Ferrate(VI)
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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.
 | It has been suggested that this article be merged into Potassium ferrate. (Discuss) Proposed since April 2025. |
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 Solutions of ferrate (left) and permanganate (right) | |
| Names | |
|---|---|
| IUPAC name
Ferrate(VI) | |
| Systematic IUPAC name
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| Other names
[FeO4]2- | |
| Identifiers | |
3D model (JSmol) |
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| ChEBI | |
| ChemSpider | |
| 2055 | |
PubChem CID |
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| Properties | |
| [FeO4]2- | |
| Molar mass | 119.843 g mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Nomenclature
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 Na2[Fe(CO)4], 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.[1]
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.[2]
For example, ferrates are produced by heating iron(III) hydroxide with sodium hypochlorite in alkaline solution:[3]
The anion is typically precipitated as the barium(II) salt, forming barium ferrate.[3]
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.
- [FeO
4]2−
+ 8 H+ + 3 e− ⇌ Fe3+
+ 4 H2O; E0 = +2.20 V (acidic medium) - [FeO
4]2−
+ 4 H2O + 3 e− ⇌ Fe(OH)
3 + 5 OH−
; E0 = +0.72 V (basic medium)
Because of this, the ferrate(VI) anion is unstable at neutral[2] or acidic pH values, decomposing to iron(III):[3] The reduction goes through intermediate species in which iron has oxidation states +5 and +4.[4] These anions are even more reactive than ferrate(VI).[5] In alkaline conditions ferrates are more stable, lasting for about 8 to 9 hours at pH 8 or 9.[5]
Aqueous solutions of ferrates are pink when dilute, and deep red or purple at higher concentrations.[4][6] The ferrate ion is a stronger oxidizing agent than permanganate,[7] and oxidizes ammonia to molecular nitrogen.[8]
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.[4]
Applications
Ferrates are excellent disinfectants, and are capable of removing and destroying viruses.[9] 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).[10]
Until now, it was impossible to produce the chemical in a sustainable and stable form, making it impossible to produce and apply it for large scale as a water treatment agent. Ferr-Tech, as a Dutch company, was the first to succeed in this and now produces FerSol® (Ferrate(VI) in liquid form) on an industrial scale.
Sodium ferrate (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]
See also
References
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