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

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 100 µg per mL.

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

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