ferrate(VI)
{{merge to|Potassium ferrate|discuss=Talk:Potassium ferrate#Merge proposal|date=April 2025}}
{{Short description|Ion}}
{{Chembox
| verifiedrevid = 443819490
| ImageFile = Ferrate ion.svg
| ImageFile_Ref = {{chemboximage|correct|??}}
| ImageSize = 121
| ImageName = Aromatic skeletal formula of ferrate
| ImageFile1 = Ferrate and permanganate solution.jpg
| ImageCaption1 = Solutions of ferrate (left)
and permanganate (right)
| IUPACName = Ferrate(VI)
| SystematicName = Tetraoxoironbis(olate){{citation needed|date = March 2011}}
| OtherNames = [FeO4]2-
|Section1={{Chembox Identifiers
| PubChem = 25000034
| ChemSpiderID = 21865127
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 30992
| Gmelin = 2055
| SMILES = [O-][Fe]([O-])(=O)=O
| SMILES1 = [O-][Fe](=O)(=O)[O-]
| StdInChI = 1S/Fe.4O/q;;;2*-1
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| InChI = 1/Fe.4O/q;;;2*-1/rFeO4/c2-1(3,4)5/q-2
| StdInChIKey = XGBDPAYTQGQHEW-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| InChIKey = XGBDPAYTQGQHEW-WTZHFVRHAI
}}
|Section2={{Chembox Properties
| Formula = [FeO4]2-
| MolarMass = 119.843 g mol−1
}}
}}
Ferrate(VI) is the inorganic anion with the chemical formula [FeO4]2−. It is photosensitive, contributes a pale violet colour to compounds and solutions containing it and is one of the strongest water-stable oxidizing species known. Although it is classified as a weak base, concentrated solutions containing ferrate(VI) are corrosive and attack the skin and are only stable at high pH. It is similar to the somewhat more stable permanganate.
Nomenclature
{{See also|Ferrate}}
The term ferrate is normally used to mean ferrate(VI), although it can refer to other iron-containing anions, many of which are more commonly encountered than salts of [FeO4]2−. These include the highly reduced species disodium tetracarbonylferrate {{chem2|Na2[Fe(CO)4]}}, {{chem2|K2[Fe(CO)4]}} and salts of the iron(III) complex tetrachloroferrate [FeCl4]− in 1-Butyl-3-methylimidazolium tetrachloroferrate. Although rarely studied, ferrate(V) [FeO4]3− and ferrate(IV) [FeO4]4− oxyanions of iron also exist. These too are called ferrates.{{cite book
| title = Chemistry in context
| author1 = Graham Hill
| author2 = John Holman
| edition = 5th
| publisher = Nelson Thornes
| year = 2000
| isbn = 0-17-448276-0
| page = 202
}}
Synthesis
Ferrate(VI) salts are formed by oxidizing iron in an aqueous medium with strong oxidizing agents under alkaline conditions, or in the solid state by heating a mixture of iron filings and powdered potassium nitrate.{{cite book
| title = Text Book Of Coordination Chemistry
| author = R. K. Sharma
| publisher = Discovery Publishing House
| year = 2007
| isbn = 978-81-8356-223-2
| pages = 124–125
}}
For example, ferrates are produced by heating iron(III) hydroxide with sodium hypochlorite in alkaline solution:
:2 {{chem|Fe(OH)|3}} + 3 {{chem|OCl|-}} + 4 {{OH-}} → 2 {{chem|[FeO|4|]|2-}} + 5 {{H2O}} + 3 {{chem|Cl|-}}
The anion is typically precipitated as the barium(II) salt, forming barium ferrate.
Properties
Fe(VI) is a strong oxidizing agent over the entire pH range, with a reduction potential (Fe(VI)/Fe(III) couple) varying from +2.2 V to +0.7 V versus SHE in acidic and basic media respectively.
