A: Dissolve 40 mg in 2.3 mL of 0.1 N hydrochloric acid in a 100-mL volumetric flask, dilute with water to volume, and mix (Stock solution). Transfer 4.0 mL of this solution to a 100-mL volumetric flask, and add 25 mL of pH 7.0 Buffer solution. (Prepare the Buffer solution by dissolving 22.73 g of anhydrous dibasic sodium phosphate and 14.52 g of monobasic potassium phosphate in water to make 1000 mL. Dilute 25 mL of the buffer with water to 100 mL: the pH, determined potentiometrically, is 7.0 ± 0.05.) Dilute with water to volume, and mix (Solution A). Transfer 20.0 mL of the Stock solution to a 100-mL volumetric flask, add 25 mL of the pH 7.0 Buffer solution, dilute with water to volume, and mix (Solution B): the UV absorption spectrum of Solution A exhibits a maximum at 234 ± 1 nm; and the UV absorption spectrum of Solution B exhibits a maximum at 285 ± 2 nm. The ratio 5A234 / A285 is about 8.8.

B: To 5 mL of a solution in 0.1 N hydrochloric acid (1 in 5000), add 2 mL of a solution of p-dimethylaminobenzaldehyde in alcohol (1 in 100): a yellow color develops immediately.

(Official January 1, 2007)