(Official January 1, 2007)

Standard preparation— Dissolve a suitable quantity of USP Tetrahydrozoline Hydrochloride RS, accurately weighed, in water, and dilute quantitatively with water to obtain a solution having a known concentration of about 500 µg per mL.

Procedure— Transfer 2.0 mL of Standard preparation to a 50-mL volumetric flask. Transfer an accurately measured volume of Ophthalmic Solution, equivalent to about 1 mg of tetrahydrozoline hydrochloride, to a second 50-mL flask, and transfer 2 mL of water to a third 50-mL volumetric flask to provide a blank. To each flask add 5.0 mL of bromophenol blue sodium salt solution (1 in 1000), dilute each flask with potassium biphthalate solution (1 in 100) to volume, and mix. Allow to stand for 20 minutes, and filter each mixture through a suitable filter paper (Whatman No. 42 or the equivalent) that does not absorb the dye, discarding the first 15 mL of the filtrate. Transfer 20.0 mL of the subsequent filtrate to separate 125-mL separators, and extract each solution with four 20-mL portions of chloroform, filtering each extract through a pledget of glass wool into a 100-mL volumetric flask. Dilute the combined extracts from each solution with chloroform to volume, and mix. Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 415 nm, with a suitable spectrophotometer, using the blank to set the instrument. Calculate the quantity, in µg, of C13H16N2·HCl in each mL of the Ophthalmic Solution taken by the formula: in which C is the concentration, in µg per mL, of USP Tetrahydrozoline Hydrochloride RS in the Standard preparation, V is the volume, in mL, of Solution taken, and AU and AS are the absorbances of the solutions from the Ophthalmic Solution and the Standard preparation, respectively.