Phenobarbital

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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C₁₂H₁₂N₂O₃ 232.24

2,4,6(1H,3H,5H)-Pyrimidinetrione, 5-ethyl-5-phenyl-.

5-Ethyl-5-phenylbarbituric acid CAS RN^®: 50-06-6; UNII: YQE403BP4D.

>> Phenobarbital contains not less than 98.0 percent and not more than 101.0 percent of C₁₂H₁₂N₂O₃, calculated on the dried basis.

1 Packaging and storage

Preserve in well-closed containers.

2 USP REFERENCE STANDARDS (11)

USP Phenobarbital RS

3 Identification

A: The IR absorption spectrum of a potassium bromide dispersion of it exhibits maxima only at the same wavelengths as that of a similar preparation of USP Phenobarbital RS. If a difference appears, dissolve portions of both the test specimen and the USP Reference Standard in a suitable solvent, evaporate the solutions to dryness, and repeat the test on the residues.

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

3.1 MELTING RANGE (741)

Between 174° and 178°, but the range between beginning and end of melting does not exceed 2°.

3.2 LOSS ON DRYING (731)

Dry it at 105° for 2 hours: it loses not more than 1.0% of its weight.

3.3 RESIDUE ON IGNITION (281)

Not more than 0.15%.

4 Assay

4.1 pH 4.5 Buffer solution

Dissolve about 6.6 g of sodium acetate trihydrate and 3.0 mL of glacial acetic acid in 1000 mL of water, and adjust, if necessary, with glacial acetic acid to a pH of 4.5 ± 0.1.

4.2 Mobile phase

Prepare a filtered and degassed mixture of pH 4.5 Buffer solution and methanol (3:2), making adjustments if necessary (see System Suitability, under Chromatography (621)).

4.3 Internal standard solution

Dissolve a sufficient quantity of caffeine in a mixture of methanol and pH 4.5 Buffer solution (1:1) to obtain a solution having a concentration of about 125 µg per mL.

4.4 Standard preparation

Dissolve about 20 mg of USP Phenobarbital RS, accurately weighed, in 15.0 mL of Internal standard solution. Sonicate if necessary.

4.5 Assay preparation

Transfer about 20 mg of Phenobarbital, accurately weighed, to a conical flask, add 15.0 mL of Internal standard solution, mix, and sonicate for 15 minutes. Filter through a membrane filter (0.5 µm or finer porosity) before use.

4.6 Chromatographic system (see CHROMATOGRAPHY (621))

The liquid chromatograph is equipped with a 254-nm detector and a 4-mm x 25-cm column that contains packing L1. The flow rate is about 2 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the resolution, R, between the analyte and the internal standard peaks is not less than 1.2, the tailing factor for the analyte and the internal standard peaks is not greater than 2.0, and the relative standard deviation for replicate injections is not more than 2.0%.

4.7 Procedure

Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. The relative retention times are about 0.6 for caffeine and 1.0 for phenobarbital. Calculate the quantity, in mg, of C₁₂H₁₂N₂O₃ in the portion of Phenobarbital taken by the formula:

W(R_U/R_S)

in which W is the weight, in mg, of USP Phenobarbital RS taken for the Standard preparation, and R_U and R_S are the peak response ratios obtained from the Assay preparation and the Standard preparation, respectively.