Characteristic Reactions of Iron (Fe³⁺)
- Page ID
- 97270
- Most common oxidation states: +2, +3
- M.P. 1535º
- B.P. 2750º
- Density 7.87 g/cm3
- Characteristics: Iron is a gray, moderately active metal.
Characteristic reactions of Fe²⁺ and Fe³⁺
The \(\ce{[Fe(H2O)6]^{3+}}\) 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 \(\ce{Fe(OH)2}\), which oxidizes to form red-brown \(\ce{Fe(OH)3}\):
\[\ce{Fe^{2+}(aq) + 2NH3(aq) + 3H2O(l) <=> Fe(OH)2(s) + 2NH4^{+}(aq)} \nonumber \]
Aqueous ammonia reacts with \(\ce{Fe(III)}\) ions to produce red-brown \(\ce{Fe(OH)3}\):
\[\ce{Fe^{3+}(aq) + 3NH3(aq) + 3H2O(l) <=> Fe(OH)3(s) + 3NH4^{+}(aq)} \nonumber \]
Both precipitates are insoluble in excess aqueous ammonia. Iron(II) hydroxide quickly oxidizes to \(\ce{Fe(OH)3}\) in the presence of air or other oxidizing agents.
Sodium Hydroxide
Sodium hydroxide also produces \(\ce{Fe(OH)2}\) and \(\ce{Fe(OH)3}\) from the corresponding oxidation states of iron in aqueous solution.
\[\ce{Fe^{2+}(aq) + 2OH^{-}(aq) <=> Fe(OH)2(s)} \nonumber \]
\[\ce{Fe^{3+}(aq) + 3OH^{-}(aq) <=> Fe(OH)3(s)} \nonumber \]
Neither hydroxide precipitate dissolves in excess sodium hydroxide.
Potassium Ferrocyanide
Potassium ferrocyanide will react with \(\ce{Fe^{3+}}\) solution to produce a dark blue precipitate called Prussian blue:
\[\ce{K^{+}(aq) + Fe^{3+}(aq) + [Fe(CN)6]^{4-}(aq) <=> KFe[Fe(CN)6](s)} \label{Prussian} \]
With \(\ce{Fe^{2+}}\) solution, a white precipitate will be formed that will be converted to blue due to the oxidation by oxygen in air:
\[\ce{2Fe^{2+}(aq) + [Fe(CN)6]^{4-}(aq) <=> Fe2[Fe(CN)6](s) } \nonumber \]
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 \(\ce{Fe^{3+}}\). With \(\ce{Fe^{2+}}\), a dark blue precipitate is formed. Although this precipitate is known as Turnbull's blue, it is identical with Prussian blue (from Equation \ref{Prussian}).
\[\ce{K+(aq) + Fe2+(aq) + [Fe(CN)6]^{3-}(aq) <=> KFe[Fe(CN)6](s)} \nonumber \]
Potassium Thiocyanate
\(\ce{KSCN}\) will give a deep red coloration to solutions containing \(\ce{Fe^{3+}}\):
\[\ce{Fe3+(aq) + NCS^{-}(aq) <=> [FeNCS]2+(aq)} \nonumber \]
No Reaction
\(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\)