Cysteine Hydrochloride

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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C3H7NO2S · HCl · H2O              175.63

C3H7NO2S · HCl            157.62

L-Cysteine hydrochloride monohydrate CAS RN^®: 7048-04-6.

Anhydrous CAS RN: 52-89-1; UNII: A9U1687S1S.

1 DEFINITION

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.

2 IDENTIFICATION

A. SPECTROSCOPIC IDENTIFICATION TESTS (197), Infrared Spectroscopy: 197K

3 ASSAY

3.1 PROCEDURE

Sample: 250 mg of Cysteine Hydrochloride

Blank: Proceed as directed in the Analysis without the Sample.

Titrimetric system

(See Titrimetry (541).)

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, while remaining in the ice bath. 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 portion of Cysteine Hydrochloride taken:

                 Result = {[(V_B - V_S) x N x F]/W) x 100

V_B = Back-titrant volume consumed by the Blank (mL)

V_S = 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

4 IMPURITIES

RESIDUE ON IGNITION (281): NMT 0.4%

CHLORIDE AND SULFATE (221), Sulfate

Standard solution: 0.10 mL of 0.020 N sulfuric acid

Sample: 0.33 g of Cysteine Hydrochloride

Acceptance criteria: NMT 0.03%

Change to read:

IRON (241), Procedures. Procedure _{▲(CN 1-JUN-2023)}: NMT 30 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 USPL-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

(See Chromatography (621), Thin-Layer Chromatography.)

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.

Analysis

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%

5 SPECIFIC TESTS

5.1 OPTICAL ROTATION (781S), Procedures. Specific Rotation

Sample solution: 80 mg/mL in 6 N hydrochloric acid

Acceptance criteria: +5.7° to +6.8"

5.2 LOSS ON DRYING (731).

Analysis: Dry at room temperature for 24 h in a vacuum desiccator using a suitable desiccant and maintaining a pressure of NMT 5 mm of mercury (Hg).

Acceptance criteria: 8.0%-12.0%

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in well-closed containers.

USP REFERENCE STANDARDS (11)

USP-Cxilteine Hydrochloride RS

USPL-Tyrosine RS