Ointments having water-soluble bases— Transfer an amount of Ointment, equivalent to about 5 mg of benzocaine, to a small beaker, add 20 mL of 0.5 N hydrochloric acid, warm gently to disperse the Ointment, cool, and filter, if necessary. To 10 mL of the filtrate add 5 drops of a solution of sodium nitrite (1 in 10) and 2 drops of methyl red TS, and neutralize with 1 N sodium hydroxide. Add 2 mL of a solution of 100 mg of 2-naphthol in 5 mL of sodium hydroxide TS: an orange-red precipitate is formed.

Ointments having water-insoluble bases— Transfer an amount of Ointment, equivalent to about 2.5 mg of benzocaine, to a small separator, and dissolve in 20 mL of ether. Extract with 10 mL of 0.5 N hydrochloric acid, and filter the aqueous phase through paper into a small beaker. Add 5 drops of a solution of sodium nitrite (1 in 10) and 2 drops of methyl red TS, and neutralize with 1 N sodium hydroxide. Add 2 mL of a solution of 100 mg of 2-naphthol in 5 mL of 1 N sodium hydroxide: an orange-red precipitate is formed.

Ointments having water-soluble bases— Accurately weigh an amount of Ointment, equivalent to about 200 mg of benzocaine, in a tared 250-mL beaker. Add 50 mL of water and 5 mL of hydrochloric acid, and stir by mechanical means until solution is effected. Cool the solution in an ice bath to about 10, and titrate slowly with 0.1 M sodium nitrite VS, determining the endpoint potentiometrically using a calomel-platinum electrode system. Perform a blank determination, and make any necessary correction. Each mL of 0.1 M sodium nitrite is equivalent to 16.52 mg of C9H11NO2.

Ointments having water-insoluble bases— Accurately weigh an amount of Ointment, equivalent to about 200 mg of benzocaine, and transfer to a 125-mL separator. Suspend the Ointment in 50 mL of ether by shaking, and extract with three successive 25-mL portions of 1 N hydrochloric acid, filtering each portion into a 250-mL beaker, and proceed as directed under Ointments having water-soluble bases, beginning with “Cool the solution.”