A: Dilute a volume of Injection with a mixture of chloroform and methanol (1:1), if necessary, to obtain a solution containing about 5 mg of methyldopate hydrochloride per mL. Apply separately 10 µL of this solution and 10 µL of a Standard solution of USP Methyldopate Hydrochloride RS in a solvent mixture of chloroform and methanol (1:1) containing 5 mg per mL to a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel. Allow the spots to dry, and develop the chromatogram in a saturated chamber with a solvent system consisting of a mixture of butyl alcohol, water, and formic acid (7:2:1), until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the spots on the plate by lightly spraying with Folin-Ciocalteu phenol TS followed by spraying with sodium carbonate solution (1 in 10): the RF value of the principal spot obtained from the test solution corresponds to that obtained from the Standard solution.

B: It responds to the tests for Chloride 191, with the exception that the 6 N ammonium hydroxide is omitted.

(Official January 1, 2007)

Buffer solution— To 214 g of monobasic potassium phosphate add 700 mL of water, and stir. Cautiously add 75 mL of sodium hydroxide solution (1 in 2), and stir until solution is complete. Adjust with sodium hydroxide solution (1 in 2) to a pH of 8.0, and dilute with water to 1000.0 mL.

Water-saturated tributyl phosphate— Shake 800 mL of tributyl phosphate with 100 mL of water, and discard the lower, aqueous phase. Filter the upper phase.

Standard preparation— Transfer about 25 mg of USP Methyldopate Hydrochloride RS, accurately weighed, to a 25-mL volumetric flask, add water to volume, and mix. Transfer 5 mL of this solution to a 100-mL volumetric flask, add 0.1 N sulfuric acid to volume, and mix. Use a freshly prepared solution. The Standard preparation contains about 50 µg per mL.

Assay preparation— Transfer to a 50-mL volumetric flask an accurately measured volume of Injection, equivalent to about 50 mg of methyldopate hydrochloride, add water to volume, and mix. Transfer a 5.0-mL aliquot of the solution to a 60-mL separator, add 15 mL of Buffer solution and 10 mL of Water-saturated tributyl phosphate, and shake for about 1 minute. Allow the phases to separate, and transfer the lower, aqueous phase to a second 60-mL separator. To this separator add a second 10-mL portion of Water-saturated tributyl phosphate, shake for about 1 minute, allow the phases to separate, discard the lower, aqueous phase, and add the upper tributyl phosphate phase to the phase retained in the first separator. Rinse the second separator with about 2 mL of Water-saturated tributyl phosphate, and add the rinsing to the first separator. Extract the phase contained in the first separator with two 25-mL portions of 0.1 N sulfuric acid. Collect the acid extracts in a 100-mL volumetric flask, add 0.1 N sulfuric acid to volume, and mix. Filter, if necessary, to obtain a clear solution.

Procedure— Concomitantly determine the absorbances of the Assay preparation and the Standard preparation in 1-cm cells at the wavelength of maximum absorbance at about 283 nm, with a suitable spectrophotometer, using 0.1 N sulfuric acid as the blank. Calculate the quantity, in mg, of methyldopate hydrochloride (C12H17NO4·HCl) in each mL of the Injection taken by the formula: in which C is the concentration, in µg per mL, of USP Methyldopate Hydrochloride RS in the Standard preparation; V is the volume, in mL, of Injection taken; and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.