Identification of Cations (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

Test for ammonium ions

• Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide and ammonia
  • Most transition metals produce hydroxides with distinctive colours

Test for metal ions with sodium hydroxide solution

• If a small amount of sodium hydroxide solution is used, the resulting metal hydroxide normally precipitates out of solution
• If excess sodium hydroxide solutionis used, some of the precipitates may re-dissolve
  • For this reason, just a few drops of sodium hydroxide solutionare added at first and very slowly
• The sodium hydroxide test for the metal ion is:
  • Add a few drops of sodium hydroxide solution
  • Record any colour changes or precipitates formed 
  • Add excess sodium hydroxide solution
  • Record any colour changes or changes to precipitates

Metal ions with NaOH results

Test for metal ions with ammonia solution 

• If a small amount of ammonia solution is used, the resulting metal hydroxide normally precipitates out of solution
• If excess ammonia solution is used, some of the precipitates may re-dissolve
  • For this reason, just a few drops of ammonia solution are added at first and very slowly
• The ammonia test for the metal ion is:
  • Add a few drops of ammonia solution
  • Record any colour changes or precipitates formed 
  • Add excess ammonia solution
  • Record any colour changes or changes to precipitates

Metal ions with ammonia results

Analysing results

• The tables above contain the results for all metal cations included in the syllabus
• If a precipitate is formed from either sodium hydroxide or ammonia solution, then the hydroxide is insoluble in water
• For example, zinc chloride:

ZnCl₂ (aq) + 2NaOH (aq) →  Zn(OH)₂ (s) + 2NaCl (aq)

• There are 3 metal ions that all form white precipitates:
  • Aluminium ions, Al³⁺ 
  • Calcium ions, Ca²⁺ 
  • Zinc ions, Zn²⁺ 
• Calcium ions, Ca²⁺, can be easily distinguished from Zn²⁺ and Al³⁺
  • The white precipitate of calcium hydroxide does not dissolve in excess sodium hydroxide solution
  • The white precipitates of zinc hydroxide and aluminium hydroxide dissolve in excess sodium hydroxide solution
• Zinc ions, Zn²⁺, can then be distinguished from Al³⁺ ions as
  • The white precipitate of zinc hydroxide dissolves in excess ammonia solution
  • The white precipitate of aluminium hydroxide does not dissolve in excess ammonia solution

Flame tests for metal ions

• The flame test is used to identify the metal cations by the colour of the flame they produce
  • Ions from different metals produce different colours
• Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid and then hold it in the blue flame of a Bunsen burner until there is no colour change
  • This is an important step as the test will only work if there is just one type of ion present
    • Two or more ions means the colours will mix, making identification erroneous
    • This cleans the wire loop and avoids contamination
• A small sample of the compound is placed on an unreactive metal wire loop such as nichrome or platinum
• Dip the loop into the solid sample / solution and place it in the edge of the blue Bunsen flame
  • Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour

Diagram showing the technique for carrying out a flame test

• The colour of the flame is observed and used to identify the metal ion present:

Metal ions form distinctive coloured flames