A: To 50 mL of Oral Solution add 50 mL of water and 50 mL of 1 N sodium hydroxide, and stir. Cool the mixture at 4 to 5 for 30 minutes: a white precipitate or cloudiness is observed. Add 100 mL of water to the cooled mixture, stir, and filter by means of vacuum through a 47-mm membrane filter of 1-µm pore size. Wash the crystals with about 100 mL of water, and allow 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 the crystalline base obtained from about 20 mg of USP Trihexyphenidyl Hydrochloride RS, similarly prepared and measured.

B: The retention time exhibited by trihexyphenidyl hydrochloride in the chromatogram of the Assay preparation corresponds to that of the Standard preparation as obtained in the Assay.

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 methanol, and dilute quantitatively, and stepwise if necessary, with methanol to obtain a solution having a known concentration of about 0.08 mg per mL.

Assay preparation— Transfer an accurately measured volume of Oral Solution, equivalent to about 2 mg of trihexyphenidyl hydrochloride, to a 25-mL volumetric flask, dilute with methanol to volume, and mix.

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