A: Reduce a number of Tablets,equivalent to 20mg of trihexyphenidyl hydrochloride,to a fine powder,and triturate with 25mLof chloroform.Filter the mixture,and evaporate the filtrate,by gently heating,to about 10mL.Add the solution to 100mLof n-hexane:a white precipitate is formed.Allow the mixture to stand for 30minutes,and collect the precipitate on a solvent-resistant membrane filter of 1-µm pore size.Wash the crystals with a small portion of n-hexane,and allow them to air-dry:the IRabsorption spectrum of a potassium bromide dispersion of the crystals so obtained exhibits maxima only at the same wavelengths as that of a similar preparation of USP Trihexyphenidyl Hydrochloride RS.

B: The precipitate obtained in Identificationtest Aresponds to the tests for Chloride á191ñ.

C: The retention time exhibited by trihexyphenidyl hydrochloride in the chromatogram of the Assay preparationcorresponds to that of the Standard preparation,both relative to the internal standard,as obtained in the Assay.

Medium: pH4.5acetate buffer,prepared by mixing 2.99g of sodium acetate trihydrate and 1.66mLof glacial acetic acid with water to obtain 1000mLof solution having a pHof 4.50±0.05;900mL.

Apparatus 1: 100rpm.

Time: 45minutes.

Determination of dissolved trihexyphenidyl hydrochloride—

Bromocresol green solution— Dissolve 250mg of bromocresol green in a mixture of 15mLof water and 5mLof 0.1Nsodium hydroxide,dilute with Mediumto 500mL,and mix.Extract 250-mLportions of this solution with two 100-mLportions of chloroform,and discard the chloroform extracts.

Procedure— Transfer an accurately measured,filtered portion of the solution under test,estimated to contain about 50µg of trihexyphenidyl hydrochloride,to a 50-mLcentrifuge tube.Transfer an equal,accurately measured volume of a Standard solution,having a known concentration of USP Trihexyphenidyl Hydrochloride RSin Medium,to a second 50-mLcentrifuge tube,and transfer an equal,accurately measured volume of Mediumto a third 50-mLcentrifuge tube to provide a blank.Add 5mLof Bromocresol green solutionand 10.0mLof chloroform to each tube,insert the stoppers into the tubes,and shake vigorously for not less than 20seconds.Centrifuge the mixtures to separate the layers,and aspirate and discard the upper aqueous layers.Filter each chloroform layer through a separate phase-separating filter paper.Determine the amount of C20H31NO·HCl dissolved from absorbances,at the wavelength of maximum absorbance at about 415nm,of the filtrate from the solution under test in comparison with that from the Standard solution,the filtrate from the blank being used to set the instrument.

Tolerances— Not less than 75%(Q)of the labeled amount of C20H31NO·HCl is dissolved in 45minutes.

Mobile phase and Chromatographic system—Prepare as directed in the Assayunder Trihexyphenidyl Hydrochloride.

Standard preparation— Dissolve an accurately weighed quantity of USP Trihexyphenidyl Hydrochloride RSin Mobile phase,and dilute quantitatively,and stepwise if necessary,with Mobile phaseto obtain a solution having a known concentration of about 0.2mg per mL.

Assay preparation— Transfer 20Tablets,accurately counted,to a volumetric flask of sufficient capacity that when diluted to volume yields a concentration of about 0.2mg of trihexyphenidyl hydrochloride per mL.Add a volume of 0.1Nhydrochloric acid equivalent to 10%of the capacity of the volumetric flask,and sonicate with occasional shaking until the Tablets have disintegrated.Add a volume of Mobile phaseequivalent to about one-half of the capacity of the volumetric flask,sonicate with frequent shaking for 10minutes,and shake by mechanical means for 10minutes.Cool,dilute with Mobile phaseto volume,mix,and filter.

Procedure— Proceed as directed for Procedurein the Assayunder Trihexyphenidyl Hydrochloride.Calculate the quantity,in mg,of C20H31NO·HCl in each Tablet taken by the formula: in which Vis the volume,in mL,of the Assay preparation,and the other terms are as defined therein.