Methyldopa Tablets

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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1 DEFINITION

Methyldopa Tablets contain NLT 90.0% and NMT 110.0% of the labeled amount of methyldopa (C₁₀H₁₃NO₄).

2 IDENTIFICATION

2.1 A.

Analysis: To about 10 mg of finely ground Tablets add 3 drops of a solution of ninhydrin in sulfuric acid (1 in 250).

Acceptance criteria: A dark purple color is produced within 5-10 min that changes to pale brownish yellow on addition of 3 drops of water.

2.2 B.

Analysis: To about 10 mg of finely ground Tablets add 2 mL of 0.1 N sulfuric acid and 2 mL of Solution A, prepared as directed in the Assay, followed by 0.25 mL of 6 N ammonium hydroxide.

Acceptance criteria: A dark purple color is produced immediately.

3 ASSAY

3.1 PROCEDURE

Solution A: Freshly dissolve 1 g of ferrous sulfate, 2 g of potassium sodium tartrate, and 100 mg of sodium bisulfite in water to make a 100-mL solution.

Solution B: 50 g/L of ammonium acetate in 20% alcohol. Adjust with 6 N ammonium hydroxide to a pH of 8.5.

Standard solution: 1 mg/mL of anhydrous methyldopa from USP Methyldopa RS in 0.1 N sulfuric acid

Sample solution: Nominally 1 mg/mL of methyldopa in 0.1 N sulfuric acid from NLT 20 powdered Tablets prepared as follows. To a 100-mL volumetric flask add 0.1 N sulfuric acid to fill about 50% of the volume of the flask, agitate by mechanical means for 15 min, and then dilute with 0.1 N sulfuric acid to volume. Filter the solution, and reject the first 20 mL of the filtrate.

Blank: Water

Instrumental conditions

Mode: UV-Vis

Analytical wavelength: 520 nm

Cell: 1 cm

Analysis

Samples: Standard solution, Sample solution, and Blank

Pipet 5 mL each of Standard solution, Sample solution, and Blank into separate 100-mL volumetric flasks. Add to each flask 5 mL of Solution A, and dilute with Solution B to volume.

Calculate the percentage of the labeled amount of methyldopa (C₁₀H₁₃NO₄) in the portion of Tablets taken:

Result = (A_U/A_S) × (C_S/C_U) × 100

A_U = absorbance of the Sample solution

A_S = absorbance of the Standard solution

C_S = concentration of USP Methyldopa RS in the Standard solution (mg/mL)

C_U = nominal concentration of methyldopa in the Sample solution (mg/mL)

Acceptance criteria: 90.0%-110.0%

4 PERFORMANCE TESTS

5 DISSOLUTION (711)

Medium: 0.1 N hydrochloric acid; 900 mL

Apparatus 2: 50 rpm

Time: 20 min

Instrumental conditions

Mode: UV

Analytical wavelength: 280 nm

Standard solution: USP Methyldopa RS in Medium

Sample solution: Sample per Dissolution (711). Dilute with Medium to a concentration similar to that of the Standard solution.

Analysis: Determine the amount of methyldopa (C₁₀H₁₃NO₄) dissolved.

Tolerances: NLT 80% (Q) of the labeled amount of methyldopa (C₁₀H₁₃NO₄) is dissolved.

5.1 UNIFORMITY OF DOSAGE UNITS (905): Meet the requirements

6 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in well-closed containers.

USP REFERENCE STANDARDS (11)

USP Methyldopa RS