Ferroin

{{Short description|Complex of Fe²⁺ by ortho-phenanthroline}} {{Chembox | verifiedrevid = 446896598 | ImageFile = Ferroin-cation-3D-balls.png{{!}}class=bg-transparent | ImageSize = 240px | ImageName = The structure of the [Fe(''o''-phen)₃]²⁺ 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 = c12c3[n+]4cccc3ccc2ccc[n+]1[Ru-4]456(c7cccc(c78)ccc(c78)ccc[n+]67)[n+]1cccc2ccc3ccc[n+1]5c3c12 }} |Section2={{Chembox Properties | Formula = C₃₆H₂₄FeN₆²⁺ | MolarMass = 596.27 g/mol }} | Section7 = {{Chembox Hazards | TLV = 1.0 mg/m³, as Fe | GHS_ref = <ref>{{Cite web |last=PubChem |title=1,10-Phenanthroline ferrous sulfate |url=https://pubchem.ncbi.nlm.nih.gov/compound/84567 |access-date=2026-05-31 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> | GHSPictograms = {{GHS06}}{{GHS07}}{{GHS09}} | GHSSignalWord = Danger | HPhrases = {{H-phrases|301|302|410|412}} | PPhrases = {{P-phrases|P264|P270|P273|P301+P316|P301+P317|P321|P330|P391|P405|P501}} }} }}

'''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.<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)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.<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 can be prepared by combining phenanthroline to ferrous sulfate dissolved in water:<ref>{{cite journal |doi=10.1002/zaac.201400137 |title=Boron Cluster Anions &#91;B''_n''H''_n''&#93;^2– ( ''n'' = 10, 12) in Reactions of Iron(II) and Iron(III) Complexation with 2,2′-Bipyridyl and 1,10-Phenanthroline |date=2014 |last1=Avdeeva |first1=Varvara V. |last2=Vologzhanina |first2=Anna V. |last3=Goeva |first3=Lyudmila V. |last4=Malinina |first4=Elena A. |last5=Kuznetsov |first5=Nikolay T. |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=640 |issue=11 |pages=2149–2160 }}</ref> : 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⁻ {{Space|6}} {{Nowrap|(E_h {{=}} +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 {{chem2|[Fe(phen)3]SO4}} precipitates {{chem2|Fe(phen)2(CN)2}}.<ref>{{cite book |doi=10.1002/9780470132432.ch43 |title=Inorganic Syntheses |year=1970 |last1=Schilt |first1=Alfred A.|chapter=Dicyanobis(1,10-phenanthroline)Iron(II) and Dicyanobis(2,2′-bipyridine)iron(II) |volume=12 |pages=247–251 |isbn=978-0-470-13171-8 }}</ref>

==Redox indicator== {{redox_indicator_template|indicator_name=''o''-Phenanthroline Fe(II) | Eh=+1.06&nbsp;V|low_E_color=red|high_E_color=blue}} This 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)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&nbsp;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&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 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.<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"/>

