Ferrate(VI)

Ferrate(VI) is the inorganic anion with the chemical formula eO₄sup>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.

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 eO₄sup>2−. These include the highly reduced species disodium tetracarbonylferrate , and salts of the iron(III) complex tetrachloroferrate eCl₄sup>− in 1-Butyl-3-methylimidazolium tetrachloroferrate. Although rarely studied, ferrate(V) eO₄sup>3− and ferrate(IV) eO₄sup>4− oxyanions of iron also exist. These too are called ferrates.

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.

For example, ferrates are produced by heating iron(III) hydroxide with sodium hypochlorite in alkaline solution:

:2 + 3 + 4 → 2 + 5 + 3

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.

: + 8 + 3 e⁻ + 4 ; ''E''₀ = +2.20 V (acidic medium)
: + 4 + 3 e⁻ + 5 ; ''E''₀ = +0.72 V (basic medium)

Because of this, the ferrate(VI) anion is unstable at neutral or acidic pH values, decomposing to iron(III): 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.

Aqueous solutions of ferrates are pink when dilute, and deep red or purple at higher concentrations. The ferrate ion is a stronger oxidizing agent than permanganate, and oxidizes ammonia to molecular nitrogen.

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. 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).

Sodium ferrate () 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.

Until now it was impossible to produce the chemical in a sustainable and stable way, which makes it impossible to produce it on a large scale and apply it as a water treatment method. 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.  

See also

* High-valent iron
* Potassium ferrate
* Barium ferrate

References

{{Iron compounds

Oxidizing agents
Transition metal oxyanions
Oxometallates