Cysteine Hydrochloride
(sis' te een hye'' droe klor' ide).
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C3H7NO2S·HCl·H2O 175.64

C3H7NO2S·HCl 157.62
l-Cysteine hydrochloride monohydrate     [7048-04-6].
Anhydrous     [52-89-1].
Cysteine Hydrochloride is l-cysteine hydrochloride monohydrate and contains NLT 98.5% and NMT 101.5% of l-cysteine hydrochloride (C3H7NO2S·HCl), calculated on the dried basis.
•  Procedure
Sample:  250 mg of Cysteine Hydrochloride
Blank:  Proceed as directed in the Analysis without the Sample.
Titrimetric system 
Mode:  Residual titration
Titrant:  0.1 N iodine VS
Back titrant:  0.1 N sodium thiosulfate VS
Endpoint detection:  Visual
Analysis:  Transfer the Sample to an iodine flask. Add 20.0 mL of water and 4 g of potassium iodide, and mix. Cool the solution in an ice bath, and add 5 mL of 3 N hydrochloric acid and 25.0 mL of 0.1 N iodine VS. Insert the stopper, and allow to stand in the dark for 20 min. Titrate the excess iodine with the Back titrant. Add 3 mL of starch TS as the endpoint is approached. Perform the Blank determination. Calculate the percentage of cysteine hydrochloride (C3H7NO2S·HCl) in the Sample taken:
Result = {[(VB VS) × N × F]/W} × 100
VB== Back titrant volume consumed by the Blank (mL)
VS== Back titrant volume consumed by the Sample (mL)
N == actual normality of the Back titrant (mEq/mL)
F== equivalency factor, 157.6 mg/mEq
W== Sample weight (mg)
Acceptance criteria:  98.5%–101.5% on the dried basis
•  Residue on Ignition 281: NMT 0.4%
•  Chloride and Sulfate, Sulfate 221
Standard solution:  0.10 mL of 0.020 N sulfuric acid
Sample:  0.33 g of Cysteine Hydrochloride
Acceptance criteria:  NMT 0.03%.
•  Iron 241: NMT 30 ppm
•  Heavy Metals, Method I 231: NMT 15 ppm
•  Related Compounds
N-Ethylmaleimide solution:  40 mg/mL of N-ethylmaleimide in alcohol
Standard stock solution:  Dissolve 20 mg of USP l-Cysteine Hydrochloride RS in 10.0 mL of water. Add 10.0 mL of N-Ethylmaleimide solution, and mix. Allow the solution to stand for 5 min before using.
Standard solution:  0.05 mg/mL from Standard stock solution in water. [Note—This solution has a concentration equivalent to 0.5% of that of the Sample solution. ]
System suitability solution:  Transfer 10 mg of USP l-Tyrosine RS and 10 mL of the Standard stock solution to a 25-mL volumetric flask. Dilute with water to volume.
Sample stock solution:  Transfer 0.2 g of Cysteine Hydrochloride to a 10-mL volumetric flask, dissolve, and dilute with water to volume.
Sample solution:  To 5.0 mL of the Sample stock solution add 5.0 mL of N-Ethylmaleimide solution, and mix. Allow the solution to stand for 5 min before using.
Chromatographic system 
Mode:  TLC
Adsorbent:  0.25-mm layer of chromatographic silica gel mixture
Application volume:  5 µL
Developing solvent system:  Butyl alcohol, glacial acetic acid, and water (3:1:1)
Spray reagent:  2 mg/mL of ninhydrin in a mixture of butyl alcohol and 2 N acetic acid (95:5)
System suitability 
Suitability requirements:  The chromatogram of the System suitability solution exhibits two clearly separated spots.
Samples:  Standard solution, System suitability solution, and Sample solution.
After air-drying the plate, spray with Spray reagent, and heat between 100 and 105 for 15 min. Examine the plate under white light.
Acceptance criteria:  Any secondary spot of the Sample solution is not larger or more intense than the principal spot of the Standard solution.
Individual impurities:  NMT 0.5%
Total impurities:  NMT 2.0%
•  Optical Rotation, Specific Rotation 781S
Sample solution:  80 mg/mL in 6 N hydrochloric acid
Acceptance criteria:  +5.7 to +6.8
•  Loss on Drying 731: Dry a sample at room temperature at a pressure not exceeding 5 mm of mercury for 24 h: it loses 8.0%–12.0% of its weight.
•  Packaging and Storage: Preserve in well-closed containers.
•  USP Reference Standards 11
USP l-Cysteine Hydrochloride RS
USP l-Tyrosine RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Huy T. Dinh, M.S.
Scientific Liaison
(DS2010) Monographs - Dietary Supplements
Reference Standards RS Technical Services
USP35–NF30 Page 2798
Pharmacopeial Forum: Volume No. 30(5) Page 1598