Zinc Oxide

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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ZnO 81.38

Zinc oxide CAS RN®: 1314-13-2; UNII: SOI2LOH54Z.

1 DEFINITION

Zinc Oxide, freshly ignited, contains NLT 98.0% and NMT 102.0% of zinc oxide (ZnO).

2 IDENTIFICATION

A. When strongly heated, it assumes a yellow color that disappears on cooling.

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

3 ASSAY

Procedure

Proceed as directed in Zinc Determination 〈591〉, Ion Chromatographic Method.

Diluent: 0.2% (w/v) hydrochloric acid

Standard solution: 20 µg/mL of USP Zinc Oxide RS in Diluent, prepared as directed in the chapter

Sample solution: 20 µg/mL of Zinc Oxide from the freshly ignited Zinc Oxide in Diluent, prepared as directed in the Standard solution Analysis

Samples: Standard solution and Sample solution

Calculate the percentage of zinc oxide (ZnO) in the portion of Zinc Oxide taken:

Result = (r_U /r_S ) × (C_S /C_U ) × 100

r_U = peak response of zinc from the Sample solution

r_S = peak response of zinc from the Standard solution

C_S = concentration of USP Zinc Oxide RS in the Standard solution (µg/mL)

C_U = concentration of Zinc Oxide in the Sample solution (µg/mL)

Acceptance criteria: 98.0%–102.0% on the ignited basis

4 IMPURITIES

Change to read:

Arsenic 〈211〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 6 ppm

4.1 Lead

Sample solution: Add 2 g to 20 mL of water, stir well, and add 5 mL of glacial acetic acid. Warm on a steam bath until dissolved.

Analysis: Add 5 drops of potassium chromate TS.

Acceptance criteria: No turbidity or precipitate is produced.

4.2 Iron and Other Heavy Metals

Sample solution: Use the solution from Carbonate and Color of Solution.

Analysis: Cool 2 separate 5-mL aliquots of the Sample solution. Add potassium ferrocyanide TS to the rst aliquot, and add sodium sulfide TS to the second aliquot.

Acceptance criteria: White precipitates are formed in both aliquots.

4.3 Loss on Ignition 〈733〉

Sample: 2 g

Analysis: Ignite the Sample at 500° to constant weight.

Acceptance criteria: NMT 1.0%

5 SPECIFIC TESTS

5.1 Alkalinity

Analysis: Mix 1.0 g with 10 mL of hot water, add 2 drops of phenolphthalein TS, and pass through a suitable filter of appropriate pore size.

Acceptance criteria: If a red color is produced, NMT 0.30 mL of 0.10 N hydrochloric acid is required to discharge it.

5.2 Carbonate and Color of Solution

Analysis: Mix 2.0 g with 10 mL of water, add 30 mL of 2 N sulfuric acid, and heat on a steam bath with constant stirring.

Acceptance criteria: No effervescence occurs, and the resulting solution is clear and colorless. [Note—Use this solution in the test for Iron and Other Heavy Metals.]

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers.