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Iron(II), Fe2+

Some pale green crystals of iron(II) sulphate-7-water. If you look carefully you can see some traces of oxidation (brown).

Iron(II) is pale green in aqueous solution due to the hexaquo iron(II) ion, [Fe(H2O)6]2+. Hydrated, solid iron(II) compounds are also pale green, but may have yellow or brown discolourations due to atmospheric oxidation to iron(III).

Addition of sodium hydroxide solution causes deprotonation and gives a gelatinous, green precipitate of iron(II) hydroxide. This precipitate is not soluble in excess sodium hydroxide.

Gelatinous green precipitate of iron(II) hydroxide

Fe2+(aq) + 2OH-(aq) ⇒ Fe(OH)2(s)

or [Fe(H2O)6]2+(aq) + 2OH-(aq) ⇒ Fe(H2O)4(OH)2(s)

Video - iron(II) and sodium hydroxide solution

Green iron(II) hydroxide is oxidized to brown iron(III) hydroxide in contact with the air

The appearance of a brown solid, particularly on the upper glass surfaces of the test tube, is caused by the atmospheric oxidation of the iron(II) hydroxide to iron(III) hydroxide. This oxidation process can be accelerated by the addition of an oxidizing agent like hydrogen peroxide.

Addition of ammonia solution gives the same green precipitate of iron(II) hydroxide (ammonia solution is alkaline as it produces hydroxide ions in aqueous solution). No ammino complex of iron(II) is formed, so the precipitate is not soluble in excess ammonia. Atmospheric oxidation will again produce brown iron(III) hydroxide.

Iron(II) salts are oxidized by a number of oxidizing agents to iron(III). For example:

Video - oxidation of iron(II) by acidified potassium manganate(VII) solution