Medium: water;500mL.

Apparatus 1: 100rpm.

Time: 45minutes.

Procedure— Filter a portion of the solution under test,and pipet a volume of the filtrate,equivalent to about 400µg of methadone hydrochloride,into a suitable separator.Add 1mLof glacial acetic acid and 20mLof a solution of bromocresol purple,prepared by dissolving 200mg of bromocresol purple in 1000mLof dilute glacial acetic acid (1in 50),mix,and extract with 20.0mLof chloroform.Determine the amount of C21H27NO·HCl dissolved from visible absorbances at the wavelength of maximum absorbance at about 405nm of the chloroform extract so obtained in comparison with the chloroform extract similarly prepared from a Standard solution having a known concentration of USP Methadone Hydrochloride RSin water.

Tolerances— Not less than 75%(Q)of the labeled amount of C21H27NO·HCl is dissolved in 45minutes.

Mobile phase— Prepare a filtered and degassed mixture of 0.03Mmonobasic potassium phosphate and acetonitrile (60:40).Adjust with phosphoric acid to a pHof 3.2.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Dissolve an accurately weighed quantity of USP Methadone Hydrochloride RSin Mobile phaseto obtain a solution having a known concentration of about 0.4mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 10mg of methadone hydrochloride,to a 25-mLvolumetric flask.Add 10mLof Mobile phase,and sonicate briefly.Shake by mechanical means for 15minutes,dilute with Mobile phaseto volume,mix,and filter.

Chromatographic system (see Chromatography á621ñ)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm ×30-cm column that contains packing L11.The flow rate is about 1.5mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed for Procedure:the tailing factor is not more than 2.0,and the relative standard deviation is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)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 methadone hydrochloride (C21H27NO·HCl)in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Methadone Hydrochloride RSin the Standard preparation;and rUand rSare the peak responses obtained from the Assay preparationand the Standard preparation,respectively.