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

Standard preparation— Dissolve a suitable quantity of USP Methdilazine Hydrochloride, accurately weighed, in chloroform, and quantitatively dilute with chloroform to obtain a solution having a known concentration of about 400 µg per mL.

Assay preparation— Transfer a volume of Oral Solution, equivalent to about 4 mg of methdilazine hydrochloride, to a 60-mL separator, add 10 mL of a saturated solution of sodium chloride, and extract with three 10-mL portions of chloroform, transferring the extracts to a 100-mL volumetric flask.

Procedure— Transfer 10.0 mL of Standard preparation to a 100-mL volumetric flask, and add 20 mL of chloroform. To this flask and to the flask containing the Assay preparation add 4.0 mL of buffered palladium chloride TS, dilute with alcohol to volume, and mix. Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 460 nm, with a suitable spectrophotometer, using a mixture of 30 mL of chloroform, 4 mL of palladium chloride TS, and 66 mL of alcohol as the blank. Calculate the quantity, in mg, of methdilazine hydrochloride (C18H20N2S·HCl ) in each mL of the Oral Solution taken by the formula: in which C is the concentration, in µg per mL, of USP Methdilazine 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 solutions from the Assay preparation and the Standard preparation, respectively.