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(Official January 1, 2007)

Mobile phase and Chromatographic system— Proceed as directed in the Assay under Dibucaine.

Standard preparation— Transfer about 55 mg of USP Dibucaine Hydrochloride RS, accurately weighed, to a 50-mL volumetric flask, add 24.0 mL of 0.1 N hydrochloric acid, and swirl to dissolve. Dilute with a mixture of methanol and 1.0 N hydrochloric acid (13:12) to volume, and mix. Transfer 10.0 mL of this solution to a 50-mL volumetric flask, dilute with methanol to volume, and mix. Pass through a suitable filter having a 0.5-µm or finer porosity.

Assay preparation— Weigh accurately a portion of Ointment, equivalent to about 50 mg of dibucaine, transfer to a separator containing 50 mL of ether, and mix to dissolve. Extract successively with 50-mL, 40-mL, and 30-mL portions of 0.1 N hydrochloric acid, combining the extracts in a 250-mL volumetric flask. Dilute with methanol to volume, and mix. Pass through a suitable filter having a 0.5-µm or finer porosity.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the area responses for the major peaks. Calculate the quantity, in mg, of dibucaine (C20H29N3O2) in the portion of Ointment taken by the formula: in which 343.46 and 379.93 are the molecular weights of dibucaine and dibucaine hydrochloride, respectively; C is the concentration, in mg per mL, of USP Dibucaine Hydrochloride RS in the Standard preparation; and rU and rS are the area responses of the dibucaine peaks obtained from the Assay preparation and the Standard preparation, respectively.