A: Place 10 mL of Oral Solution in a separator containing 20 mL of water, add 5 mL of 1 N sodium hydroxide, and extract with two 10-mL portions of chloroform, discarding the chloroform extracts. Add 5 mL of 3 N hydrochloric acid, and extract with two 25-mL portions of chloroform, filtering the extracts through paper into a beaker. Remove the chloroform by evaporation on a steam bath, and dry the residue at 105 for 2 hours: the residue so obtained meets the requirements for Identification test A under Phenobarbital.

B: The retention time of the major peak in the chromatogram of the Assay preparation corresponds to that in the chromatogram of the Standard preparation, both relative to the internal standard, as obtained in the Assay.

pH 4.5 Buffer solution, Mobile phase, and Chromatographic system— Prepare as directed in the Assay under Phenobarbital.

Diluent— Prepare a mixture of methanol and pH 4.5 Buffer solution (2:1).

Internal standard solution— Dissolve a sufficient quantity of caffeine in Diluent to obtain a solution having a concentration of about 1.7 mg per mL.

Standard preparation— Transfer about 33 mg of USP Phenobarbital RS, accurately weighed, to a 25-mL volumetric flask containing 2.0 mL of Internal standard solution. Dilute with Diluent to volume, and mix.

Assay preparation— Transfer a quantity of Oral Solution, equivalent to about 33 mg of phenobarbital, to a 25-mL volumetric flask containing 2.0 mL of Internal standard solution. Dilute with Diluent to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Phenobarbital. Calculate the quantity, in mg, of phenobarbital (C12H12N2O3) in the portion of the Oral Solution taken by the formula: in which the terms are as defined therein.