Formation of Coloured Ions

Transition metal ions can be identified by their colour.

Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.

d electrons move from the ground state to an excited state when light is absorbed.

The energy difference between the ground state and the excited state of the d electrons is given by:

∆E = hν = hc/λ

Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.

The absorption of visible light is used in spectroscopy.

A simple colorimeter can be used to determine the concentration of coloured ions in solution.