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

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

Ointments having water-soluble bases— Accurately weigh an amount of Ointment,equivalent to about 200mg of benzocaine,in a tared 250-mLbeaker.Add 50mLof water and 5mLof 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 0.1Msodium nitrite VS,determining the endpoint potentiometrically using a calomel-platinum electrode system.Perform a blank determination,and make any necessary correction.Each mLof 0.1Msodium nitrite is equivalent to 16.52mg of C9H11NO2.

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