A previous post looked at the colours of transition metals, and the origin of their colours – this graphic, on the other hand, looks at how transition metals (and some non-transition metals) can be identified by the precipitates they form with sodium hydroxide and ammonia solutions. I’m going to keep the explanation of the reasons for the colour changes and precipitates fairly simple here, but I’ve provided links at the bottom of the page if you want to read about them in more detail.
In the case of the d block metals (all commonly referred to as transition metals, although according to the definition zinc is not a transition metal), the reactions of their aqueous ions with sodium hydroxide and ammonia solutions are due, in some cases, to ligand exchange reactions. The d block metals form metal aquo complexes, like that shown below, in water:
In this complex, water acts as a ‘ligand’, binding to the central metal atom. But water isn’t the only molecule that can act as a ligand; both OH- ions and ammonia can also do so. In the case of hydroxide ions, they can react with the metal aquo complex and take the place of one or more of the ligands. When this happens, the metal hydroxide can be formed. Only group 1 metal hydroxides are soluble in aqueous solution, so hydroxides of any other metals form solid precipitates. Also, only transition metals have the partially filled d orbitals required to appear coloured, so the hydroxides of non-transition metals are colourless.
Ammonia can act as a base as well as a ligand. Added in a small amount, it acts as a base; it reacts with hydrogen ions in water to produce ammonium ions, with the metal aquo complex losing a hydrogen to the solution from one of its water ligands to form the metal hydroxide – so initially the result seen is the same as for with OH- ions.
However, with excess ammonia, we see ligand exchange reactions occur, where ammonia molecules can take the place of the water molecules around the metal ion. This can lead to the drastic colour changes seen in the graphic above. The ligand exchange process works better for some metals than others, which is why we don’t see a drastic change for all of the metals shown.
As was discussed in the previous transition metal graphic, different ligands can alter the colour of transition metal complexes as they affect the splitting in energy of the d orbitals, in turn affecting the wavelengths of light which the complex absorbs.
If you want to read in more detail about ligands and how these precipitates are formed, below are a few links.
References & Further Reading
- Reactions of metal complexes with hydroxide ions – Chemguide
- Reactions of metal complexes with ammonia solution – Chemguide
- Metal & ammonia complexes – chem.colorado.edu