Ferric Oxide
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
1 DEFINITION
Ferric Oxide contains NLT 97.0% and NMT 100.5% of ferric oxide (Fe₂O₃), calculated on the ignited basis.
2 IDENTIFICATION
A. Identification Tests—General, Iron 〈191〉
Sample solution: Dissolve 0.5 g in 50 mL of hydrochloric acid, and dilute with water to 200 mL.
Acceptance criteria: Meets the requirements of the test for Ferric Salts
3 ASSAY
Procedure
To calculate the percentage of ferric oxide on the ignited basis, ignite about 2 g at 800 ± 25° to constant weight as directed in Loss on Ignition 〈733〉.
[Note—Ignited Ferric Oxide is hygroscopic.]
Sample: 1.5 g
Blank: 25 mL of hydrochloric acid
Titrimetric system
Mode: Direct titration
Titrant: 0.1 N sodium thiosulfate VS
Endpoint detection: Visual
Analysis:
Digest the Sample in 25 mL of hydrochloric acid on a water bath until dissolved. Add 10 mL of Hydrogen peroxide TS, and evaporate on a water bath almost to dryness to volatilize all hydrogen peroxide. Dissolve the residue by warming with 5 mL of hydrochloric acid; add 25 mL of water; filter into a 250-mL volumetric flask, washing the filter with water; and add water to volume.
Transfer a 50-mL aliquot to a glass-stoppered flask, add 3 g of potassium iodide and 5 mL of hydrochloric acid, and insert the stopper into the flask. Allow the mixture to stand for 15 min, add 50 mL of water, and titrate the liberated iodine with Titrant, using starch TS as the indicator. Perform a blank determination in the same manner.
Calculate the percentage of ferric oxide (Fe₂O₃) in the portion of the sample taken:
Result = {[(Vs − VB ) × N × F] / W} × 100
Vs = Titrant volume consumed by the Sample (mL)
VB = Titrant volume consumed by the Blank (mL)
N = actual normality of the Titrant (mEq/mL)
F = equivalency factor, 79.85 mg/mEq
W = Sample weight, calculated with a correction for loss on ignition (mg)
Acceptance criteria: 97.0%–100.5% on the ignited basis
4 IMPURITIES
Change to read:
▲Mercury 〈261〉, Procedures, Procedure 2 and Procedure 3, Procedure 2▲ (CN 1-Jun-2023)
Test preparation: Combine 0.67 g of Ferric Oxide and 35 mL of 0.5 N hydrochloric acid. Heat to boiling, and allow to cool.
Analysis: Proceed as directed for Test preparation in the chapter, beginning with “Add 2 drops of phenolphthalein”.
Acceptance criteria: NMT 3 µg/g
Limit of Arsenic
Lead acetate cotton: Immerse absorbent cotton pledgets in a mixture of lead acetate TS and 2 N acetic acid (10:1). Free the cotton pledgets from excess liquid by expression, and allow to air-dry.
Sodium borohydride solution: 30 mg/mL of sodium borohydride in 0.25 N sodium hydroxide. Store in a loosely covered container protected from direct sunlight.
Mercuric bromide paper: Immerse several filter paper disks with a 15-mm diameter in alcoholic mercuric bromide TS, remove the disks from the solution, and allow to dry, protected from light. Store in a glass-stoppered container protected from light.
Arsenic trioxide stock solution: Dissolve 132.0 mg of arsenic trioxide in 2.0 mL of 2 N sodium hydroxide, and dilute with water to 100 mL.
Standard stock solution: On the day of use, dilute 1.0 mL of Arsenic trioxide stock solution with water to 1000 mL.
Standard solution: Dilute 1.5 mL of the Standard stock solution with hydrochloric acid to 10 mL. This solution contains 0.15 µg/mL of arsenic.
Sample solution: Dissolve 0.5 g of Ferric Oxide in several mL of hydrochloric acid with the aid of heat, and dilute with hydrochloric acid to 10.0 mL.
