Methylbenzethonium Chloride Lotion
» Methylbenzethonium Chloride Lotion is an emulsion containing not less than 90.0 percent and not more than 110.0 percent of the labeled amount of methylbenzethonium chloride (C28H44ClNO2·H2O).
Packaging and storage Preserve in tight containers.
Identification Suspend about 0.5 mL of Lotion in 20 mL of water, add 0.1 g of sodium carbonate, 1 mL of bromophenol blue TS, and 10 mL of chloroform, and shake the mixture: the chloroform layer is blue.
pH 791: between 5.2 and 6.0.
0.0001 N Docusate sodium Dissolve an accurately weighed quantity of USP Docusate Sodium RS in isopropyl alcohol, and dilute quantitatively with isopropyl alcohol to obtain a solution having a concentration of 4.446 mg of anhydrous docusate sodium per mL. Store this solution in a tightly stoppered glass container. On the day of use, pipet 10 mL of this solution into a 1000-mL volumetric flask, add water to volume, and mix to obtain a 0.0001 N solution.
Procedure Transfer an accurately weighed portion of Lotion, equivalent to about 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 safranin O solution (1 in 20,000). Titrate with 0.0001 N Docusate sodium until about 1 mL from the endpoint, then shake the stoppered tube vigorously for about 2 minutes, 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. Each mL of 0.0001 N Docusate sodium is equivalent to 48.01 µg of methylbenzethonium chloride (C28H44ClNO2·H2O).
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USP32NF27 Page 2938Pharmacopeial Forum: Volume No. 30(2) Page 515