Iron, Carbonyl
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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)
Fe 55.85 CAS RN®: 7439-89-6; UNII: E1UOL152H7.
1 DEFINITION
Carbonyl Iron is elemental iron produced by chemical decomposition of purified iron pentacarbonyl. It is a powder, composed of spherical microparticles. It contains NLT 98.0% of iron (Fe), calculated on the as-is basis.
2 IDENTIFICATION
A.
Analysis: Dissolve a sample in a dilute mineral acid.
Acceptance criteria: Hydrogen is evolved, and the resulting solutions give a positive test for Identication Tests—General 〈191〉, Iron, Ferrous Salts.
B.
Analysis: View a sample under a microscope having a magnifying power of 500 or greater.
Acceptance criteria: It appears as spheres built up with concentric shells. Its particle size is 45–75 µm.
3 ASSAY
Procedure
Sample: 200 mg
Blank: Proceed as directed in the Analysis, omitting use of the Sample.
Titrimetric system
(See Titrimetry 〈541〉.)
Mode: Direct titration
Titrant: 0.1 N ceric sulfate VS
Endpoint detection: Potentiometric
Electrode system: Platinum indicating electrode and a silver–silver chloride reference electrode (or an equivalent combination electrode) Analysis: Transfer the Sample into a 300-mL Erlenmeyer flask. Add 50 mL of 2 N sulfuric acid, and close the flask with a stopper containing a Bunsen valve (made by inserting a glass tube connected to a short piece of rubber tubing with a slit on the side and a glass rod inserted in the other end and arranged so that gases can escape but air cannot enter). Heat on a steam bath to completely dissolve the Sample. [Note —The solution should be clear.] Remove the flask from the steam bath, and allow the solution to cool at room temperature with the stopper in place.
Add a stir bar and 50 mL of recently boiled and cooled water to the flask. Titrate the solution with the Titrant through the injection point. Perform a blank determination.
Calculate the percentage of iron (Fe) in 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 Titrant normality (mEq/mL)
F = equivalency factor, 55.85 mg/mEq
W = Sample weight (mg)
Acceptance criteria: NLT 98.0% on the as-is basis
4 IMPURITIES
Acid-Insoluble Substances
Sample: 1 g of Carbonyl Iron
Analysis: Dissolve the Sample in 25 mL of 2 N sulfuric acid, and heat on a steam bath until the evolution of hydrogen ceases. Filter through a tared filter crucible, wash the residue with water until free from sulfate, dry at 105° for 1 h, cool to room temperature, and weigh. Acceptance criteria: NMT 0.2%. The residue weighs NMT 2 mg.
Elemental Impurities, Procedures〈233〉
Acceptance criteria
Arsenic: NMT 3 µg/g
Lead: NMT 4 µg/g
Mercury: NMT 2 µg/g
5 SPECIFIC TESTS
Particle Size Distribution Estimation By Analytical Sieving, Test Sieves〈786〉: NLT 100% passes through a 200-mesh sieve and NLT 95% passes through a 325-mesh sieve.
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in well-closed containers.
