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	Small amount of NaOH(aq)	Excess NaOH(aq)
Cu2+	pale blue precipitate	no change
Fe2+	green precipitate, turns orange-brown at the surface in air	no change
Fe3+	orange-brown precipitate	no change
Mn2+	light brown precipitate, darkens in air	no change
Cr3+	grey-green precipitate	dissolves to form a dark green solution of [Cr(OH)6]3-