note—Throughout the following procedures, protect test or assay specimens, the Reference Standard, and solutions containing them, by conducting the procedures without delay, under subdued light, or using low-actinic glassware.

Ammoniacal chloroform— Shake 125 mL of chloroform with 5 mL of ammonium hydroxide in a separator, slowly filter the bottom layer through filter paper containing anhydrous sodium sulfate, and discard the top layer.

Standard preparation— Dissolve a suitable quantity of USP Thioridazine Hydrochloride RS, accurately weighed, in Ammoniacal chloroform to obtain a solution having a known concentration of about 6 µg per mL.

Assay preparation— Pipet a portion of Oral Solution, equivalent to about 120 mg of thioridazine hydrochloride, into a separator containing 15 mL of water. Render alkaline with ammonium hydroxide, and extract with three 25-mL portions of Ammoniacal chloroform. Filter the extracts through a pledget of glass wool into a 200-mL volumetric flask. Rinse the filter, add chloroform to volume, and mix. Dilute 2.0 mL of this solution with Ammoniacal chloroform to 200.0 mL, and mix.

Procedure— Concomitantly determine the absorbances of the Standard preparation and the Assay preparation in 1-cm cells at the wavelength of maximum absorbance at about 265 nm, with a suitable spectrophotometer, using Ammoniacal chloroform as the blank. Calculate the quantity, in mg, of C21H26N2S2·HCl in each mL of the Oral Solution taken by the formula: in which C is the concentration, in µg per mL, of USP Thioridazine Hydrochloride RS in the Standard preparation, V is the volume, in mL, of Oral Solution taken, and AU and AS are the absorbances of the Assay preparation and the Standard preparation, respectively.