Identifying Cations

Test with Sodium Hydroxide and Ammonia Solution

  1. Cations can be identified by their reaction with aqueous sodium hydroxide and aqueous ammonia.
  2. Sodium hydroxide and aqueous ammonia produce hydroxide ion which will react with most anion to form precipitate.
    NaOH + H2O → Na+ + 2OH + H+

     

    NH3 + H2O → NH4+ + OH 
  3. Different cations like aluminium Al3+, calcium Ca2+, copper(II) Cu2+, iron(II) Fe2+, iron(III) Fe3+, lead(II) Pb2+, zinc Zn2+ produce different coloured precipitates, which may or may not dissolve in excess alkali.
  4. Zn(OH)2, Al(OH)3 and Pb(OH)3 dissolve in excess NaOH solution, this is because Zn(OH)2, Al(OH)3 and Pb(OH)3 are amphoteric, they can react with NaOH to form salt and water.
    Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O
    Al2(OH)3 + 3NaOH → Na3Al2O3 + 3H2O
    Pb(OH)2 + 2NaOH → Na2PbO2 + 2H2O
  5. Table below shows the summary of the precipitate form by different cation.
NaOH(ak)
NH3(ak)
Na+
Ca2+
White precipitate.
Mg2+
White precipitate.
White precipitate.
Al3+
White precipitate.
Dissolve in excess NaOH solution.
White precipitate.
Zn2+
White precipitate.
Dissolve in excess NaOH solution.
White precipitate.
Dissolve in excess NH3 solution.
Pb2+
White precipitate.
Dissolve in excess NaOH solution.
White precipitate.
Fe2+
Dirty green precipitate.
Dirty green precipitate.
Fe3+
Red brown precipitate.
Red brown precipitate.
Cu2+
Blue precipitate.
Blue precipitate.
Dissolve in excess NH3 solution and form a blue solution.
NH4+

Test with Chloride Ions

  1. Out of the 10 cations, only lead(II) ions will form a precipitate with chloride ions.
  2. This is because lead(II) chloride is insoluble in water.
  3. The chemical reaction is a double decomposition reaction.
    Pb2+ + 2Cl → PbCl2
  4. Lead(II) chloride will dissolve in hot water.

HCl or NaCl
Na+
Ca2+
Mg2+
Al3+
.-
Zn2+
Pb2+
White precipitate.
Dissolve in hot water
Fe2+
Fe3+
Cu2+
NH4+

Test with Sulphate Ions

  1. Out of the 10 cations, only calcium ions and lead(II) ions will form a precipitate with sulphate ions.
  2. This is because both calcium sulphate and lead(II) sulphate are insoluble in water.
  3. The chemical reaction is a double decomposition reaction.
    Pb2+ + SO42- → PbSO4
    Ca2+ + SO42- → CaSO4


H2SO4 or Na2SO4
Na+
Ca2+
White precipitate.
Mg2+
Al3+
Zn2+
Pb2+
White precipitate.
Fe2+
Fe3+
Cu2+
NH4+

Test with Carbonate Ions

  1. All ions, except sodium ions and ammonium ions will form precipitate with carbonate.
  2. This is because sodium carbonate and ammonium carbonate are soluble in water.



Na2CO3
Na+
Ca2+
White precipitate.
Mg2+
White precipitate.
Al3+
White precipitate.
Zn2+
White precipitate.
Pb2+
White precipitate.
Fe2+
Green precipitate.
Fe3+
Brown precipitate.
Cu2+
Blue precipitate.
NH4+


Test with Iodide Ions

  1. Iodide ions will form precipitate with lead(II) ions and copper(II) ions.
  2. However, in SPM you only need to know the reaction between lead(II) ions and iodide ions.
  3. The yellow precipitate formed will dissolve in hot water.
    Pb2+ + 2I → PbI2


KI
Na+
Ca2+
Mg2+
Al3+
Zn2+
Pb2+
Yellow precipitate. Dissolve in hot water
Fe2+
Fe3+
A red brown solution formed.
Cu2+
White precipitate form in brown solution
NH4+

Tests to Distinguish Iron(II) and Iron(III) ions

  1. The presence of Fe2+ ion and Fe3+ ion in a salt can be confirmed by using solution of potassium hexacyanoferrate (II), solution of potassium hexacyanoferrate (III) and potassium thiocyanate.
  2. Table below shows the observation of the tests.
Reagent
Observation
Ion presents
Solution of potassium hexacyanoferrate (II)Light blue precipitateFe2+
Dark Blue precipitateFe3+
Solution of potassium hexacyanoferrate (III)Dark blue precipitateFe2+
Greenish brown solutionFe3+
Potassium thiocyanatePinkish solutionFe2+
Blood red solutionFe3+