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).