Zinc Oxide Neutral

ZnO     81.39
DEFINITION
Zinc Oxide Neutral, freshly ignited, contains NLT 95.0% and NMT 98.0% of ZnO.
IDENTIFICATION
•  A. When strongly heated, it assumes a yellow color that disappears on cooling.
•  B. Identification Tests—General, Zinc 191
Sample solution:   Dissolve in a slight excess of 3 N hydrochloric acid.
Acceptance criteria:  Meets the requirements
ASSAY
•  Procedure
Sample:  1.5 g, freshly ignited
Analysis:  Dissolve the Sample and 2.5 g of ammonium chloride in 50.0 mL of 1 N sulfuric acid VS, with the aid of gentle heat if necessary. When dissolution is complete, add methyl orange TS, and titrate the excess sulfuric acid with 1 N sodium hydroxide VS. Each mL of 1 N sulfuric acid is equivalent to 40.69 mg of ZnO.
Acceptance criteria:  95.0%–98.0%
IMPURITIES
•  Chloride and Sulfate, Sulfate 221: A 0.1-g portion shows no more sulfate than corresponds to 2.3 mL of 0.020 N sulfuric acid (2.2%).
•  Arsenic, Method I 211: NMT 2 ppm
•  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.
•  Mercury
Nitric acid solution 1:  Carefully add 50 mL of nitric acid to 450 mL of water, and mix.
Nitric acid solution 2:  Carefully add 10 mL of nitric acid to 490 mL of water, and mix.
Hydrochloric acid–nitric acid solution:  [Note—Prepare immediately before use. ] Concentrated hydrochloric acid and concentrated 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 Nitric acid solution 1
Standard working mercury solution:  0.5 µg/mL of mercury in Nitric acid solution 2, 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 Hydrochloric acid–nitric acid solution. 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.
Blank solution:  To a 300-mL BOD bottle add 5 mL of water and 5 mL of Hydrochloric acid–nitric acid solution. 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.
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 Hydrochloric acid–nitric acid solution. 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.
Mercury detection instrument and Aeration apparatus:  Proceed as directed for Mercury 261, Method IIa and Method IIb.
Analysis:  Add 5 mL of Stannous sulfate solution to a Standard solution, and immediately insert the bottle into the Aeration apparatus. Measure the absorbance of the Standard solution. Repeat with the remaining Standard solutions and Sample solution. Perform a blank determination using 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
•  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 first aliquot, and add sodium sulfide TS to the second aliquot.
Acceptance criteria:  White precipitates are formed in both aliquots.
•  Magnesium Oxide
Nitric acid solution:  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 Nitric acid solution
Sample solution:  4 mg/mL of Zinc Oxide Neutral in Nitric acid solution
Blank:  Nitric acid solution
Instrumental conditions 
Mode:  Inductively coupled plasma–atomic emission spectrometry
Analytical wavelength:  279.1 nm
RF power:  1.2 KW
Flow rate 
Argon torch:  17 L/min
Argon nebulizer:  1.0 L/min
Argon auxiliary:  1.4 L/min
Analysis 
Samples:  Standard solution, Sample solution, and Blank
Calculate the percentage of magnesium oxide in the portion of ZnO taken:
Result = (CU/CS) × (Mr1/Mr2) × 100
CU== concentration of magnesium in the Sample solution, determined from the instrument (µg/mL)
CS== concentration of zinc oxide neutral in the Sample solution (µg/mL)
Mr1== molecular weight of magnesium oxide, 40.30
Mr2== molecular weight of magnesium, 24.31
Acceptance criteria:  NMT 0.7%
•  Loss on Ignition 733: Weigh 1 g and ignite at 750 for 15 min: it loses NMT 5.0% of its weight.
SPECIFIC TESTS
•  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.
•  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. ]
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in well-closed containers, and store at room temperature.
•  Labeling: Label it to indicate that it is for use in sunscreen preparations only.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Feiwen Mao, M.S.
Senior Scientific Liaison
1-301-816-8320
(SM32010) Monographs - Small Molecules 3
USP35–NF30 Page 5073
Pharmacopeial Forum: Volume No. 31(1) Page 80