A: Reduce a number of Tablets, equivalent to 20 mg of trihexyphenidyl hydrochloride, to a fine powder, and triturate with 25 mL of chloroform. Filter the mixture, and evaporate the filtrate, by gently heating, to about 10 mL. Add the solution to 100 mL of n-hexane: a white precipitate is formed. Allow the mixture to stand for 30 minutes, 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 IR absorption 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 Identification test A responds to the tests for Chloride 191.

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

Medium: pH 4.5 acetate buffer, prepared by mixing 2.99 g of sodium acetate trihydrate and 1.66 mL of glacial acetic acid with water to obtain 1000 mL of solution having a pH of 4.50 ± 0.05; 900 mL.

Apparatus 1: 100 rpm.

Time: 45 minutes.

Determination of dissolved trihexyphenidyl hydrochloride—

Bromocresol green solution— Dissolve 250 mg of bromocresol green in a mixture of 15 mL of water and 5 mL of 0.1 N sodium hydroxide, dilute with Medium to 500 mL, and mix. Extract 250-mL portions of this solution with two 100-mL portions of chloroform, and discard the chloroform extracts.

Procedure— Transfer an accurately measured, filtered portion of the solution under test, estimated to contain about 50 µg of trihexyphenidyl hydrochloride, to a 50-mL centrifuge tube. Transfer an equal, accurately measured volume of a Standard solution, having a known concentration of USP Trihexyphenidyl Hydrochloride RS in Medium, to a second 50-mL centrifuge tube, and transfer an equal, accurately measured volume of Medium to a third 50-mL centrifuge tube to provide a blank. Add 5 mL of Bromocresol green solution and 10.0 mL of chloroform to each tube, insert the stoppers into the tubes, and shake vigorously for not less than 20 seconds. 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 415 nm, 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 45 minutes.

(Official January 1, 2007)

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

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

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

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