Question

Distinguish between the following metal salt solutions.

Iron and copper salts

Answer 1

Add ammonium hydroxide solution dropwise and then in excess to both the salt solutions.

Iron salt:

If it is a Ferrous (Fe²⁺) salt, it forms a dirty green precipitate which remains insoluble in excess.

If it is a Ferric (Fe³⁺) salt, it forms a reddish-brown precipitate which remains insoluble in excess.

\[\ce{FeSO4 + 2NH4OH -> (NH4)2SO4 + \underset{(Dirty green ppt.)}{Fe(OH)2↓}}\]

Copper salt:

Forms a pale blue precipitate which dissolves in excess to form a deep inky-blue solution.

\[\ce{CuSO4 + 2NH4OH -> (NH4)2SO4 + \underset{(Pale blue ppt.)}{Cu(OH)2↓}}\]

\[\ce{Cu(OH)2 + (NH4)2SO4 + 2NH4OH -> \underset{(Deep inky-blue solution)}{[Cu(NH3)4]SO4} + 4H2O}\]