: When heated, it fuses. At a high temperature it decomposes, emitting flammable vapors and the odor of burning fat, finally leaving a residue that, when moistened with water, is alkaline to litmus paper, effervesces with acids, and colors a nonluminous flame intensely yellow.
B: Dissolve 25 g in 300 mL of hot water, add 60 mL of 2 N sulfuric acid, and heat the solution, with frequent stirring, until the separated fatty acid layer is clear. Wash the fatty acids with boiling water until they are free from sulfate, collect in a small beaker, and warm on a steam bath until the water has settled and the fatty acids are clear. Allow the acids to cool, pour off the water layer, then melt the acids, filter into a dry beaker while hot, and dry at 105 for 20 minutes: the solidification temperature of the fatty acids is not less than 54.
Acid value of fatty acids 401: between 196 and 211, determined on 1 g of the fatty acids obtained in Identification test B.
Iodine value of fatty acids 401: not more than 4.0, determined on the fatty acids obtained in Identification test B.
Acidity— Heat 50 mL of alcohol to the same temperature, ±5, as that attained when the pink endpoint is reached in the titration of the test specimen. Add 3 drops of phenolphthalein TS and sufficient 0.020 N sodium hydroxide to produce a faint pink color. Add 2.00 g of Sodium Stearate, and dissolve with the aid of a small amount of heat: no pink color is produced. Titrate the solution with 0.020 N sodium hydroxide until a pink color is produced: between 1.00 mL and 4.25 mL of 0.020 N sodium hydroxide is required (between 0.28% and 1.2% as stearic acid).
Loss on drying 731 Tare a beaker containing about 1 g of washed sand, previously dried at 105, add about 500 mg of Sodium Stearate, and again weigh. Add 10 mL of alcohol, evaporate the mixture at about 80 to dryness, and dry at 105 for 4 hours: it loses not more than 5.0% of its weight.
Alcohol-insoluble substances— Reflux 1.0 g with 25 mL of alcohol: it dissolves completely, and the resulting solution is clear or not more than slightly opalescent.
Assay— Proceed with Sodium Stearate as directed in the Assay under Stearic Acid, using about 100 mg of Sodium Stearate, accurately weighed. Determine the percentage of C18H35NaO2 in the portion of Sodium Stearate 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 fatty acid ester peaks in the chromatogram. Similarly, determine the percentage of C16H31NaO2.
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Monograph Robert H. Lafaver, M.S.
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(EXC2010) Monographs - Excipients
Reference Standards RS Technical Services
USP35–NF30 Page 1960