Ferroin
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![The structure of the [Fe(o-phen)3]2+ complex cation in ferroin](#/media/File:Ferroin-cation-3D-balls.png) | |
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3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.035.145 |
PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| C36H24FeN62+ | |
| Molar mass | 596.27 g/mol |
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.[1] Ferroin is one of many transition metal complexes of 1,10-phenanthroline.
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]
Redox indicator
| 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]