A: Place about 20 g of Ophthalmic Ointment in a beaker, add about 25 mL of 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-mL portion, and reserve for Identification test B: the solution in the separator meets the requirements of the test for Identification—Organic Nitrogenous Bases 181, USP Physostigmine Salicylate RS being used, and 1 g of sodium bicarbonate being used in place of the 2 mL of 1 N sodium hydroxide specified.

B: A 2-mL portion of the aqueous solution obtained in Identification test A responds to the tests for Sulfate 191.

0.05 M Ammonium acetate and Mobile phase— Prepare as directed in the Assay under Physostigmine Salicylate Injection.

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

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

Chromatographic system (see Chromatography 621)— Proceed as directed for Chromatographic system in the Assay under Physostigmine Salicylate Ophthalmic Solution.

Procedure— Proceed as directed for Procedure in the Assay under Physostigmine Salicylate Injection. Calculate the quantity, in mg, of (C15H21N3O2)2·H2SO4 in the portion of the Ophthalmic Ointment taken by the formula: in which 648.77 and 413.47 are the molecular weights of physostigmine sulfate and physostigmine salicylate, respectively, C is the concentration, in µg per mL, of USP Physostigmine Salicylate RS in the Standard preparation.