:{{chem|[FeO|4|]|2-}} + 8 {{H+}} + 3 e− {{eqm}} {{chem|Fe|3+}} + 4 {{H2O|nolink=y}}; E0 = +2.20 V (acidic medium)
:{{chem|[FeO|4|]|2-}} + 4 {{H2O|nolink=y}} + 3 e− {{eqm}} {{chem|Fe|(OH)|3}} + 5 {{Chem|OH|-}}; E0 = +0.72 V (basic medium)
Because of this, the ferrate(VI) anion is unstable at neutral or acidic pH values, decomposing to iron(III):{{cite book
| title = Principles of descriptive inorganic chemistry
| author = Gary Wulfsberg
| publisher = University Science Books
| year = 1991
| isbn = 0-935702-66-0
| pages = 142–143
}} The reduction goes through intermediate species in which iron has oxidation states +5 and +4. These anions are even more reactive than ferrate(VI). In alkaline conditions ferrates are more stable, lasting for about 8 to 9 hours at pH 8 or 9.{{cite book
| title = The Development of Iron Chelators for Clinical Use
| editor = Raymond J. Bergeron
| author = Gary M. Brittenham
| publisher = CRC Press
| year = 1994
| isbn = 0-8493-8679-9
| pages = 37–38
}}
Aqueous solutions of ferrates are pink when dilute, and deep red or purple at higher concentrations.{{cite book
| title = Inorganic chemistry
| author1 = Egon Wiberg
| author2 = Nils Wiberg
| author3 = Arnold Frederick Holleman
| publisher = Academic Press
| year = 2001
| isbn = 0-12-352651-5
| pages = 1457–1458
| title = Oxford dictionary of chemistry
| editor = John Daintith
| edition = 5th
| publisher = Oxford University Press
| year = 2004
| isbn = 0-19-860918-3
| page = 235
}} The ferrate ion is a stronger oxidizing agent than permanganate,{{cite book
| title = Introduction to modern inorganic chemistry
| author1 = Kenneth Malcolm Mackay
| author2 = Rosemary Ann Mackay
| author3 = W. Henderson
| edition = 6th
| publisher = CRC Press
| year = 2002
| isbn = 0-7487-6420-8
| pages = 334–335
}} and oxidizes ammonia to molecular nitrogen.{{cite web | url = https://kb.osu.edu/dspace/bitstream/1811/36335/1/OH_WRC_444.pdf | title = Oxidation of Ammonia in Water by Ferrates(VI) and (IV) | author = Karlis Svanks | date = June 1976 | page = 3 | publisher = Water Resources Center, Ohio State University | access-date = 2010-05-04 }}
The ferrate(VI) ion has two unpaired electrons and is thus paramagnetic. It has a tetrahedral molecular geometry, isostructural with the chromate and permanganate ions.
Applications
Ferrates are excellent disinfectants, and are capable of removing and destroying viruses.{{cite book
| title = Environmental chemistry
| author = Stanley E. Manahan
| edition = 8th
| publisher = CRC Press
| year = 2005
| isbn = 1-56670-633-5
| page = 234
}} They are also of interest as potential as an environmentally friendly water treatment chemical, as the byproduct of ferrate oxidation is the relatively benign iron(III).{{Cite journal|last1=Sharma|first1=Virender K.|last2=Zboril|first2=Radek|last3=Varma|first3=Rajender S.|date=2015|title=Ferrates: Greener Oxidants with Multimodal Action in Water Treatment Technologies|url=https://pubs.acs.org/doi/10.1021/ar5004219|journal=Accounts of Chemical Research|language=en|volume=48|issue=2|pages=182–191|doi=10.1021/ar5004219|pmid=25668700|issn=0001-4842}}
Sodium ferrate ({{chem2|Na2FeO4}}) is a useful reagent with good selectivity and is stable in aqueous solution of high pH, remaining soluble in an aqueous solution saturated with sodium hydroxide.{{Citation needed|date=October 2020}}