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Ferroin
Complex of Fe2+ by ortho-phenanthroline From Wikipedia, the free encyclopedia
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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.[1] Ferroin is one of many transition metal complexes of 1,10-phenanthroline.
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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.[2]
Preparation and reactions
Ferroin sulfate can be prepared by combining phenanthroline to ferrous sulfate dissolved in water:[3]
- 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.[4]
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Redox indicator
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| o-Phenanthroline Fe(II) (Redox indicator) | ||
| E0= +1.06 V | ||
| Reduced. | ↔ | Oxidized |
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 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.[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
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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+.[7][8] The maximum absorbance of the Fe(II) o-phenanthroline complex is at 511 nm.[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 562 nm.[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]
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Related complexes
References
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