Schikorr reaction

The Schikorr reaction formally describes the conversion of the iron(II) hydroxide (Fe(OH)2) into iron(II,III) oxide (Fe3O4). This transformation reaction was first studied by Gerhard Schikorr. The global reaction follows:

3

Fe

(

OH

)

2

ferrous

hydroxide

⟶

Fe

3

O

4

magnetite

+

H

2

hydrogen

+

2

H

2

O

water

{\displaystyle {\ce {{\underset {ferrous\ hydroxide}{3Fe(OH)2}}->{\underset {magnetite}{Fe3O4}}+{\underset {hydrogen}{H2}}+{\underset {water}{2H2O}}}}}

It is of special interest in the context of the serpentinization, the formation of hydrogen by the action of water on a common mineral.

Reaction mechanism

The Schikorr reaction can be viewed as two distinct processes:

the anaerobic oxidation of two Fe(II) (Fe2+) into Fe(III) (Fe3+) by the protons of water. The reduction of two water protons is accompanied by the production of molecular hydrogen (H2), and;

the loss of two water molecules from the iron(II) and iron(III) hydroxides giving rise to its dehydration and to the formation of a thermodynamically more stable phase iron(II,III) oxide.

The global reaction can thus be decomposed in half redox reactions as follows:

2 (Fe2+ → Fe3+ + e−) (oxidation of 2 iron(II) ions)

2 (H2O + e− → ½ H2 + OH−) (reduction of 2 water protons)

to give:

2 Fe2+ + 2 H2O → 2 Fe3+ + H2 + 2 OH−

Adding to this reaction one intact iron(II) ion for each two oxidized iron(II) ions leads to:

3 Fe2+ + 2 H2O → Fe2+ + 2 Fe3+ + H2 + 2 OH−

Electroneutrality requires the iron cations on both sides of the equation to be counterbalanced by 6 hydroxyl anions (OH−):

3 Fe2+ + 6 OH− + 2 H2O → Fe2+ + 2 Fe3+ + H2 + 8 OH−

3 Fe(OH)2 + 2 H2O → Fe(OH)2 + 2 Fe(OH)3 + H2

For completing the main reaction, two companion reactions have still to be taken into account:

The autoprotolysis of the hydroxyl anions; a proton exchange between two OH−, like in a classical acid–base reaction:

OH− + OH− → O2− + H2O

acid 1 + base 2 → base 1 + acid 2, or also,

2 OH− → O2− + H2O

it is then possible to reorganize the global reaction as:

3 Fe(OH)2 + 2 H2O → (FeO + H2O) + (Fe2O3 + 3 H2O) + H2

3 Fe(OH)2 + 2 H2O → FeO + Fe2O3 + 4 H2O + H2

3 Fe(OH)2 → FeO + Fe2O3 + 2 H2O + H2

Considering then the formation reaction of iron(II,III) oxide:

Fe

II

O

+

Fe

2

III

O

3

⟶

Fe

3

O

4

{\displaystyle {\ce {Fe^{II}O + Fe^{III}2O3 -> Fe3O4}}}

it is possible to write the balanced global reaction:

3 Fe(OH)2 → (FeO·Fe2O3) + 2 H2O + H2

in its final form, known as the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + 2 H2O + H2

Occurrences

The Schikorr reaction can occur in the process of anaerobic corrosion of iron and carbon steel in various conditions.

Anaerobic corrosion of metallic iron to give iron(II) hydroxide and hydrogen:

3 (Fe + 2 H2O → Fe(OH)2 + H2)

followed by the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + 2 H2O + H2

give the following global reaction:

3 Fe + 6 H2O → Fe3O4 + 2 H2O + 4 H2

3 Fe + 4 H2O → Fe3O4 + 4 H2

At low temperature, the anaerobic corrosion of iron can give rise to the formation of "green rust" (fougerite) an unstable layered double hydroxide (LDH). In function of the geochemical conditions prevailing in the environment of the corroding steel, iron(II) hydroxide and green rust can progressively transform in iron(II,III) oxide, or if bicarbonate ions are present in solution, they can also evolve towards more stable carbonate phases such as iron carbonate (FeCO3), or iron(II) hydroxycarbonate (Fe2(OH)2(CO3), chukanovite) isomorphic to copper(II) hydroxycarbonate (Cu2(OH)2(CO3), malachite) in the copper system.

Application fields

Anaerobic oxidation of iron and steel commonly finds place in oxygen-depleted environments, such as in permanently water-saturated soils, peat bogs or wetlands in which archaeological iron artefacts are often found.

Anaerobic oxidation of carbon steel of canisters and overpacks is also expected to occur in deep geological formations in which high-level radioactive waste and spent fuels should be ultimately disposed.