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Test for Transition Metal Ions

If sodium hydroxide or ammonia solution is added to complex ion with water ligands insoluble hydroxides are precipitated. The precipitates of Mn(OH)2 and Fe(OH)2 darken as they oxidise in air. The metal ions can be identified by the colour of their precipitates. The table shows the colours of the precipitates and the result of adding excess NaOH.
Ion    Few drops of NaOH    Excess NaOH
Cr3+    grey-green precipitate    dissolves to give a dark green solution
Mn2+    off-white precipitate, darkens to brown in air    no change
Fe2+    green precipitate, turns brown in air    no change
Fe3+    orange-brown precipitate    no change
Co2+    blue precipitate, turns pink on standing    no change
Ni2+    green precipitate    no change
Cu2+    pale blue precipitate    no change

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Specification: Edexcel Chemistry (9CH0), A-level, Topic 15A: Principles of transition metal chemistry. Topic: Transition Metals.

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