A: Disperse a portion of powdered Tablets,equivalent to about 100mg of chlorzoxazone,in 100mLof methanol,shake for 15minutes,and filter.Transfer 2.0mLof the filtrate to a 100-mLvolumetric flask,dilute with methanol to volume,and mix:the UVabsorption 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 preparationobtained in the Assayexhibits a major peak for chlorzoxazone,the retention time of which corresponds to that exhibited in the chromatogram of the Standard preparationobtained in the Assay.

Medium: pH6.8phosphate buffer (see under Buffer Solutionsin the section Reagents,Indicators,and Solutions);1800mL.

Apparatus 2: 75rpm.

Time: 60minutes.

Procedure— Determine the amount of C7H4ClNO2dissolved by employing UVabsorption at the wavelength of maximum absorbance at about 284nm on filtered portions of the solution under test,diluted with Dissolution Medium,if necessary,in comparison with a Standard solution of USP Chlorzoxazone RSin the same Medium.

Tolerances— Not less than 75%(Q)of the labeled amount of C7H4ClNO2is dissolved in 60minutes.

1%Acetic acid solution— Dilute 10mLof glacial acetic acid with water to make 1000mLof 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 Suitabilityunder Chromatography á621ñ).

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

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

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

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

Chromatographic system (see Chromatography á621ñ)—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 1.5mLper minute.Chromatograph the Resolution solution,and record the peak responses as directed under Procedure:the relative retention times are about 0.7for phenacetin,1.0for chlorzoxazone,and 1.2for 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 not more than 2.0%.

Procedure— [NOTE—Use peak areas where peak responses are indicated.]Separately inject equal volumes (about 20µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the responses for the major peaks.Calculate the quantity,in mg,of C7H4ClNO2in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Chlorzoxazone RSin the Standard preparation,and RUand RSare the peak response ratios of the chlorzoxazone peak to the phenacetin peak obtained from the Assay preparationand the Standard preparation,respectively.