Iron(II) hydroxide or ferrous hydroxide is a compound produced when iron(II) ions, from a compound such as iron (II) sulfate react with hydroxide ions. Iron(II) hydroxide itself is practically white, but even traces of oxygen impart it with a greenish tinge. If the solution was not deoxygenated and the iron reduced, the precipitate can vary in color starting from green to reddish brown depending on the iron(III) content. This precipitate is also known as "green rust" in the crystal lattice of which Fe²⁺ ions are easily substituted by Fe³⁺ ions produced by its progressive oxidation. In the presence of oxygen the color changes quickly. Green rust is a powerful reducing agent and also a layer double hydroxides (LDH) capable to sorb anions because of the presence of positive electrical charges borne on its surface. The mineralogical form of green rust is a recently discovered fougerite. All forms of green rust (including fougerite) does not correspond the ideal Fe(OH)₂ compound, as their structure is more complex and variable. The natural analogue of Fe(OH)₂ compound is a very rare mineral amakinite, (Fe,Mg)(OH)₂.

Iron(II) hydroxide is poorly soluble (1.43 × 10⁻³ g/L). It precipitates from from the reaction of iron(II) sulfate and hydroxide ions (from a soluble compound containing hydroxide ion):

FeSO₄ + 2OH⁻ → Fe(OH)₂ + SO₄²⁻

It is also easily formed as an undesirable by-product of other reactions, a.o., in the synthesis of siderite, an iron carbonate (FeCO₃), if the crystal growth conditions are poorly controlled (reagent concentrations, addition rate, addition order, pH, pCO₂, T, ageing time, ...).

Uses

Anions such as selenite and selenate can be easily adsorbed on the positively charged surface of green rust where they are subsequently reduced by Fe²⁺. The resulting products being poorly soluble (Se⁰, FeSe, or FeSe₂).

Ferrous hydroxide has also been investigated as an agent for the removal of toxic selenate and selenite ions from water systems such as wetlands. The ferrous hydroxide reduces these ions to elemental selenium, which is insoluble in water and precipitates out.

Note: pK_sp = 15.097 where p is the -log and K_sp is the Solubility Product Constant. This means it has a low tendency to dissolve, but is not entirely insoluble. An acidic solution would allow this to disassociate more because the H⁺ would react with the OH^- in the compound.

In a basic solution (potassium hydroxide), ferrous hydroxide is the electrochemically active material of the negative electrode of the Nickel-iron battery.

See also

• Rust
• Green rust
• Layered double hydroxides
• Periodic table

References