Oxidized nitroprusside reagent— Dissolve 1.0g of sodium nitroferricyanide in water to make 10.0mL(Solution A).Dissolve 1.0g of potassium ferricyanide in water to make 10.0mL(Solution B).Transfer 1.0mLeach of Solution Aand Solution Bto a 100-mLvolumetric flask,add 1mLof sodium hydroxide solution (1in 10),and allow to stand until the solution changes to a light yellow color (about 20to 30minutes).Dilute with water to volume,and mix.Store in a refrigerator or keep in an ice bath,and use within 4hours.

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 100µg per mL.

Assay preparation— Transfer an accurately measured volume of Nasal Solution,equivalent to about 10mg of tetrahydrozoline hydrochloride,to a 100-mLvolumetric flask,dilute with water to volume,and mix.

Procedure— Transfer 5.0mLeach of the Standard preparationand the Assay preparationto separate glass-stoppered test tubes.Pipet 5mLof water into a third tube to provide a blank.To each tube add 4.0mLof Oxidized nitroprusside reagent,mix,and allow to stand at 30for 15minutes.Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 570nm,with a suitable spectrophotometer,using the blank to set the instrument.Calculate the quantity,in mg,of C13H16N2·HCl in each mLof the Nasal 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 Nasal Solution taken,and AUand ASare the absorbances of the solutions from the Assay preparationand the Standard preparation,respectively.