0.0001N Docusate sodium— Dissolve an accurately weighed quantity of USP Docusate Sodium RSin isopropyl alcohol,and dilute quantitatively with isopropyl alcohol to obtain a solution having a concentration of 4.446mg of anhydrous docusate sodium per mL.Store this solution in a tightly stoppered glass container.On the day of use,pipet 10mLof this solution into a 1000-mLvolumetric flask,add water to volume,and mix to obtain a 0.0001Nsolution.

Procedure— Transfer an accurately weighed portion of Lotion,equivalent to about 0.5mg of methylbenzethonium chloride,to a glass-stoppered,50-mLcylinder.Add 5mLof chloroform (freshly purified by shaking 100mLwith 10g of silica gel,allowing to settle,and withdrawing the supernatant),5mLof phosphoric acid solution (1in 10),and 1mLof safranin Osolution (1in 20,000).Titrate with 0.0001N Docusate sodiumuntil about 1mLfrom the endpoint,then shake the stoppered tube vigorously for about 2minutes,and continue the titration in 0.1-mLincrements,shaking vigorously after each addition,until a pink color appears in the chloroform layer.Perform a blank determination,and make any necessary correction.Each mLof 0.0001N Docusate sodiumis equivalent to 48.01µg of methylbenzethonium chloride (C28H44ClNO2·H2O).