A: Disperse a portion of powdered Tablets, equivalent to about 100 mg of chlorzoxazone, in 100 mL of methanol, shake for 15 minutes, and filter. Transfer 2.0 mL of the filtrate to a 100-mL volumetric flask, dilute with methanol to volume, and mix: the UV absorption spectrum of this solution exhibits maxima and minima at the same wavelengths as that of a similar solution of USP Chlorzoxazone RS, concomitantly measured.

B: The chromatogram of the Assay preparation obtained in the Assay exhibits a major peak for chlorzoxazone, the retention time of which corresponds to that exhibited in the chromatogram of the Standard preparation obtained in the Assay.

Medium: pH 6.8 phosphate buffer (see under Buffer Solutions in the section Reagents, Indicators, and Solutions); 1800 mL.

Apparatus 2: 75 rpm.

Time: 60 minutes.

Procedure— Determine the amount of C7H4ClNO2 dissolved by employing UV absorption at the wavelength of maximum absorbance at about 284 nm on filtered portions of the solution under test, diluted with Medium, if necessary, in comparison with a Standard solution of USP Chlorzoxazone RS in the same Medium.

Tolerances— Not less than 75% (Q) of the labeled amount of C7H4ClNO2 is dissolved in 60 minutes.

1% Acetic acid solution— Dilute 10 mL of glacial acetic acid with water to make 1000 mL of solution.

Mobile phase— Prepare a filtered and degassed mixture of water, acetonitrile, and glacial acetic acid (70:30:1). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Prepare a solution of phenacetin in acetonitrile containing about 1.25 mg per mL.

Standard preparation— Dissolve an accurately weighed quantity of USP Chlorzoxazone RS quantitatively in Mobile phase to obtain a stock solution having a known concentration of about 1.25 mg per mL. Transfer 5.0 mL of this stock solution to a 50-mL volumetric flask containing 10.0 mL of Internal standard solution, dilute with 1% Acetic acid solution to volume, and mix.

Resolution solution— Prepare a solution of p-chlorophenol in acetonitrile containing about 8.5 mg per mL. Transfer 1 mL of this solution to a 50-mL volumetric flask containing 4 mL of the stock solution used to prepare the Standard preparation and 10 mL of Internal standard solution, dilute with 1% Acetic acid solution to volume, and mix.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 125 mg of chlorzoxazone, to a 100-mL volumetric flask, add about 70 mL of acetonitrile, and shake by mechanical means for about 30 minutes. Dilute with acetonitrile to volume, and mix. Filter a portion of this solution, discarding the first 10 mL of the filtrate. Transfer 5.0 mL of the clear filtrate to a 50-mL volumetric flask containing 10.0 mL of Internal standard solution, dilute with 1% Acetic acid solution to volume, and mix.

Chromatographic system (see Chromatography 621)—The liquid chromatograph is equipped with a 280-nm detector and a 4-mm × 30-cm column containing packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Resolution solution, and record the peak responses as directed under Procedure: the relative retention times are about 0.7 for phenacetin, 1.0 for chlorzoxazone, and 1.2 for p-chlorophenol; and the resolution, R, between the chlorzoxazone peak and the p-chlorophenol peak is not less than 2.0. Chromatograph the Standard preparation, and record the responses as directed under Procedure: the relative standard deviation for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the area responses for the major peaks. Calculate the quantity, in mg, of C7H4ClNO2 in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Chlorzoxazone RS in the Standard preparation; and RU and RS are the peak response ratios of the chlorzoxazone peak to the phenacetin peak obtained from the Assay preparation and the Standard preparation, respectively.