Distinguishing between cations
Qualitative analysis may be used to establish the identity of a cation in a solution. This method depends on observable differences in the properties of these cations. The cations considered are in three groups. The way in which these can be distinguished is given in words below and summarised in a flow chart on the next page.
The first group (Cu2+, Fe2+, Ag+ and Fe3+) can be distinguished based
The second set of ions (Mg2+, Ba2+, Al3+, Pb2+, Zn2+) have colourless solutions and give white precipitates with hydroxide ion.
The cations can be divided into subgroups on the basis of the solubility of the precipitate with aqueous hydroxide ion or ammonia. Three of these ions dissolve because of reaction to form complex ions.
The third set of cations (NH4+, Na+) are colourless and do not form precipitates with hydroxide.
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The first group (Cu2+, Fe2+, Ag+ and Fe3+) can be distinguished based
on the colour of their solution OR
on the colour of the precipitate that they give on mixing with a solution containing hydroxide ion .
Cu2+blue solution Fe2+ green solution
Ag+ (colourless solution)
Ag2O brown precipitate
Fe3+ (pale yellow)
on the colour of the precipitate that they give on mixing with a solution containing hydroxide ion .
Cu2+blue solution Fe2+ green solution
Ag+ (colourless solution)
Ag2O brown precipitateFe3+ (pale yellow)
Fe(OH)3 orange precipitate
The second set of ions (Mg2+, Ba2+, Al3+, Pb2+, Zn2+) have colourless solutions and give white precipitates with hydroxide ion.
The cations can be divided into subgroups on the basis of the solubility of the precipitate with aqueous hydroxide ion or ammonia. Three of these ions dissolve because of reaction to form complex ions.
Hydroxides of Mg2+ and Ba2+ are insoluble in both aqueous ammonia and sodium hydroxide.
Hydroxides of Pb2+ and Al3+ are soluble in aqueous sodium hydroxide but insoluble in aqueous ammonia.
The hydroxide of Zn2+ dissolves in both hydroxide ion AND in aqueous ammonia.
Hydroxides of Pb2+ and Al3+ are soluble in aqueous sodium hydroxide but insoluble in aqueous ammonia.
The hydroxide of Zn2+ dissolves in both hydroxide ion AND in aqueous ammonia.
The two pairs can be distinguished by adding H2SO4 to a solution of the cation.
Both PbSO4 and BaSO4 are insoluble.
The third set of cations (NH4+, Na+) are colourless and do not form precipitates with hydroxide.
NH4+ is an acidic cation and solutions of NH4+ turn blue litmus red.