A: To a portion of powdered Tablets, equivalent to about 300 mg of metronidazole, add 20 mL of dilute hydrochloric acid (1 in 100), shake for several minutes, and filter: suitable aliquots of the filtrate respond to Identification test B under Metronidazole.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that of the Standard preparation as obtained in the Assay.

Medium: 0.1 N hydrochloric acid; 900 mL.

Apparatus 1: 100 rpm.

Time: 60 minutes.

Procedure— Determine the amount of C6H9N3O3 dissolved from UV absorbances at the wavelength of maximum absorbance at about 278 nm of filtered portions of the solution under test, suitably diluted with 0.1 N hydrochloric acid, in comparison with a Standard solution having a known concentration of USP Metronidazole RS in the same medium.

Tolerances— Not less than 85% (Q) of the labeled amount of C6H9N3O3 is dissolved in 60 minutes.

Procedure for content uniformity— Transfer one Tablet to a 250-mL volumetric flask, add about 100 mL of dilute hydrochloric acid (1 in 100), and shake for 30 minutes. Dilute with dilute hydrochloric acid (1 in 100) to volume, and mix. Filter, discarding the first 15 mL of the filtrate. Dilute the filtrate quantitatively with dilute hydrochloric acid (1 in 100), to obtain a solution having a concentration of about 0.2 mg of metronidazole per mL. Pipet 10 mL of this solution into a 100-mL volumetric flask, dilute with dilute hydrochloric acid (1 in 100) to volume, and mix. Concomitantly determine the absorbance of this test solution and that of a similarly prepared Standard solution of USP Metronidazole RS, having a known concentration of about 20 µg per mL, in 1-cm matched cells, at the wavelength of maximum absorbance at about 278 nm, with a suitable spectrophotometer, using dilute hydrochloric acid (1 in 100) as the blank. Calculate the quantity, in mg, of C6H9N3O3 in the Tablet taken by the formula: in which T is the labeled quantity, in mg, of the metronidazole in the Tablet; C is the concentration, in µg per mL, of USP Metronidazole RS in the Standard solution; D is the concentration, in µg per mL, of metronidazole in the test solution, on the basis of the labeled quantity per Tablet and the extent of dilution; and AU and AS are the absorbances of the test solution and the Standard solution, respectively.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of water and methanol (80:20), making adjustments if necessary (see System Suitability under Chromatography 621).

Standard preparation— Dissolve an accurately weighed quantity of USP Metronidazole RS in Mobile phase to obtain a solution having a known concentration of about 0.5 mg per mL.

Assay preparation— Transfer to a suitable size volumetric flask 10 Tablets, whole or ground, which when diluted with methanol will yield a solution having a concentration of about 10 mg per mL. Add methanol, and shake by mechanical means for 30 minutes or until the Tablets are disintegrated. Dilute with methanol to volume, and allow the solution to stand until the insoluble material has settled. Pipet 5.0 mL of the clear supernatant into a 100-mL volumetric flask, dilute with Mobile phase to volume, and mix. Filter the solution.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 254-nm detector and a 4.6-mm × 15-cm column that contains packing L7. The flow rate is about 1 mL per minute. Chromatograph the Standard preparation, and record the peak response as directed under Procedure: the tailing factor is not more than 2, and the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the quantity, in mg, of metronidazole (C6H9N3O3) in the portion of Tablets taken by the formula: in which L is the labeled amount, in mg, of Metronidazole in each Tablet; D is the concentration, in mg per mL, of Metronidazole in the Assay preparation based on the labeled quantity per Tablet and the extent of dilution; C is the concentration, in mg per mL, of USP Metronidazole RS in the Standard preparation; and rU and rS are the metronidazole peak responses obtained from the Assay preparation and the Standard preparation, respectively.