A: The solution employed for measurement of absorbance in the Assay exhibits a maximum at 310 ± 2 nm.

B: Dissolve 5 g in 50 mL of ether, extract the ether solution with 5 mL of 3 N hydrochloric acid, and filter the acid extract. Add 2 mL of potassium thiocyanate solution (1 in 2) to the filtrate: a crystalline precipitate is formed, and when recrystallized from water and dried at 80 for 2 hours, it melts between 130 and 132 (see Melting Range or Temperature 741).

Standard preparation— Transfer about 20 mg of USP Tetracaine Hydrochloride RS, accurately weighed, to a 100-mL volumetric flask, dissolve in water, add water to volume, and mix. Transfer 5.0 mL of this solution to a second 100-mL volumetric flask, add 5 mL of dilute hydrochloric acid (1 in 240) and 10 mL of Buffer No. 6, 10 percent, pH 6.0 (see Phosphate Buffers 81), dilute with water to volume, and mix. The concentration of USP Tetracaine Hydrochloride RS in the Standard preparation is about 10 µg per mL.

Assay preparation— Transfer an accurately weighed portion of Ointment, equivalent to about 9 mg of tetracaine, to a separator, and dissolve in 15 mL of ether. Extract with one 20-mL portion and two 10-mL portions of dilute hydrochloric acid (1 in 240), collecting the acid extracts in a second separator. Render the aqueous solution alkaline by the addition of 5 mL of sodium carbonate TS, and extract immediately with two 50-mL portions of ether, collecting the ether extracts in another separator. Wash the ether solution with 20 mL of water, discard the washing, and extract the ether solution with two 20-mL portions and one 5-mL portion of dilute hydrochloric acid (1 in 240), collecting the acid extracts in a 50-mL volumetric flask. Dilute with water to volume, and mix. Transfer 5.0 mL of this solution to a 100-mL volumetric flask, add 10 mL of Buffer No. 6, 10 percent, pH 6.0 (see Phosphate Buffers 81), dilute with water to volume, and mix.

Procedure— Concomitantly determine the absorbances of the Assay preparation and the Standard preparation in 1-cm cells at the wavelength of maximum absorbance at about 310 nm, with a suitable spectrophotometer, using water as the blank. Calculate the quantity, in mg, of C15H24N2O2 in the portion of Ointment taken by the formula: in which 264.36 and 300.82 are the molecular weights of tetracaine and tetracaine hydrochloride, respectively; C is the concentration, in µg per mL, of USP Tetracaine Hydrochloride RS in the Standard preparation; and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.