» Methyldopa Tablets contain not less than 90.0 percent and not more than 110.0 percent of the labeled amount of C10H13NO4.
Packaging and storage Preserve in well-closed containers.
USP Reference standards 11
A: To about 10 mg of finely ground Tablets add 3 drops of a solution of ninhydrin in sulfuric acid (1 in 250): a dark purple color is produced within 5 to 10 minutes. Add 3 drops of water: the color changes to pale brownish yellow.
B: To about 10 mg of finely ground Tablets add 2 mL of 0.1 N sulfuric acid and 2 mL of Ferrous tartrate solution prepared as directed in the Assay, then add 0.25 mL of 6 N ammonium hydroxide, and mix: a dark purple color is produced immediately.
Medium: 0.1 N hydrochloric acid; 900 mL.
Apparatus 2: 50 rpm.
Time: 20 minutes.
Procedure Determine the amount of C10H13NO4 dissolved from UV absorbances at the wavelength of maximum absorbance at about 280 nm of filtered portions of the solution under test, suitably diluted with Medium, if necessary, in comparison with a Standard solution having a known concentration of USP Methyldopa RS in the same medium.
Tolerances Not less than 80% (Q) of the labeled amount of C10H13NO4 is dissolved in 20 minutes.
Uniformity of dosage units 905: meet the requirements.
Ferrous tartrate solution Dissolve 1 g of ferrous sulfate, 2 g of potassium sodium tartrate, and 100 mg of sodium bisulfite in water to make 100 mL. Prepare this solution fresh.
Buffer solution Dissolve 50 g of ammonium acetate in 1000 mL of 20 percent alcohol. Adjust with 6 N ammonium hydroxide to a pH of 8.5.
Standard preparation Dissolve a suitable quantity of USP Methyldopa RS in 0.1 N sulfuric acid to obtain a solution having a known concentration of about 1 mg of anhydrous methyldopa per mL.
Assay preparation Weigh and finely powder not less than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 100 mg of methyldopa, to a 100-mL volumetric flask, add 50 mL of 0.1 N sulfuric acid, agitate by mechanical means for 15 minutes, add the dilute acid to volume, and mix. Filter the solution, rejecting the first 20 mL of the filtrate.
Procedure Pipet 5 mL each of the Assay preparation and the Standard preparation into separate 100-mL volumetric flasks. To a third 100-mL volumetric flask add 5 mL of water to provide a blank. Add to each flask 5 mL of Ferrous tartrate solution, and dilute with Buffer solution to volume. Determine the absorbances of both solutions at the wavelength of maximum absorbance at about 520 nm, with a suitable spectrophotometer, using the blank in the reference cell. Calculate the quantity, in mg, of C10H13NO4 in the portion of Tablets taken by the formula:
100C(AU / AS)in which C is the concentration, in mg per mL, of USP Methyldopa RS in the Standard solution; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.
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USP35NF30 Page 3871Pharmacopeial Forum: Volume No. 28(3) Page 770