View source for Ferroin

{{Short description|Complex of Fe<sup>2+</sup> by ortho-phenanthroline}}
{{chembox
| verifiedrevid = 446896598
| ImageFile = Ferroin-cation-3D-balls.png
| ImageSize = 240px
| ImageName = The structure of the [Fe(''o''-phen)<sub>3</sub>]<sup>2+</sup> complex cation in ferroin
|Section1={{Chembox Identifiers
| 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
| InChIKey = CIWXFRVOSDNDJZ-NUQVWONBAU
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 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
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = CIWXFRVOSDNDJZ-UHFFFAOYSA-L
| CASNo = 14634-91-4
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = YP88WTW2E4
| PubChem = 84567
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 76289
| SMILES = [Fe+2].[O-]S([O-])(=O)=O.n3c2c1ncccc1ccc2ccc3.n3c2c1ncccc1ccc2ccc3.n1c3c(ccc1)ccc2cccnc23
}}
|Section2={{Chembox Properties
| Formula = C<sub>36</sub>H<sub>24</sub>FeN<sub>6</sub><sup>2+</sup>
| MolarMass = 596.27 g/mol
}}
}}
'''Ferroin''' is the [[chemical compound]] with the formula [Fe(''o''-phen)<sub>3</sub>]SO<sub>4</sub>, 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.<ref>{{cite journal|doi=10.1021/ed100797s|title=A Unified Kinetics and Equilibrium Experiment: Rate Law, Activation Energy, and Equilibrium Constant for the Dissociation of Ferroin|journal=Journal of Chemical Education|volume=88|issue=4|pages=457–460|year=2011|last1=Sattar|first1=Simeen|bibcode=2011JChEd..88..457S}}</ref>   Ferroin is one of many [[transition metal complexes of 1,10-phenanthroline]].
==Structure==
Many salts of  [Fe(''o''-phen)<sub>3</sub>]2+ have been characterized by [[X-ray crystallography]]. The structures of  [Fe(''o''-phen)<sub>3</sub>]<sup>2+</sup> and [Fe(''o''-phen)<sub>3</sub>]<sup>3+</sup> are almost identical, consistent with both being low-spin. These cations are octahedral with D<sub>3</sub> [[symmetry group]].   The Fe-N distances are 197.3 [[picometer|pm]].<ref>{{cite journal |doi=10.1039/DT9750000530 |title=Crystal Structure, Electron Spin Resonance, and Magnetism of Tris(o-Phenanthroline)Iron(III) Perchlorate Hydrate |date=1975 |last1=Baker |first1=Joe |last2=Engelhardt |first2=Lutz M. |last3=Figgis |first3=Brian N. |last4=White |first4=Allan H. |journal=Journal of the Chemical Society, Dalton Transactions |issue=6 |page=530 }}</ref>
==Preparation and reactions==
Ferroin sulfate may be prepared by combining [[phenanthroline]] to ferrous sulfate in water.
:3&nbsp;phen + Fe<sup>2+</sup> → [Fe(phen)<sub>3</sub>]<sup>2+</sup>
The main reaction is 1-electron oxidation.
[Fe(phen)<sub>3</sub>]<sup>2+</sup>  →  [Fe(phen)<sub>3</sub>]<sup>3+</sup>  +  1 e<sup>−</sup>
Addition of [[sulfuric acid]] to an aqueous solution of [Fe(phen)<sub>3</sub>]<sup>2+</sup> causes hydrolysis:
:[Fe(phen)<sub>3</sub>]<sup>2+</sup> + 3 H<sub>2</sub>SO<sub>4</sub>  +  6 H<sub>2</sub>O → [Fe(OH<sub>2</sub>)<sub>6</sub>]<sup>2+</sup>  +  3 [phenH]HSO<sub>4</sub><sup>−</sup>
==Redox indicator==
{{redox_indicator_template|indicator_name=Phenanthroline Fe(II) | Eh=1.06&nbsp;V|low_E_color=red|high_E_color=blue}}
This [[coordination compound|complex]] is used as an indicator in [[analytical chemistry]].<ref>{{cite book|first=D. C.|last=Harris|title=Quantitative Chemical Analysis|edition=4th|publisher=W. H. Freeman|location=New York, NY|isbn=978-0-7167-2508-4|year=1995|url-access=registration|url=https://archive.org/details/quantitativechem00harr_0}}</ref>  The active ingredient is the [Fe(''o''-phen)<sub>3</sub>]<sup>2+</sup> ion, which is a [[chromophore]] that can be oxidized to the ferric derivative [Fe(''o''-phen)<sub>3</sub>]<sup>3+</sup>. The potential for this redox change is +1.06 volts in 1&nbsp;M H<sub>2</sub>SO<sub>4</sub>. It is a popular redox indicator for visualizing oscillatory [[Belousov–Zhabotinsky reaction]]s.
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&nbsp;°C. It is the main indicator used in [[cerimetry]].<ref name="physchemrare">{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths|url=https://books.google.com/books?id=_Ro3Fqtz4xgC&pg=PA289|date= 2006|publisher=Elsevier|isbn=978-0-08-046672-9|pages=289–}}</ref>
'''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 Ce<sup>4+</sup> 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.<ref name="physchemrare"/>
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.<ref name="physchemrare"/>
==Related complexes==
*[[Tris(bipyridine)iron(II) chloride|[Tris(bipyridine)iron(II)]]
*[[Tris(bipyridine)ruthenium(II) chloride|[Tris(bipyridine)ruthenium(II)]]
==References==
<references/>
[[Category:Iron complexes]]
[[Category:Phenanthrolines]]