Methylbenzethonium Chloride Ointment
DEFINITION
Methylbenzethonium Chloride Ointment contains NLT 90.0% and NMT 110.0% of the labeled amount of methylbenzethonium chloride (C28H44ClNO2·H2O).
IDENTIFICATION
• 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.
ASSAY
• 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).
541). Each mL of Standard solution is equivalent to 48.01 µg of methylbenzethonium chloride (C28H44ClNO2·H2O).
Acceptance criteria:
90.0%–110.0%
SPECIFIC TESTS
• pH 791:
5.0–7.0, in a dispersion of 10 mg/mL of the sample in carbon dioxide-free water
791:
5.0–7.0, in a dispersion of 10 mg/mL of the sample in carbon dioxide-free water
ADDITIONAL REQUIREMENTS
• Packaging and Storage:
Preserve in collapsible tubes or in tight containers.
• USP Reference Standards 11
11
USP Docusate Sodium RS 
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Auxiliary Information—
Please check for your question in the FAQs before contacting USP.
| Topic/Question | Contact | Expert Committee |
|---|---|---|
| Monograph | Leonel M. Santos, Ph.D.
Senior Scientific Liaison 1-301-816-8168 |
(SM12010) Monographs - Small Molecules 1 |
| Reference Standards | RS Technical Services 1-301-816-8129 rstech@usp.org |
USP35–NF30 Page 3867