Identification—
Treat 25 mL of Oral Solution as directed in the
Assay, ending with “using a current of air only.” Dissolve the residue in 2.5 mL of carbon disulfide, filter through paper if necessary, and determine the IR absorption spectrum as directed under
Identification—Organic Nitrogenous Bases 
181
, obtaining the spectrum of
USP Promethazine Hydrochloride RS as directed: the Oral Solution meets the requirements of the test.
Assay—
[NOTE—Use low-actinic glassware in this assay.
] Transfer an accurately measured volume of Oral Solution, equivalent to about 25 mg of promethazine hydrochloride, to a 250-mL separator. Add 10 mL of ammonium hydroxide, and extract the promethazine base with six 40-mL portions of chloroform. Wash the combined chloroform extracts with 25 mL of dilute hydrochloric acid (1 in 9). Wash the acid solution with 25 mL of chloroform, and add the washings to the main chloroform extract. Evaporate the chloroform extract on a steam bath, with the aid of a current of air, to a volume of 5 to 10 mL, and finally evaporate, using only a current of air, to dryness. Dissolve the residue, with slight warming, in dilute sulfuric acid (1 in 100), and transfer to a 500-mL volumetric flask with the aid of additional acid. Cool, add dilute sulfuric acid (1 in 100) to volume, mix, and filter, rejecting the first half of the filtrate. Dissolve an accurately weighed quantity of
USP Promethazine Hydrochloride RS in dilute sulfuric acid (1 in 100), and dilute quantitatively and stepwise with the dilute acid to obtain a Standard solution having a known concentration of about 50 µg per mL. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 298 nm, with a suitable spectrophotometer, using dilute sulfuric acid (1 in 100) as the blank. Calculate the quantity, in mg, of promethazine hydrochloride (C
17H
20N
2S·HCl) in each mL of the Oral Solution taken by the formula:
500(C / V)(AU / AS),
in which
C is the concentration, in mg per mL, of
USP Promethazine Hydrochloride RS in the Standard solution;
V is the volume, in mL, of Oral Solution taken; and
AU and
AS are the absorbances of the solution from the Oral Solution and the Standard solution, respectively.
Promethazine Hydrochloride Syrup