The colour of a ‘normal’ aqueous solution of a copper II salt is due to the presence of a coordination complex between the copper ion and water molecules. Copper is, however, capable of forming complexes with a wide array of compounds – the deep blue complex with ammonia is perhaps on of the most familiar.
For each complex, there is an equilibrium in relation to the complex with water, some being slightly more stable, some much so. Using this information, it is possible to sequentially add ligands of greater affinity for copper to an aqueous solution and see a series of colour changes as each ligans is displaced by the next which has a greater affinity for the copper ion.
Ligands of copper complexes
Ligands of copper complexes – risk assessment