A: To 2 mL of Injection add 3 mL of water, and mix. Add 10 mL of cupric sulfate TS: a bluish-gray precipitate is formed.

B: To 2 mL of Injection add 3 mL of water, and mix. Add 2 mL of 3 N sodium hydroxide and 2 drops of sodium nitroferricyanide solution (1 in 20): a red-purple color is produced, and it rapidly changes to yellow.

(Official January 1, 2007)

Standard solution— Dissolve an accurately weighed quantity of USP L-Cysteine Hydrochloride RS in nitrogen-saturated water to obtain a solution having a known concentration of about 1 mg per mL.

Standard preparation— Transfer 20.0 mL of Standard solution to a 200-mL volumetric flask, dilute with nitrogen-saturated 1.0 N sodium hydroxide to volume, and mix.

Assay preparation— Dilute an accurately measured volume of Injection, equivalent to about 250 mg of cysteine hydrochloride, quantitatively and stepwise with nitrogen-saturated 1.0 N sodium hydroxide, to obtain a solution having a concentration of about 0.1 mg per mL.

Procedure— Transfer a suitable amount of Standard preparation to a polarographic cell. With mercury dropping from the electrode, lower the dropping mercury electrode of a polarograph so that the end is submerged in the liquid. Bubble oxygen-free, water-saturated nitrogen through the liquid for 15 minutes. Record the polarogram from 0.2 volt to 1.10 volts, using a saturated calomel electrode as the reference electrode. In a similar manner, record the polarograms obtained using portions of the Assay preparation and of the nitrogen-saturated 1.0 N sodium hydroxide. Determine the height of the diffusion current wave at 0.4 volt. Calculate the quantity, in mg, of C3H7NO2S·HCl·H2O in each mL of the Injection taken by the formula: in which C is the concentration, in mg per mL, of USP L-Cysteine Hydrochloride RS. In the Standard preparation, V is the volume, in mL, of Injection taken; and (id)U and (id)S are the observed diffusion currents, corrected for the diffusion current of the 0.1 N sodium hydroxide, of the Assay preparation and the Standard preparation, respectively.