Ferrosoferric 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)
Fe3O4(FeO · Fe2O3) 231.54
Magnetite;
Ferrous-ferric oxide;
Iron (II, III) oxide CAS RN®: 1317-61-9.
1 DEFINITION
Ferrosoferric Oxide contains NLT 97.0% and NMT 100.5% of Fe2O3 calculated on the ignited basis.
2 IDENTIFICATION
A.
Sample solution: Dissolve 0.1 g in 5 mL of hydrochloric acid, and dilute with water to 50 mL.
Analysis 1: Transfer 5 mL of the Sample solution to a test tube, and add a few drops of potassium ferrocyanide TS.
Analysis 2: Transfer 5 mL of the Sample solution to a test tube, and add a few drops of potassium ferricyanide TS.
Acceptance criteria: For both Analysis 1 and Analysis 2, a blue precipitate (Prussian blue) is formed, which does not dissolve in dilute hydrochloric acid subsequently added. The precipitate dissolves in oxalic acid and sodium or potassium hydroxide.
B. Ferrosoferric Oxide presents as black powder, which is distinguished from ferric oxide exhibiting two basic colors (red and yellow). It is attracted to a magnet.
3 ASSAY
3.1 PROCEDURE
To enable the calculation of the percentage of Fe2O3 on the ignited basis, ignite about 2 g at 800 ± 25° to constant weight as directed in Loss on Ignition (733). [NOTE-Ignited Ferrosoferric Oxide is hygroscopic.]
Sample: 1.5 g
Blank: 25 mL of hydrochloric acid
Titrimetric system
(See Titrimetry (541)..)
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 in order 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 0.1 N sodium thiosulfate VS, using starch TS as the indicator. Perform a blank determination in the same manner.
Calculate the percentage of the labeled amount as ferric oxide (Fe2O3) in the portion of the Sample taken:
Result = {[(VS - VB) x N x F]/W} x 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 = weight of the Sample, calculated with a correction for loss on ignition (mg)
Acceptance criteria: 97.0%-100.5% on the ignited basis
4 IMPURITIES
4.1 LIMIT OF ARSENIC (AS)
[NOTE-Select all reagents to have as low contents of elemental impurities as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse all glassware before use by soaking in warm 8 N nitric acid for 30 min and by rinsing with deionized water.]
Solvent: 10% hydrochloric acid in water
Standard stock solution: Prepare a solution containing 2 ppm of arsenic from an Arsenic ICP/AA standard solution in Solvent. [NOTE-A commercially available certified standard solution can be used.]
Sample stock solution: In a 100-mL volumetric flask, dissolve 5.0 g of Ferrosoferric Oxide in 50 mL of hydrochloric acid by heating without boiling for about 1 h until the sample is dissolved. Cool down to room temperature. Pass through a filter, if necessary. Dilute with water to volume. Transfer 20.0 mL of the solution to a 100-mL volumetric flask, add 40 mL of ascorbic acid solution (200 mg/mL of ascorbic acid in water) and 20 mL of potassium iodide solution (400 mg/mL of potassium iodide in water), and dilute with water to volume. [NOTE-Reduce arsenic to the lowest possible oxidation state.] The resulting solution contains 10 mg/mL (g/L) of Ferrosoferric Oxide.
Sample solution A: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Dilute with Solvent to volume. The resulting solution contains 2 mg/mL (g/L) of Ferrosoferric Oxide.
Sample solution B: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 25 mL of the Standard stock solution, and dilute with Solvent to volume.
Sample solution C: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 50 mL of the Standard stock solution, and dilute with Solvent to volume.
Blank: Solvent
Instrumental conditions
(See Atomic Absorption Spectroscopy (852).)
Mode: Atomic absorption spectrophotometer
Lamp: Arsenic hollow-cathode
Flame: Air-acetylene
Analytical wavelength: 193.696 nm
Analysis
Samples: Sample solution A, Sample solution B, and Sample solution C
[NOTE-To minimize matrix interference, it is recommended that the method of standard additions be used.]
Determine the absorbances of the Samples against the Blank, and plot the corrected absorbances versus their added content of arsenic (0, 0.5, and 1.0 ppm). Draw the straight line best fitting the three points, and extrapolate the line until it meets the concentration axis. The distance between this point and the intersection of the axes represents the concentration of arsenic, CA, in ppm (µg/g) in the 2 mg/mL of Ferrosoferric Oxide in Solvent.
Calculate the content of arsenic, in µg/g, in the portion of Ferrosoferric Oxide taken:
Result = CA x D
D = dilution factor, 25
Acceptance criteria: NMT 2 ppm
4.2 LIMIT OF CADMIUM (CD) AND CHROMIUM (CR)
[NOTE-Select all reagents to have as low contents of elemental impurities as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse all glassware before use by soaking in warm 8 N nitric acid for 30 min and by rinsing with deionized water.]
