Methylbenzethonium Chloride Ointment
Methylbenzethonium Chloride Ointment contains NLT 90.0% and NMT 110.0% of the labeled amount of methylbenzethonium chloride (C28H44ClNO2·H2O).
•  A.
Sample:  Suspend 0.5 g of methylbenzethonium chloride ointment in 10 mL of water.
Analysis:  Add 0.1 g of sodium carbonate, 1 mL of bromophenol blue TS, and 10 mL of chloroform to the Sample, and shake the mixture.
Acceptance criteria:  The chloroform layer is blue.
•  Procedure
Standard stock solution:  4.446 mg/mL of USP Docusate Sodium RS in isopropyl alcohol (equivalent to 0.01 M of USP Docusate Sodium RS). Store this solution in a tightly-stoppered glass container. Prepare the Standard solution on the day of use.
Standard solution  44.46 µg/mL of USP Docusate Sodium RS in water from the Standard stock solution (equivalent to 0.1 mM of USP Docusate Sodium RS)
Sample solution:  Transfer an amount of Ointment, equivalent to 0.5 mg of methylbenzethonium chloride, to a glass-stoppered, 50-mL cylinder. Add 5 mL of chloroform (freshly purified by shaking 100 mL with 10 g of silica gel, allowing to settle, and withdrawing the supernatant), 5 mL of phosphoric acid solution (1 in 10), and 1 mL of 0.05 mg/mL of safranin O solution.
Analysis:  Titrate the Sample solution with the Standard solution until 1 mL from the endpoint, then shake the stoppered tube vigorously for about 2 min, and continue the titration in 0.1-mL increments, shaking vigorously after each addition, until a pink color appears in the chloroform layer. Perform a blank determination, and make any necessary correction (see Titrimetry 541). Each mL of Standard solution is equivalent to 48.01 µg of methylbenzethonium chloride (C28H44ClNO2·H2O).
Acceptance criteria:  90.0%–110.0%
•  pH 791: 5.0–7.0, in a dispersion of 10 mg/mL of the sample in carbon dioxide-free water
•  Packaging and Storage: Preserve in collapsible tubes or in tight containers.
•  USP Reference Standards 11
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Monograph Leonel M. Santos, Ph.D.
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(SM12010) Monographs - Small Molecules 1
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USP35–NF30 Page 3867