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 500µg per mL.

Procedure— Transfer 2.0mLof Standard preparationto a 50-mLvolumetric flask.Transfer an accurately measured volume of Ophthalmic Solution,equivalent to about 1mg of tetrahydrozoline hydrochloride,to a second 50-mLflask,and transfer 2mLof water to a third 50-mLvolumetric flask to provide a blank.To each flask add 5.0mLof bromophenol blue sodium salt solution (1in 1000),dilute each flask with potassium biphthalate solution (1in 100)to volume,and mix.Allow to stand for 20minutes,and filter each mixture through a suitable filter paper (Whatman No.42or the equivalent)that does not absorb the dye,discarding the first 15mLof the filtrate.Transfer 20.0mLof the subsequent filtrate to separate 125-mLseparators,and extract each solution with four 20-mLportions of chloroform,filtering each extract through a pledget of glass wool into a 100-mLvolumetric 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 415nm,with a suitable spectrophotometer,using the blank to set the instrument.Calculate the quantity,in µg,of C13H16N2·HCl in each mLof the Ophthalmic Solution taken by the formula: in which Cis the concentration,in µg per mL,of USP Tetrahydrozoline Hydrochloride RSin the Standard preparation,Vis the volume,in mL,of Solution taken,and AUand ASare the absorbances of the solutions from the Ophthalmic Solution and the Standard preparation,respectively.