Ferroin

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.{{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 }}

Ferroin sulfate can be prepared by combining phenanthroline to ferrous sulfate dissolved in water:{{cite journal |doi=10.1002/zaac.201400137 |title=Boron Cluster Anions [B_nH_n]^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 }}

: 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}}.{{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 }}

{{redox_indicator_template|indicator_name=o-Phenanthroline Fe(II) | Eh=+1.06 V|low_E_color=red|high_E_color=blue}}

This complex is used as an indicator in analytical chemistry.{{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}} 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.{{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–}}

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.

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.

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+)}}.{{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}}{{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}} The maximum absorbance of the Fe(II) o-phenanthroline complex is at 511 nm.{{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}} However, another related N-ligand called ferrozin (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid monosodium salt hydrate){{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}} is also used and must not be confused with ferroin. Ferrozin was specifically synthesised in the 1970s to obtain a less expensive reagent for automated chemical analysis.{{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}} Ferrozine reacts with {{Chem2|Fe(2+)}} to form a relatively stable magenta-colored complex with a maximum absorbance at 562 nm.{{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}} The ferrozin method allows the determination of Fe(II)/Fe(III) speciation in natural fresh or marine waters at the submicromolar level.{{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}}

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.{{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 | page=111460}}

• [Tris(bipyridine)iron(II)
• [Tris(bipyridine)ruthenium(II)

Category:Iron complexes

Category:Phenanthrolines