A: Place about 20g of Ophthalmic Ointment in a beaker,add about 25mLof water,and heat gently on a steam bath,with continuous stirring,until the ointment base has melted.Cool to congeal the ointment base,and decant the aqueous solution through a filter into a separator.Draw off a 2-mLportion,and reserve for Identificationtest B:the solution in the separator meets the requirements of the test for Identification—Organic Nitrogenous Bases á181ñ,USP Physostigmine Salicylate RSbeing used,and 1g of sodium bicarbonate being used in place of the 2mLof 1Nsodium hydroxide specified.

B: A2-mLportion of the aqueous solution obtained in Identificationtest Aresponds to the tests for Sulfate á191ñ.

0.05M Ammonium acetateand Mobile phase— Prepare as directed in the Assayunder Physostigmine Salicylate Injection.

Standard preparation— Dissolve an accurately weighed quantity of USP Physostigmine Salicylate RSin acetonitrile,and dilute quantitatively,and stepwise if necessary,with acetonitrile,to obtain a solution having a known concentration of about 40µg per mL.

Assay preparation— Transfer an accurately weighed quantity of Ophthalmic Ointment,equivalent to about 3mg of physostigmine sulfate,to a 60-mLseparator.Add 20mLof spectrophotometric grade n-hexane,and extract with four 20-mLportions of acetonitrile.Collect the acetonitrile extracts in a 100-mLvolumetric flask,dilute with acetonitrile to volume,and mix.

Chromatographic system (see Chromatography á621ñ)— Proceed as directed for Chromatographic systemin the Assayunder Physostigmine Salicylate Ophthalmic Solution.

Procedure— Proceed as directed for Procedurein the Assayunder Physostigmine Salicylate Injection.Calculate the quantity,in mg,of (C15H21N3O2)2·H2SO4in the portion of the Ophthalmic Ointment taken by the formula: in which 648.77and 413.47are the molecular weights of physostigmine sulfate and physostigmine salicylate,respectively,Cis the concentration,in µg per mL,of USP Physostigmine Salicylate RSin the Standard preparation.