Ferroin

Ferroin

Identifiers
CAS Number	14634-91-4
3D model (Jmol)	Interactive image
ChemSpider	76289
ECHA InfoCard	100.035.145
PubChem	84567
InChI 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 Key: CIWXFRVOSDNDJZ-UHFFFAOYSA-L 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
SMILES [Fe+2].[O-]S([O-])(=O)=O.n3c2c1ncccc1ccc2ccc3.n3c2c1ncccc1ccc2ccc3.n1c3c(ccc1)ccc2cccnc23
Properties
Chemical formula	C36H24FeN6^2+
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.

Redox indicator

Phenanthroline Fe(II) (Redox indicator)
E^0= 1.06 V
Reduced	↔	Oxidized

This coordination compound is used as an indicator in analytical chemistry.^[1] 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 making oscillatory Belousov–Zhabotinsky reactions visible.

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.^[2]

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.^[2]

The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by position and number of methyl groups on the phenanthroline core.^[2]

Preparation

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

3 phen + Fe²⁺ → [Fe(phen)₃]²⁺

The iron is low spin and octahedral with D₃ symmetry. The intense color of this ferrous complex arises from a metal-to-ligand charge-transfer transition.

References

1. Harris, D. C. Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 0-7167-2508-8.
2. https://books.google.cz/books?id=_Ro3Fqtz4xgC&pg=PA289&lpg=PA289&dq=alternative+to+ferroin&source=bl&ots=kXHliTI_a_&sig=JeKKZEvsW23OLYTr_qIipxI1rcM&hl=en&sa=X&redir_esc=y#v=onepage&q=alternative%20to%20ferroin&f=false