Iron, Fe3+

Most common oxidation states: +2, +3

M.P. 1535o

B.P. 2750o

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-, SO42-

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