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 summarsed 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 in aqueous solutions containing hydroxide ion or ammonia. Three of these ions form precipitates that dissolve because of reaction to form complex ions.
The last cation (Na+) is colourless and does not form a precipitate with added hydroxide because its hydroxide is water soluble.
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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+ light green solution
Ag+ (colourless solution)
Fe3+ (pale yellow solution)
on the colour of the precipitate that they give on mixing with a solution containing hydroxide ion .
Cu2+ blue solution
Fe2+ light green solution
Ag+ (colourless solution)
Ag2O brown precipitate
Fe3+ (pale yellow solution)
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 in aqueous solutions containing hydroxide ion or ammonia. Three of these ions form precipitates that dissolve because of reaction to form complex ions.
Mg2+ and Ba2+
These hydroxides are insoluble in both aqueous ammonia and aqueous sodium hydroxide.
Pb2+ and Al3+
These hydroxides are soluble in aqueous sodium hydroxide and insoluble in aqueous ammonia.
Zn2+
This hydroxide dissolves in both aqueous sodium hydroxide AND in aqueous ammonia.
These hydroxides are insoluble in both aqueous ammonia and aqueous sodium hydroxide.
Pb2+ and Al3+
These hydroxides are soluble in aqueous sodium hydroxide and insoluble in aqueous ammonia.
Zn2+
This hydroxide dissolves in both aqueous sodium hydroxide 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 last cation (Na+) is colourless and does not form a precipitate with added hydroxide because its hydroxide is water soluble.
Indeed an aqueous solution of NaOH is used in the tests!!