Solvent: 10% hydrochloric acid in water
Standard stock solution: Prepare a solution containing 2 ppm each of cadmium and chromium from an ICP standard solution containing cadmium and chromium in Solvent. [NOTE-A commercially available certified standard solution can be used.]
Sample stock solution: In a 100-mL volumetric flask, dissolve 5.0 g of Ferrosoferric Oxide in 50 mL of hydrochloric acid by heating without boiling for about 1 h until the sample is dissolved. Cool down to room temperature. Pass through a filter, if necessary. Dilute with water to volume. Transfer 20.0 mL of the solution to a 100-mL volumetric flask, and add water to volume. The resulting solution contains 10 mg/mL of Ferrosoferric Oxide.
Sample solution A: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Dilute with Solvent to volume. The resulting solution contains 2 mg/mL of Ferrosoferric Oxide.
Sample solution B: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 25 mL of the Standard stock solution, and dilute with Solvent to volume.
Sample solution C: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 50 mL of the Standard stock solution, and dilute with Solvent to volume.
Blank: Solvent
Instrumental conditions
(See Plasma Spectrochemistry (730).)
Mode: Inductively coupled plasma-atomic emission spectrometer (ICP-AES)
Analytical wavelength
Cadmium: 228.802 nm
Chromium: 267.716 nm
Analysis
Samples: Sample solution A, Sample solution B, and Sample solution C
[NOTE-The operating conditions may be developed and optimized based on the manufacturer's recommendation.]
[NOTE-To minimize matrix interference, it is recommended that the method of standard additions be used.] Instrument performance must be verified to conform to the manufacturer's specifications for resolution and sensitivity. Before analyzing samples, the instrument must pass a suitable performance check. Aspirate the Samples, at least in duplicate, and calculate the average absorbances for each against the Blank. Plot the average absorbances of the Samples versus their added content (0, 0.5, and 1.0 ppm) of cadmium (or chromium). Draw the straight line best fitting the three points, and extrapolate the line until it intercepts the concentration axis. The distance between this point and the intersection of the axes represents the concentration of cadmium (or chromium), C in ppm (µg/g) in the 2 mg/mL of Ferrosoferric Oxide in Solvent.
Calculate the content of cadmium (or chromium), in µg/g, in the portion of Ferrosoferric Oxide taken:
Result = CC x D
D = dilution factor, 25
Acceptance criteria
Cadmium: NMT 1 ppm
Chromium: NMT 100 ppm
4.3 LIMIT OF LEAD (PB)
[NOTE-Select all reagents to have as low contents of elemental impurities as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse all glassware before use by soaking in warm 8 N nitric acid for 30 min and by rinsing with deionized water.]
Solvent: 4% hydrochloric acid and 8% nitric acid in water
Standard solution: Prepare a solution containing 25 ppb of lead in Solvent. [NOTE-A commercially available certified standard solution can be used.]
Sample stock solution: Dissolve 1.0 g of Ferrosoferric Oxide in 4 mL of hydrochloric acid and 8 mL of nitric acid in a 100-mL volumetric flask by heating without boiling until the sample is dissolved. Cool down to room temperature. Pass through a filter, if necessary. Dilute with water to volume. The resulting solution in the flask contains 10 mg/mL of Ferrosoferric Oxide.
Sample solution A: Transfer 10 mL of the Sample stock solution to a 100-mL volumetric flask, and dilute with Solvent to volume. The solution contains 1 mg/mL of Ferrosoferric Oxide.
Sample solution B: Transfer 10 mL of the Sample stock solution to a 100-mL volumetric flask. Add 4 mL of the Standard solution, and dilute with Solvent to volume.
Sample solution C: Transfer 10 mL of the Sample stock solution to a 100-mL volumetric flask. Add 12 mL of the Standard solution, and dilute with Solvent to volume.
Sample solution D: Transfer 10 mL of the Sample stock solution to a 100-mL volumetric flask. Add 20 mL of the Standard solution, and dilute with Solvent to volume.
Blank: Solvent
Instrumental conditions
(See Atomic Absorption Spectroscopy (852).)
Mode: Atomic absorption spectrophotometer equipped with graphite heating device
Analytical wavelength: 283.3 nm
Lamp: Lead hollow-cathode
Analysis
Samples: Sample solution A, Sample solution B, Sample solution C, and Sample solution D
[NOTE-TO minimize matrix interference, it is recommended that the method of standard additions be used.]
