Iron, Fe3+

Iron, Fe³⁺

Most common oxidation states: +2, +3

M.P. 1535^o

B.P. 2750^o

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:

Aqueous ammonia reacts with Fe(II) ions to produce white gelatinous Fe(OH)₂, which oxidizes to form red-brown Fe(OH)₃:

Fe²⁺(aq) + 2NH₃(aq) + 3H₂O(l) <==> Fe(OH)₂(s) + 2NH₄⁺(aq)

Aqueous ammonia reacts with Fe(III) ions to produce red-brown Fe(OH)₃:

Fe³⁺(aq) + 3NH₃(aq) + 3H₂O(l) <==> Fe(OH)₃(s) + 3NH₄⁺(aq)

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:

Sodium hydroxide also produces Fe(OH)₂ and Fe(OH)₃ from the corresponding oxidation states of iron in aqueous solution.

Fe²⁺(aq) + 2OH^-(aq) <==> Fe(OH)₂(s)

Fe³⁺(aq) + 3OH^-(aq) <==> Fe(OH)₃(s)

Neither hydroxide precipitate dissolves in excess sodium hydroxide.

Potassium Ferrocyanide:

Potassium ferrocyanide will react with Fe³⁺ solution to produce a dark blue precipitate called Prussian blue:

K⁺(aq) + Fe³⁺(aq) + [Fe(CN)₆]^4-(aq) <==> KFe[Fe(CN)₆](s)

With Fe²⁺ solution, a white precipitate will be formed that will be converted to blue due to the oxidation by oxygen in air:

2Fe²⁺(aq) + [Fe(CN)₆]^4-(aq) <==> Fe₂[Fe(CN)₆](s) 

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:

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.

K⁺(aq) + Fe²⁺(aq) + [Fe(CN)₆]^3-(aq) <==> KFe[Fe(CN)₆](s)

 Potassium Thiocyanate:

KSCN will give a deep red coloration to solutions containing Fe³⁺:

Fe³⁺(aq) + NCS^-(aq) <==> [FeNCS]²⁺(aq)

No Reaction:

Cl^-, SO₄^2-