Ferroin
The structure of the [Fe(o-phen)3]2+ complex cation in ferroin
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
  • [Fe+2].[O-]S([O-])(=O)=O.n3c2c1ncccc1ccc2ccc3.n3c2c1ncccc1ccc2ccc3.n1c3c(ccc1)ccc2cccnc23
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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Infobox references

Ferroin is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.[1]

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 may be prepared by combining phenanthroline to ferrous sulfate in water.

3 phen + Fe2+ → [Fe(phen)3]2+

The main reaction is 1-electron oxidation. [Fe(phen)3]2+ → [Fe(phen)3]3+ + 1 e− Addition of sulfuric acid to an aqueous solution of [Fe(phen)3]2+ causes hydrolysis:

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

Redox indicator

Phenanthroline Fe(II) (Redox indicator)
E0= 1.06 V
Reduced. ↔ Oxidized

This complex is used as an indicator in analytical chemistry.[3] 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.[4]

Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has transition potential of +1.25 volts. It is more stable than ferroin, but in sulfuric acid with Ce4+ ion it requires significant excess of the titrant. It is however useful for titration in perchloric acid or nitric acid solution, where cerium redox potential is higher.[4]

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.[4]

References

  1. ↑ 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.
  2. ↑ 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.
  3. ↑ Harris, D. C. (1995). Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 978-0-7167-2508-4.
  4. 1 2 3 Handbook on the Physics and Chemistry of Rare Earths. Elsevier. 2006. pp. 289–. ISBN 978-0-08-046672-9.
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