Benzoyl Peroxide Gel

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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1 DEFINITION

Benzoyl peroxide Gel is benzoyl peroxide in a suitable gel base. It contains NLT 90.0% and NMT 125.0% of the labeled amount of benzoyl peroxide (C₁₄H₁₀O₄).

2 IDENTIFICATION

A. The retention time of the major peak for benzoyl peroxide of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.

3 ASSAY

Procedure

Mobile phase: Acetonitrile in water (5 in 10)

Internal standard solution: 3.6 mg/mL of ethyl benzoate in acetonitrile

Standard stock solution: 0.8 mg/mL of benzoyl peroxide prepared as follows. Transfer a suitable quantity of benzoyl peroxide, recently subjected to the Assay in Hydrous Benzoyl Peroxide, into a weighed conical flask fitted with a glass stopper. Weigh again to obtain the weight of the specimen, and quantitatively dissolve in acetonitrile.

Standard solution: 0.32 mg/mL of benzoyl peroxide prepared as follows. Mix 10 mL of Standard stock solution and 5 mL of Internal standard solution, and dilute with acetonitrile to 25 mL.

Sample stock solution: Transfer an equivalent to 40 mg of benzoyl peroxide from Gel into a 50-mL volumetric flask, add 40 mL of acetonitrile, and shake until the material is thoroughly dispersed. Sonicate the mixture for 5 min, dilute with acetonitrile to volume, mix, and filter.

Sample solution: 10 mL of Sample stock solution and 5 mL of Internal standard solution; dilute with acetonitrile to 25 mL.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 254 nm

Column: 3.9-mm × 30-cm; packing L1

Flow rate: 1 mL/min

Injection volume: 10 µL

System suitability

Sample: Standard solution (three replicate injections)

[Note—The retention times for ethyl benzoate and benzoyl peroxide are 7 and 14 min, respectively.]

Suitability requirements

Resolution: NLT 2.0 between ethyl benzoate and benzoyl peroxide

Tailing factors: NMT 2.0 for the ethyl benzoate and benzoyl peroxide peaks

Peak response ratios: The lowest and highest peak response ratios (R) agree within 2.0%.

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of the labeled amount of benzoyl peroxide (C₁₄H₁₀O₄) in the portion of Gel taken:

Result = (Rᵤ/Rₛ) × (Cₛ/Cᵤ) × 100

Rᵤ = peak response ratio of benzoyl peroxide to ethyl benzoate from the Sample solution

Rₛ = peak response ratio of benzoyl peroxide to ethyl benzoate from the Standard solution

Cₛ = concentration of benzoyl peroxide in the Standard solution (mg/mL)

Cᵤ = nominal concentration of benzoyl peroxide in the Sample solution (mg/mL)

Acceptance criteria: 90.0%–125.0%

4 IMPURITIES

Organic Impurities

Solution A: Acetonitrile and glacial acetic acid (1000:1)

Solution B: Water and glacial acetic acid (1000:1)

Mobile phase: See Table 1.

System suitability solution: 100 µg/mL of benzoic acid and 60 µg/mL of methylparaben in acetonitrile

Standard solution A: 500 µg/mL of benzoic acid in acetonitrile

Standard solution B: 20 µg/mL of ethyl benzoate in acetonitrile

Standard solution C: 20 µg/mL of benzaldehyde in acetonitrile

Standard solution D: Equivalent to 40 µg/mL of anhydrous benzoyl peroxide in acetonitrile, prepared from hydrous benzoyl peroxide, which has been analyzed as follows. Place 300 mg of previously mixed hydrous benzoyl peroxide in a conical flask fitted with a ground-glass stopper. Weigh again to obtain the weight of the sample. Add 30 mL of glacial acetic acid, previously sparged with carbon dioxide for NLT 2 min just before use, and swirl the flask gently to dissolve. Add 5 mL of potassium iodide solution (1 in 2), and mix. Allow the solution to stand for 1 min. Titrate the liberated iodine with 0.1 N sodium thiosulfate VS. As the endpoint is approached, add 1 drop of starch iodide paste TS, or equivalent, and continue the titration to the discharge of the blue color. Perform a blank determination, and make any necessary correction (see Titrimetry 〈541〉). Each mL of 0.1 N sodium thiosulfate is equivalent to 12.11 mg of anhydrous benzoyl peroxide (C₁₄H₁₀O₄).

Sample solution: Transfer an amount of Gel equivalent to 100 mg of benzoyl peroxide to a 50-mL volumetric flask, and add 25 mL of acetonitrile. Shake vigorously to disperse the specimen, sonicate for 5 min, dilute with acetonitrile to volume, and filter.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 235 nm

Column: 4.6-mm × 25-cm; packing L1

Flow rate: 1.2 mL/min

Injection volume: 10 µL

System suitability

Sample: System suitability solution

Suitability requirements

Resolution: NLT 2.0 between benzoic acid and methylparaben

Tailing factor: NMT 2.0 for the benzoic acid and methylparaben peaks

Analysis

Samples: Standard solution and Sample solution

Acceptance criteria: The responses of any peaks from the Sample solution corresponding to benzoic acid, ethyl benzoate, and benzaldehyde are NMT those of the main peaks from Standard solution A (25%), Standard solution B (1%), and Standard solution C (1%), respectively. The response of any other impurity peak from the Sample solution—other than the main benzoyl peroxide peak, any benzoic acid, ethyl benzoate, benzaldehyde, methylparaben, or Propylparaben peak, and any solvent peak—is NMT that from Standard solution D (2%); and the sum of the responses of all the impurity peaks—other than those of benzoic acid, ethyl benzoate, and benzaldehyde—is NMT that from Standard solution D (2%).

5 SPECIFIC TESTS

pH 〈791〉: 2.8–6.6

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.