Most common oxidation states: +2, +3
Density 7.87 g/cm3
Characteristics: Gray, moderately active metal.
Characteristic reactions of Fe2+ and Fe3+:
The [Fe(H2O)6]3+ ion is colorless (or pale pink), but many solutions containing this ion are yellow or amber-colored because of hydrolysis. Iron in both oxidation states forms many complex ions.
Aqueous ammonia reacts with Fe(II) ions to produce white gelatinous Fe(OH)2, which oxidizes to form red-brown Fe(OH)3:
Aqueous ammonia reacts with Fe(III) ions to produce red-brown Fe(OH)3:
Both precipitates are insoluble in excess aqueous ammonia. Iron(II) hydroxide quickly oxidizes to Fe(OH)3 in the presence of air or other oxidizing agents.
Sodium hydroxide also produces Fe(OH)2 and Fe(OH)3 from the corresponding oxidation states of iron in aqueous solution.
Neither hydroxide precipitate dissolves in excess sodium hydroxide.
Potassium ferrocyanide will react with Fe3+ solution to produce a dark blue precipitate called Prussian blue:
With Fe2+ solution, a white precipitate will be formed that will be converted to blue due to the oxidation by oxygen in air:
Note: Many metal ions form ferrocyanide precipitates, so potassium ferrocyanide is not a good reagent for separating metal ions. It is used more commonly as a confirmatory test.
Potassium ferricyanide will give a brown coloration but no precipitate with Fe3+. With Fe2+, a dark blue precipitate is formed. Although this precipitate is known as Turnbull's blue, it is identical with Prussian blue.
KSCN will give a deep red coloration to solutions containing Fe3+:
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