Article · Wikipedia archive · Last revised Jul 4, 2026

Ferroin

Ferroin, also known as tris(o-phenanthroline)iron(II), is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is the abbreviation of ortho-phenanthroline for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride. Ferroin is one of many transition metal complexes of 1,10-phenanthroline.

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Ferroin
The structure of the [Fe(o-phen)3]2+ complex cation in ferroin
The structure of the [Fe(o-phen)3]2+ complex cation in ferroin source ↗
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.035.145
UNII
  • InChI=1S/3C12H8N2.Fe.H2O4S/c3*1-3-9-5-6-10-4-2-8-14-12(10)11(9)13-7-1;;1-5(2,3)4/h3*1-8H;;(H2,1,2,3,4)/q;;;+2;/p-2 checkY
    Key: CIWXFRVOSDNDJZ-UHFFFAOYSA-L checkY
  • InChI=1/3C12H8N2.Fe.H2O4S/c3*1-3-9-5-6-10-4-2-8-14-12(10)11(9)13-7-1;;1-5(2,3)4/h3*1-8H;;(H2,1,2,3,4)/q;;;+2;/p-2
    Key: CIWXFRVOSDNDJZ-NUQVWONBAU
  • c12c3[n+]4cccc3ccc2ccc[n+]1[Ru-4]456(c7cccc(c78)ccc(c78)ccc[n+]67)[n+]1cccc2ccc3ccc[n+1]5c3c12
Properties
C36H24FeN62+
Molar mass 596.27 g/mol
Hazards
GHS labelling:1
GHS06: ToxicGHS07: Exclamation markGHS09: Environmental hazard
Danger
H301, H302, H410, H412
P264, P270, P273, P301+P316, P301+P317, P321, P330, P391, P405, P501
1.0 mg/m3, as Fe
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Ferroin, also known as tris(o-phenanthroline)iron(II), is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is the abbreviation of ortho-phenanthroline for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.2 Ferroin is one of many transition metal complexes of 1,10-phenanthroline.

Structure

Many salts of [Fe(o-phen)3]2+ have been characterized by X-ray crystallography. The structures of [Fe(o-phen)3]2+ and [Fe(o-phen)3]3+ are almost identical, consistent with both being low-spin. These cations are octahedral with D3 symmetry group. The Fe-N distances are 197.3 pm.3

Preparation and reactions

Ferroin sulfate can be prepared by combining phenanthroline to ferrous sulfate dissolved in water:4

3 phen + Fe2+ → [Fe(phen)3]2+

The oxidation of this complex from Fe(II) to Fe(III), involving the fast and reversible transfer of only one electron, makes it a useful redox indicator in aqueous solution:

[Fe(phen)3]2+ → [Fe(phen)3]3+ + 1 e−        (Eh = +1.06 V)

Addition of sulfuric acid to an aqueous solution of [Fe(phen)3]2+ causes its hydrolysis and the formation of a neutral ion pair [phenH]HSO4:

[Fe(phen)3]2+ + 3 H2SO4 + 6 H2O → [Fe(OH2)6]2+ + 3 [phenH]+HSO4−

Addition of cyanide to an aqueous solution of [Fe(phen)3]SO4 precipitates Fe(phen)2(CN)2.5

Redox indicator

o-Phenanthroline Fe(II) (Redox indicator)
E0= +1.06 V
Reduced. ↔ Oxidized
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This complex is used as an indicator in analytical chemistry.6 The active ingredient is the [Fe(o-phen)3]2+ ion, which is a chromophore that can be oxidized to the ferric derivative [Fe(o-phen)3]3+. The potential for this redox change is +1.06 volts in 1 M H2SO4. It is a popular redox indicator for visualizing oscillatory Belousov–Zhabotinsky reactions.

Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in cerimetry.7

Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has a transition potential of +1.25 volt. It is more stable than ferroin, but in sulfuric acid with Ce4+ ion, it requires a significant excess of titrant. It is, however, useful for titration in perchloric acid or nitric acid solution, where the cerium redox potential is higher.7

The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by adjusting the position and number of methyl groups on the phenanthroline core.7

Fe2+ direct UV-visible spectrophotometric determination

In analytical chemistry, the red color specific for the reduced form of ferroin was once used for the direct UV-visible spectrophotometric determination of Fe2+.89 The maximum absorbance of the Fe(II) o-phenanthroline complex is at 511 nm.10 However, another related N-ligand called ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate)11 is also used and must not be confused with ferroin. Ferrozine was specifically synthesised in the 1970s to obtain a less expensive reagent for automated chemical analysis.12 Ferrozine reacts with Fe2+ to form a relatively stable magenta-colored complex with a maximum absorbance at 562 nm.1213 The ferrozine method allows the determination of Fe(II)/Fe(III) speciation in natural fresh or marine waters at the submicromolar level.14

