Salicylic Acid Plaster

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

Salicylic Acid Plaster is a uniform mixture of Salicylic Acid in a suitable base, spread on paper, cotton cloth, or other suitable backing material. The plaster mass contains NLT 90.0% and NMT 110.0% of the labeled amount of salicylic acid (C₇H₆O₃).

2 IDENTIFICATION

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

3 ASSAY

3.1 Procedure

Solution A: Methanol and 2% (v/v) phosphoric acid in water (1:1)

Solution B: Methanol

Mobile phase: See Table 1.

Table 1

Diluent: Tetrahydrofuran and hydrochloric acid (99:1)

Standard solution: 0.2 mg/mL of USP Salicylic Acid RS prepared as follows. Transfer USP Salicylic Acid RS to a suitable volumetric flask, and add Diluent equivalent to 12.5% of the final volume to dissolve. Dilute with Solution A to volume. Protect from light.

Sample solution: Transfer Plaster (scrape it from the fabric if needed), equivalent to 40 mg of salicylic acid, to a 200-mL volumetric flask, and add 25 mL of Diluent to dissolve. Sonicate if necessary to facilitate dissolution. Dilute with Solution A to volume. Mix, filter, and discard the first few mL. Protect from light.

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: UV 306 nm

Column: 4.6-mm × 15-cm; 5-µm packing L11

Injection volume: 10 µL

System suitability

Sample: Standard solution

Suitability requirements

Tailing factor: NMT 2.0

Relative standard deviation: NMT 2.0%

Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of salicylic acid (C₇H₆O₃) in the portion of Plaster taken:

Result = (r_U/r_S) × (C_S/C_U) × 100

r_U = peak area from the Sample solution

r_S = peak area from the Standard solution

C_S = concentration of USP Salicylic Acid RS in the Standard solution (mg/mL)

C_U = nominal concentration of salicylic acid in the Sample solution (mg/mL)

Acceptance criteria: 90.0%–110.0%

4 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed containers. Store between 20° and 25°.

USP Reference Standards 〈11〉

USP Salicylic Acid RS