Determine the absorbances of the Samples against the Blank, and plot the corrected absorbances versus their added content (0, 1, 3, and 5 ppb) of lead. Draw the straight line best fitting the four points, and extrapolate the line until it intercepts the concentration axis. The distance between this point and the intersection of the axes represents the concentration of lead, CL, in ppm (µg/g) in the 1 mg/mL of Ferrosoferric Oxide.
Calculate the content of lead, in ug/g, in the portion of Ferrosoferric Oxide taken:
Result = CL x D
D = dilution factor, 10
Acceptance criteria: NMT 10 ppm
4.4 LIMIT OF MERCURY (HG) AND NICKEL (NI)
[NOTE-Select all reagents to have as low contents of elemental impurities as practicable, and store all reagent solutions in containers of borosilicate glass. Cleanse all glassware before use by soaking in warm 8 N nitric acid for 30 min and by rinsing with deionized water.]
Solvent: 10% hydrochloric acid in water
Standard solution: Prepare a solution containing 2 ppm of mercury (or nickel) from a Mercury (or Nickel) ICP/AA standard solution in Solvent. [NOTE-A commercially available certified standard solution can be used.]
Sample stock solution: In a 100-mL volumetric flask, dissolve 5.0 g of Ferrosoferric Oxide in 50 mL of hydrochloric acid by heating without boiling for about 1 h until the sample is dissolved. Cool down to room temperature. Pass through a filter, if necessary. Dilute with water to volume. Transfer 20.0 mL of the solution to a 100-mL volumetric flask, and add water to volume. The resulting solution contains 10 mg/mL of Ferrosoferric Oxide.
Sample solution A: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Dilute with Solvent to volume. The resulting solution contains 2 mg/mL of Ferrosoferric Oxide.
Sample solution B: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 25 mL of the Standard solution, and dilute with Solvent to volume.
Sample solution C: Transfer 20 mL of the Sample stock solution to a 100-mL volumetric flask. Add 50 mL of the Standard solution, and dilute with Solvent to volume.
Blank: Solvent
Instrumental conditions
(See Atomic Absorption Spectroscopy (852).)
Mode: Atomic absorption spectrophotometer
Detection
Mercury: Flameless
Nickel: Air-acetylene flame
Lamp: Mercury hollow-cathode or nickel hollow-cathode
Analytical wavelength
Mercury: 253.7 nm
Nickel: 231.604 nm
Analysis
Samples: Sample solution A, Sample solution B, and Sample solution C
NOTE-To minimize matrix interference, it is recommended that the method of standard additions be used.]
[ Determine the absorbances of the Samples against the Blank, and plot the corrected absorbances versus their content (0, 0.5, and 1.0 ppm) of mercury (or nickel). Draw the straight line best fitting the three points, and extrapolate the line until it intercepts the concentration axis. The distance between this point and the intersection of the axes represents the concentration of mercury (or nickel), CS, in ppm (µg/g) in the 2 mg/mL of Ferrosoferric Oxide in Solvent.
Calculate the content of mercury (or nickel), in µg/g, in the portion of Ferrosoferric Oxide taken:
Result = CS x D
D = dilution factor, 25
Acceptance criteria
Mercury: NMT 1 ppm
Nickel: NMT 200 ppm
5 SPECIFIC TESTS
5.1 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: The weight of the dried residue is NMT 20 mg, corresponding to NMT 1.0%.
5.2 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 filtering1 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: The weight of the residue is NMT 6 mg, corresponding to NMT 0.3%.
5.3 ORGANIC COLORS AND LAKES
Analysis: Place 1.0 g in each of 3 beakers, and add 25 mL of each of the following reagents, respectively: 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. 2 If any of the filtrates show visible turbidity, centrifuge for 15 min. Record the spectra against respective solvent blanks in 1-cm cells from 350 to 750 nm.
Acceptance criteria: Absorbance in the range of 350-750 nm is NMT 0.01 AU.
5.4 MICROBIAL ENUMERATION TESTS (61), and TESTS FOR SPECIFIED MICROORGANISMS (62)
The total aerobic microbial count does not exceed 103 cfu/g, and the total combined molds and yeasts count does not exceed 102 cfu/g.
5.5 PH (791)
6-9 in a 10% suspension
5.6 LOSS ON DRYING (731)
Analysis: Dry a sample at 105° for 1 h.
Acceptance criteria: NMT 1.0% of its weight
6 ADDITIONAL REQUIREMENTS
PACKAGING AND STORAGE: Preserve in well-closed containers. Store at room temperature.
1 A glass-filter disc (P4; pore size diameter 9 to 15 µm) can be used, or equivalent.
2 A PTFE filter (diameter 25 mm, 0.2-µm pore size), or equivalent.
