Qualitative Analysis is the process of identifying unknown substances in a solution.
Students are required to describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the following aqueous cations: aluminium (Al3+), ammonium (NH4+), calcium (Ca2+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+), lead(II) (Pb2+) and zinc (Zn2+) (formulae of complex ions are not required).
We will focus on the use of aqueous ammonia in this video.
For easy memory, you may wish to first divide the cations into 2 groups – precipitate formed and no precipitate formed.
No Precipitate formed
If no precipitate is formed, we can confirm that the ion present is calcium ion.
Calcium hydroxide is partially soluble/ sparingly soluble. This means that it will be soluble if the concentration of calcium and hydroxide ions are low. If the concentration of these ions are high, white precipitate will form.
Aqueous ammonia is a weak alkali. It partially ionizes in aqueous solution to form low concentration of hydroxide ions.
Since concentration of hydroxide ions is low, calcium hydroxide remains soluble. Hence, no precipitate is formed.
Further divide the cations under ‘precipitate formed’ into white precipitate and coloured precipitate.
White Precipitate formed
Aluminium, zinc and lead(II) ions give white precipitate with a few drops of aqueous ammonia. If the precipitate dissolves in excess to form a colourless solution, then zinc ion is present. If the precipitate remains insoluble in excess aqueous ammonia, aluminium ion or lead(II) ions could be present. Further tests can be done to identify which of these two ions are present.
Coloured Precipitate formed
Things get easier with coloured precipitate.
Light blue precipitate is formed if copper(II) ion is present. Precipitate dissolves in excess aqueous ammonia to form a deep blue solution.
Green precipitate is formed if iron(II) ion is present. Precipitate is insoluble in excess aqueous ammonia.
Reddish brown precipitate is formed if iron(III) ion is present. Precipitate is insoluble in excess aqueous ammonia.
The precipitate formed in each of the reactions is the hydroxide of the metal ion.