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.
![The structure of the [Fe(o-phen)3]2+ complex cation in ferroin](http://wiki.nitrosworld.org/wiki/File:Ferroin-cation-3D-balls.png) | |
| Identifiers | |
|---|---|
3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.035.145 |
PubChem CID |
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| UNII | |
CompTox Dashboard (EPA) |
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| Properties | |
| C36H24FeN62+ | |
| Molar mass | 596.27 g/mol |
| Hazards | |
| GHS labelling:[1] | |
   | |
| Danger | |
| H301, H302, H410, H412 | |
| P264, P270, P273, P301+P316, P301+P317, P321, P330, P391, P405, P501 | |
Threshold limit value (TLV) |
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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Structure
editMany 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
editFerroin 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
edit| o-Phenanthroline Fe(II) (Redox indicator) | ||
| E0= +1.06 V | ||
| Reduced. | ↔ | Oxidized |
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
editIn analytical chemistry, the red color specific for the reduced form of ferroin was once used for the direct UV-visible spectrophotometric determination of Fe2+.[8][9] 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.[12][13] 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]
Related complexes
editReferences
edit- ↑ PubChem. "1,10-Phenanthroline ferrous sulfate". pubchem.ncbi.nlm.nih.gov. Retrieved 2026-05-31.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ Harris, D. C. (1995). Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 978-0-7167-2508-4.
- 1 2 3 Handbook on the Physics and Chemistry of Rare Earths. Elsevier. 2006. pp. 289–. ISBN 978-0-08-046672-9.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ "Ferrozine". Sigma-Aldrich. Retrieved 2025-03-10.
- 1 2 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.