Zinc Oxide Compounded Ointment

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

Change to read:

Zinc oxide Compounded Ointment contains NLT 18.5% and NMT 21.5% of zinc oxide (ZnO).

Prepare Zinc Oxide Compounded Ointment as follows.

Levigate the Zinc Oxide with the Mineral oil to a smooth paste, and incorporate the White Ointment [see General Notices, 5.20.20 Added Substances (Excipients and Ingredients) in Ocial Products].

2 IDENTIFICATION

A. The residue obtained in the Assay is yellow when hot and white when cool.

3 ASSAY

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Procedure

Sample solution: Weigh 700 mg of Ointment in a porcelain crucible, heat gently until melted, and continue heating, gradually raising the temperature until the mass is thoroughly charred. Ignite the mass until the residue is uniformly yellow, and cool. Dissolve the residue in 10 mL of 2 N sulfuric acid, warming if necessary to dissolve.

Titrimetric system

(See Titrimetry 〈541〉.) (USP 1-May-2019)

Mode: Direct titration

Titrant: 0.05 M edetate disodium VS

Endpoint detection: Visual

Analysis: Transfer the Sample solution to a beaker, and rinse the crucible with small portions of water until the combined solution and rinsings measure 50 mL. Add 15 mL of ammonia–ammonium chloride buffer TS and 1 mL of eriochrome black TS, and titrate with Titrant until the solution is blue in color. Each milliliter of Titrant is equivalent to 4.069 mg of zinc oxide (ZnO).

Acceptance criteria: 18.5%–21.5%

4 PERFORMANCE TESTS

Minimum Fill 〈755〉: Meets the requirements

5 SPECIFIC TESTS

Calcium, Magnesium, and Other Foreign Substances

Sample: Heat 2 g gently until melted, and continue heating, gradually raising the temperature until the mass is thoroughly charred. Ignite the mass until the residue is uniformly yellow.

Analysis 1: To the Sample add 6 mL of 3 N hydrochloric acid.

Acceptance criteria 1: No effervescence occurs.

Analysis 2: Heat the mixture obtained in Analysis 1 on a steam bath for 10–15 min.

Acceptance criteria 2: NMT a trace of insoluble residue remains.

Analysis 3: Filter the solution from Analysis 2, dilute with water to 10 mL, add 6 N ammonium hydroxide until the precipitate rst formed redissolves, and add 2 mL each of ammonium oxalate TS and dibasic sodium phosphate TS.

Acceptance criteria 3: NMT a slight turbidity is produced in 5 min.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Package in well-closed containers, and avoid prolonged exposure to temperatures exceeding 30°.