A ferrate is a salt of the extremely unstable ferric acid H2FeO4, a compound of hexavalent iron. The anion FeO42- is a very strong oxidizer that readily reacts with organic compounds, ammonium and cations of metals that aren't in their highest state of oxidation. It is colored purple-red.
The existence of ferrates
The Russian amateur chemist "Odin" experimented with ferrates a lot, and his research sheds much light on various salts of the ferrate ion. According to him, the following ferrates exist:
- Lithium ferrate Li2FeO4 (stable, soluble)
- Sodium ferrate Na2FeO4 (stable, soluble)
- Potassium ferrate K2FeO4 (stable, soluble)
- Rubidium ferrate Rb2FeO4 (stable, low solubility)
- Cesium ferrate Cs2FeO4 (stable, soluble)
- Magnesium ferrate MgFeO4 (unstable, low solubility)
- Calcium ferrate CaFeO4 (stable, low solubility)
- Barium ferrate BaFeO4 (stable, almost insoluble)
Odin also tried to synthesize ferrates of rare earth metals, but the results aren't unambiguous. His research suggests that rare earth ferrates likely exist, are insoluble and unstable.
The following ferrates do not exist:
- Ammonium ferrate (undergoes reduction of ferrate by ammonium upon formation);
- Ferrate salts of transition metal hydroxides (the ferrate ion oxidizes them to non-basic states of oxidation)
The possibility of organic esters of ferric acid is unknown but, most likely, only the most oxidation-resistant organic radicals such as perfluorocarbon radicals are capable of forming these esters.
Iron can form salt-like compounds with two more states of oxidation, +4 and +5. These compounds are usually called ferrates (IV) and ferrates (V), or hypoferrates. All hypoferrates are stable when anhydrous but disproportionate rapidly in water into iron +6 and iron +3.
The +4 hypoferrates can be prepared by calcining alkali metal oxides and iron (III) oxide together in an oxygen atmosphere. The +5 hypoferrates can be prepared by thermal decomposition of +6 ferrates (melting them together with alkali increases the yield).
There are a few reports on synthesizing ferrates (VII) and (VIII), sometimes called perferrates. However, these reports remain unconfirmed and unrepeated. At least one report claims that the perferrate (VIII) ion is colored green.