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 D3symmetry group. The Fe-N distances are 197.3 pm.[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:
This complex is used as an indicator in analytical chemistry.[5] 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 1M 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.[6]
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.[6]
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.[6]
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-visiblespectrophotometric determination of Fe2+.[7][8] The maximum absorbance of the Fe(II) o-phenanthroline complex is at 511nm.[9] However, another related N-ligand called ferrozin (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate)[10] is also used and must not be confused with ferroin. Ferrozin was specifically synthesised in the 1970s to obtain a less expensive reagent for automated chemical analysis.[11] Ferrozine reacts with Fe2+ to form a relatively stable magenta-colored complex with a maximum absorbance at 562nm.[11][12] The ferrozin method allows the determination of Fe(II)/Fe(III) speciation in natural fresh or marine waters at the submicromolar level.[13]
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.[14]
↑ 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.
↑ 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. ISSN0096-4484.
↑ 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. ISSN0096-4484.
↑ 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.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.