Solutions of ferrate (left)
and permanganate (right)
|Systematic IUPAC name
|Jmol 3D model||Interactive image
|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).
|what is ?)(|
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 oxidising 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.
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] and salts of the iron(III) complex tetrachloroferrate [FeCl4]−. Although rarely studied, ferrate(V) [FeO4]3− and ferrate(IV) [FeO4]4− oxyanions of iron also exist. These too are called ferrates.
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 Fe(OH)
3 + 3 OCl−
+ 4 OH−
→ 2 [FeO
+ 5 H
2O + 3 Cl−
+ 3 e- + 8 H+
+ 4 H
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 will oxidize chromium(III) to dichromate, and ammonia to molecular nitrogen.
The ferrate(VI) ion has two unpaired electrons, and is thus paramagnetic. It has a tetrahedral molecular geometry. Fe (VI) was a dominant oxidizing agent during the entire pH range with a reduction potential (Fe(VI)/Fe(III) couple) varying from +2.2 V to +0.7 V versus NHE in acidic and basic medium respectively. The approach for a clean and selective oxidations was to make use of Fe(VI) as the oxidant For this cause the notice has been directed to sodium ferrate (Na2FeO4), which has performance different from those of other ferrates and remains soluble in an aqueous solution saturated in sodium hydroxide. Iron usually exists in the +2 and +3 oxidation states; however, in a strong oxidizing environment, higher oxidation states of iron such as +4, +5 and +6 can also be obtained. Ferrate was one of the most powerful, yet environmentally friendly, water treatment chemicals known. The byproduct of ferrate oxidation was safe Fe (III) or ferric oxide or rust. Fe (III) was found in the human body and plant and animal forms. Ferrate was a tetrahedral ion, is isostructural with K2CrO4 and KMnO4.39
Eo = +2.20 V + 8H++ 3e- Fe3+ + 4H2O (acidic medium) Eo = +0.72 V + 4H2O+ 3e- Fe(OH)3 + 5OH- (basic medium)
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