Phosphate buffer–bromocresol purple solution— Prepare as directed in the Assay under Biperiden Hydrochloride Tablets.

Standard preparation— Transfer about 80 mg of USP Biperiden RS, accurately weighed, to a 100-mL volumetric flask, add methanol to volume, and mix. Transfer 5.0 mL of this solution to a second 100-mL volumetric flask, add 25 mL of water, dilute with methanol to volume, and mix to obtain a Standard preparation having a known concentration of about 40 µg per mL.

Assay preparation— Transfer an accurately measured volume of Injection, equivalent to about 5 mg of biperiden lactate, to a 100-mL volumetric flask, add 25 mL of water, dilute with methanol to volume, and mix.

Procedure— Proceed as directed in the Assay under Biperiden Hydrochloride Tablets. Calculate the quantity, in mg, of C21H29NO·C3H6O3 in each mL of the Injection taken by the formula: in which 401.55 and 311.47 are the molecular weights of biperiden lactate and biperiden, respectively; C is the concentration, in µg per mL, of USP Biperiden RS in the Standard preparation; V is the volume, in mL, of Injection taken; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.