In 2021, Smith et al. reexamined the formation kinetics and stability of ferroin and ferrozine Fe(II) complexes. They have found that while the kinetics of Fe2+ binding by o-phenanthroline are very fast, the kinetics of Fe2+ complexation by ferrozine depend on ligand concentration. An excess ligand concentration provides a more stable absorbance, while the formation of Fe(II) complexes is pH-independent.15

References

References

  1. PubChem. "1,10-Phenanthroline ferrous sulfate". pubchem.ncbi.nlm.nih.gov. Retrieved 2026-05-31.
  2. Sattar, Simeen (2011). "A unified kinetics and equilibrium experiment: Rate law, activation energy, and equilibrium constant for the dissociation of ferroin". Journal of Chemical Education. 88 (4): 457–460. Bibcode:2011JChEd..88..457S. doi:10.1021/ed100797s.
  3. Baker, Joe; Engelhardt, Lutz M.; Figgis, Brian N.; White, Allan H. (1975). "Crystal structure, electron spin resonance, and magnetism of tris(o-phenanthroline)iron(III) perchlorate hydrate". Journal of the Chemical Society, Dalton Transactions (6): 530. doi:10.1039/DT9750000530.
  4. Avdeeva, Varvara V.; Vologzhanina, Anna V.; Goeva, Lyudmila V.; Malinina, Elena A.; Kuznetsov, Nikolay T. (2014). "Boron Cluster Anions [BnHn]2– ( n = 10, 12) in Reactions of Iron(II) and Iron(III) Complexation with 2,2′-Bipyridyl and 1,10-Phenanthroline". Zeitschrift für Anorganische und Allgemeine Chemie. 640 (11): 2149–2160. doi:10.1002/zaac.201400137.
  5. Schilt, Alfred A. (1970). "Dicyanobis(1,10-phenanthroline)Iron(II) and Dicyanobis(2,2′-bipyridine)iron(II)". Inorganic Syntheses. Vol. 12. pp. 247–251. doi:10.1002/9780470132432.ch43. ISBN 978-0-470-13171-8.
  6. Harris, D. C. (1995). Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 978-0-7167-2508-4.
  7. Handbook on the Physics and Chemistry of Rare Earths. Elsevier. 2006. pp. 289–. ISBN 978-0-08-046672-9.
  8. Fortune, W. B.; Mellon, M. G. (1938-02-01). "Determination of iron with o-phenanthroline: A spectrophotometric study". Industrial & Engineering Chemistry Analytical Edition. 10 (2): 60–64. doi:10.1021/ac50118a004. ISSN 0096-4484.
  9. Bandemer, Selma L.; Schaible, P J. (1944-05-19). "Determination of iron. A study of the o-phenanthroline method". Industrial & Engineering Chemistry Analytical Edition. 16 (5): 317–319. doi:10.1021/i560129a013. ISSN 0096-4484.
  10. Tripathi, Atri Deo; Gupta, K.A.; Malik, Shally (2019). "Iron determination by colorimetric method using o-phenanthroline". Bulletin of Pure & Applied Sciences – Chemistry. 38c (2): 171. doi:10.5958/2320-320X.2019.00018.9. ISSN 0970-4620.
  11. "Ferrozine". Sigma-Aldrich. Retrieved 2025-03-10.
  12. Stookey, Lawrence L. (1970-06-01). "Ferrozine—a new spectrophotometric reagent for iron" (PDF). Analytical Chemistry. 42 (7): 779–781. doi:10.1021/ac60289a016. ISSN 0003-2700. Retrieved 2025-03-10.
  13. Huang, Wenjuan; Hall, Steven J. (2017). "Optimized high-throughput methods for quantifying iron biogeochemical dynamics in soil". Geoderma. 306: 67–72. doi:10.1016/j.geoderma.2017.07.013. Retrieved 2025-03-12.
  14. Viollier, E.; Inglett, P.W.; Hunter, K.; Roychoudhury, A.N.; Van Cappellen, P. (2000). "The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters". Applied Geochemistry. 15 (6): 785–790. doi:10.1016/S0883-2927(99)00097-9.
  15. Smith, Gideon L.; Reutovich, Aliaksandra A.; Srivastava, Ayush K.; Reichard, Ruth E.; Welsh, Cass H.; Melman, Artem; Bou-Abdallah, Fadi (2021). "Complexation of ferrous ions by ferrozine, 2,2′-bipyridine and 1,10-phenanthroline: Implication for the quantification of iron in biological systems". Journal of Inorganic Biochemistry. 220 111460. doi:10.1016/j.jinorgbio.2021.111460.