==Fe²⁺ direct UV-visible spectrophotometric determination== In analytical chemistry, the red color specific for the reduced form of ferroin was once used for the direct UV-visible spectrophotometric determination of {{Chem2|Fe(2+)}}.<ref name="Fortune1938">{{cite journal | last1=Fortune | first1=W. B. | last2=Mellon | first2=M. G. | title=Determination of iron with o-phenanthroline: A spectrophotometric study | journal=Industrial & Engineering Chemistry Analytical Edition | volume=10 | issue=2 | date=1938-02-01 | issn=0096-4484 | doi=10.1021/ac50118a004 | pages=60–64}}</ref><ref name="Bandemer1944">{{cite journal | last1=Bandemer | first1=Selma L. | last2=Schaible | first2=P J. | title=Determination of iron. A study of the o-phenanthroline method | journal=Industrial & Engineering Chemistry Analytical Edition | volume=16 | issue=5 | date=1944-05-19 | issn=0096-4484 | doi=10.1021/i560129a013 | pages=317–319}}</ref> The maximum absorbance of the Fe(II) ''o''-phenanthroline complex is at 511&nbsp;nm.<ref name="Tripathi2019">{{cite journal | last1=Tripathi | first1=Atri Deo | last2=Gupta | first2=K.A. | last3=Malik | first3=Shally | title=Iron determination by colorimetric method using o-phenanthroline | journal=Bulletin of Pure & Applied Sciences – Chemistry | volume=38c | issue=2 | date=2019 | issn=0970-4620 | doi=10.5958/2320-320X.2019.00018.9 | page=171| doi-access=free }}</ref> However, another related N-ligand called ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate)<ref name="SigmaAldrich">{{cite web | title=Ferrozine | website=Sigma-Aldrich | url=https://www.sigmaaldrich.com/US/en/search/ferrozine?focus=products&page=1&perpage=30&sort=relevance&term=ferrozine&type=product | access-date=2025-03-10}}</ref> is also used and must not be confused with ferroin. Ferrozine was specifically synthesised in the 1970s to obtain a less expensive reagent for automated chemical analysis.<ref name="Stookey1970">{{cite journal | last=Stookey | first=Lawrence L. | title=Ferrozine—a new spectrophotometric reagent for iron | journal=Analytical Chemistry | volume=42 | issue=7 | date=1970-06-01 | issn=0003-2700 | doi=10.1021/ac60289a016 | doi-access=free | pages=779–781 | url=https://epic.awi.de/id/eprint/32905/1/Stookey-1970.pdf | access-date=2025-03-10}}</ref> Ferrozine reacts with {{Chem2|Fe(2+)}} to form a relatively stable magenta-colored complex with a maximum absorbance at 562&nbsp;nm.<ref name="Stookey1970" /><ref name="Huang2017">{{cite journal | last1=Huang | first1=Wenjuan | last2=Hall | first2=Steven J. | title=Optimized high-throughput methods for quantifying iron biogeochemical dynamics in soil | journal=Geoderma | volume=306 | date=2017 | doi=10.1016/j.geoderma.2017.07.013 | doi-access=free | pages=67–72 | url=https://dr.lib.iastate.edu/bitstreams/f595e6f6-c567-447d-a9f5-875efaa2a05f/download | access-date=2025-03-12}}</ref> The ferrozine method allows the determination of Fe(II)/Fe(III) speciation in natural fresh or marine waters at the submicromolar level.<ref name="Viollier2000">{{cite journal | last1=Viollier | first1=E. | last2=Inglett | first2=P.W. | last3=Hunter | first3=K. | last4=Roychoudhury | first4=A.N. | last5=Van Cappellen | first5=P. | title=The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters | journal=Applied Geochemistry | volume=15 | issue=6 | date=2000 | doi=10.1016/S0883-2927(99)00097-9 | pages=785–790}}</ref>

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 {{Chem2|Fe(2+)}} binding by ''o''-phenanthroline are very fast, the kinetics of {{Chem2|Fe(2+)}} 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.<ref name="Smith2021">{{cite journal | last1=Smith | first1=Gideon L. | last2=Reutovich | first2=Aliaksandra A. | last3=Srivastava | first3=Ayush K. | last4=Reichard | first4=Ruth E. | last5=Welsh | first5=Cass H. | last6=Melman | first6=Artem | last7=Bou-Abdallah | first7=Fadi | title=Complexation of ferrous ions by ferrozine, 2,2′-bipyridine and 1,10-phenanthroline: Implication for the quantification of iron in biological systems | journal=Journal of Inorganic Biochemistry | volume=220 | date=2021 | doi=10.1016/j.jinorgbio.2021.111460 | article-number=111460}}</ref>

==Related complexes== * Tris(bipyridine)iron(II) * Tris(bipyridine)ruthenium(II)

==References== <references/>

Category:Iron complexes Category:Phenanthrolines