Zinc Oxide Neutral

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 81.39

1 DEFINITION

Zinc oxide Neutral, freshly ignited, contains NLT 95.0% and NMT 98.0% 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 Neutral from the freshly ignited Zinc Oxide Neutral 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 Neutral 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 Neutral in the Sample solution (µg/mL)

Acceptance criteria: 95.0%–98.0%

4 IMPURITIES

4.1 Chloride and Sulfate 〈221〉, Sulfate

Standard solution: 0.020 N sulfuric acid

Sample: 0.1 g

Acceptance criteria: The Sample shows no more sulfate than corresponds to 2.3 mL of the Standard solution (2.2%). Change to read:

Arsenic 〈211〉, Procedures, Procedure 1 (CN 1-Jun-2023) : NMT 2 ppm

4.2 Lead

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

Analysis: Add 5 drops of potassium chromate TS.

Acceptance criteria: No turbidity or precipitate is produced.

Change to read:

4.3 Mercury

Diluent A: Carefully add 50 mL of nitric acid to 450 mL of water, and mix.

Diluent B: Carefully add 10 mL of nitric acid to 490 mL of water, and mix.

Aqua regia: [Note—Prepare immediately before use.]Hydrochloric acid and nitric acid (3:1)

Stannous sulfate solution: [Note—The mixture is a suspension and should be stirred continuously during use.] Add 25 g of stannous sulfate to 250 mL of 0.5 N sulfuric acid.

Sodium chloride–hydroxylamine sulfate solution: 120 mg/mL each of sodium chloride and hydroxylamine sulfate in water Potassium permanganate solution: 50 mg/mL of potassium permanganate in water

Standard stock mercury solution: [Note—Use of a commercially prepared mercury standard is recommended.] 1.0 mg/mL of mercury from mercuric chloride in Diluent A

Standard working mercury solution: 0.5 µg/mL of mercury in Diluent B, from Standard stock mercury solution

Standard solutions: Transfer 1-, 2-, 3-, and 4-mL aliquots of Standard working mercury solution to four separate 300-mL biological oxygen demand (BOD) bottles. To each bottle add 5 mL of water and 5 mL of Aqua regia. Heat the bottle for 2 min in a water bath at 95°. Cool, and add 50 mL of water and 15 mL of Potassium permanganate solution. Mix thoroughly, and place in a water bath for 30 min at 95°. Cool, add 5 mL of Sodium chloride–hydroxylamine sulfate solution, and dilute with water to 200 mL. These solutions contain the equivalent of 2.5, 5, 7.5, and 10 ng/mL of mercury, respectively.

Sample solution: Transfer 2.0 g of Zinc Oxide Neutral into a 300-mL BOD bottle. To the bottle add 5 mL of water and 5 mL of Aqua regia. Heat the sample in a water bath for 2 min at 95°. Cool, and add 50 mL of water and 15 mL of Potassium permanganate solution. Mix thoroughly, and place in a water bath for 30 min at 95°. Cool, add 5 mL of Sodium chloride–hydroxylamine sulfate solution, and dilute with water to 200 mL.

Blank solution: To a 300-mL BOD bottle add 5 mL of water and 5 mL of Aqua regia. Heat the solution for 2 min in a water bath at 95°. Cool, and add 50 mL of water and 15 mL of Potassium permanganate solution. Mix thoroughly, and place in a water bath for 30 min at 95°. Cool, add 5 mL of Sodium chloride–hydroxylamine sulfate solution, and dilute with water to 200 mL.

Mercury detection instrument and Aeration apparatus: Proceed as directed in Mercury 〈261〉, Procedures, Procedure 2 and Procedure 3 (CN 1-Jun-2023) .

Analysis: Add 5 mL of Stannous sulfate solution to a Standard solution, and immediately insert the bottle into the Aeration apparatus. Measure the absorbances of the Standard solutions. Repeat with the remaining Standard solutions and Sample solution. Perform a blank determination using the Blank solution, and make any necessary corrections. Plot the absorbances of the Standard solutions versus concentrations, in µg/mL, and draw the straight line best fitting the plotted points. From the graph so obtained, determine the concentration, in ppm of mercury, in the Sample solution.

Acceptance criteria: NMT 1 ppm

4.4 Iron and Other Heavy Metals

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

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

Acceptance criteria: White precipitates are formed in both aliquots.

4.5 Magnesium Oxide

Blank: Carefully add 10 mL of concentrated nitric acid to 490 mL of water, and mix.

Standard solution: [Note—Use of a commercially prepared magnesium–inductively coupled plasma standard solution is recommended.] 25 µg/mL of magnesium in Blank

Sample solution: 4 mg/mL of Zinc Oxide Neutral in Blank

4.5.1 Instrumental conditions

(See Plasma Spectrochemistry 〈730〉.)

Mode: Inductively coupled plasma–atomic emission spectrometry

Analytical wavelength: 279.1 nm

4.5.2 Analysis

Samples: Standard solution, Sample solution, and Blank

Calculate the percentage of magnesium oxide in the portion of Zinc Oxide Neutral taken:

Result = (C_U /C_S ) × (M_r1 /M_r2 ) × 100

C_U = concentration of magnesium in the Sample solution, determined from the instrument (µg/mL)

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

M_r1 = molecular weight of magnesium oxide, 40.30

M_r2 = molecular weight of magnesium, 24.31

Acceptance criteria: NMT 0.7%

4.6 Loss on Ignition 〈733〉 

Sample: 1 g 

Analysis: Ignite the Sample at 750° for 15 min. 

Acceptance criteria: NMT 5.0% 

5 SPECIFIC TESTS 

5.1 Alkalinity 

Analysis: Mix 1.0 g with 10 mL of hot water, and add two drops of phenolphthalein TS. 

Acceptance criteria: No color change occurs. 

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, and store at room temperature. 

Labeling: Label it to indicate that it is for use in sunscreen preparations only.