Apparatus:
Prepare a 300-mL, side-arm conical flask containing a magnetic stirring bar. Attach to the conical flask a ground-glass stopper. Pass through the ground-glass stopper a glass tube 20 cm long with an internal diameter of 5 mm. The lower end of this tube is inside the conical flask, and it has been drawn to a tip with an internal diameter of 1 mm. There is an orifice, 2.5 mm in diameter, 15 mm from the tip, and at least 3 mm below the lower surface of the stopper. The upper end of the tube has a flat ground surface at a right angle to the axis of the tube.
A second glass tube, 30 mm long with an internal diameter of 5 mm and with a similar flat ground surface, is placed in contact with the ground surface of the first tube and is held in position by a clamp and springs.
Into the lower tube insert 55 mg of loosely packed Lead acetate cotton. Between the flat surfaces of the tubes place a disk of Mercuric bromide paper.
Analysis:
Before placing the tube assembly into the flask, transfer the Sample solution to the flask, and add 5.0 mL of potassium iodide TS and 20 mL of water. Assemble the apparatus immediately, and stir while slowly adding, over a period of 20 min, 40 mL of Sodium borohydride solution through the side arm of the flask. Examine the stain produced on the Mercuric bromide paper. Perform the same Analysis using the Standard solution.
Acceptance criteria: NMT 3 µg/g; the stain produced on the Mercuric bromide paper from the Sample solution is not more intense than that from the Standard solution.
Limit of Lead
Lead nitrate stock solution: 1.598 mg/mL of lead nitrate in 0.5 M nitric acid. Prepare and store this solution in glass containers free from soluble lead salts.
Standard stock solution: On the day of use, combine 5.0 mL of Lead nitrate stock solution and 10 mL of 1 N hydrochloric acid, and dilute with water to 100 mL.
Standard solution: Transfer 1.0 mL of the Standard stock solution to a 100-mL volumetric flask, add 10 mL of 1 N hydrochloric acid, and dilute with water to volume. This solution contains 0.5 µg/mL of lead.
Sample solution: Transfer 2.5 g of Ferric Oxide to a 100-mL, glass-stoppered conical flask. Add 35 mL of 0.1 N hydrochloric acid, and stir for 1 h. Filter, collecting the filtrate in a 50-mL volumetric flask, and dilute with 0.1 N hydrochloric acid to volume.
Instrumental conditions
(See Atomic Absorption Spectroscopy 〈852〉.)
Mode: Atomic absorption spectrophotometer equipped with a flow spoiler
Analytical wavelength: 217.0 nm (lead emission line)
Lamp: Lead hollow-cathode
Flame: Air–acetylene oxidizing
Analysis
Samples: Standard solution and Sample solution
Acceptance criteria: 0.001%; the absorbance of the Sample solution does not exceed that of the Standard solution.
5 SPECIFIC TESTS
Water-Soluble Substances
Sample: 2.0 g
Analysis: Digest the Sample in 100 mL of water on a boiling water bath for 2 h, filter, and wash the filter with water. Evaporate the filtrate and washings, and dry the residue at 105° for 1 h.
Acceptance criteria: 1.0%; NMT 20 mg of residue
Acid-Insoluble Substances
Sample: 2.0 g
Analysis: Digest the Sample in 25 mL of hydrochloric acid by boiling for 20 min. Add 100 mL of hot water, and filter quantitatively through a tared filtering crucible, with the aid of hot wash water, until the filtrate tests negative for chloride. Dry the crucible and contents at 105° for 1 h.
Acceptance criteria: 0.3%; the residue weighs NMT 6 mg.
Organic Colors and Lakes
Sample: 3.0 g
Analysis: Place 1.0 g of the Sample in each of 3 beakers, and add 25 mL of each of the following reagents, one reagent to each beaker: 1-chloronaphthalene, alcohol, and chloroform. Heat the beakers containing alcohol and chloroform just to boiling. Heat the other beaker on a boiling water bath for 15 min, with occasional swirling. Pass the contents of the beakers through retentive, solvent-resistant filter paper. If any of the filtrates shows visible turbidity, centrifuge for 15 min. Record the spectra against respective solvent blanks in 1-cm cells from 350 to 750 nm.
Acceptance criteria: No peak above the noise level with a slope greater than +0.001 absorbance unit/nm is found.
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in well-closed containers.
