Sunday, 16 May 2010

Ion Analysis : Testing for Anions/Cations

Testing for Cations

Cations are positively charged (+) ions. There are 2 basic tests for them:

  • Flame test
  • Adding NaOH (aq) to solution of ionic compound
The flame test involves a clean nichrome wire first being dipped into a solution of Hydrochloric acid. This is then dipped into the solid powdered compound of the metal being tested. This is then held over a Bunsen Burner and the flame colour is observed.

The following results are gained for each ion:

  • Na+ - Yellow flame
  • K+ - Lilac (light purple) flame
  • Ca2+ - Red flame
  • Cu2+ - Blue / green flame
The second test of adding NaOH (sodium hydroxide) gives a coloured precipitate. The colour of this precipitate reveals which ion is present. The following results are gained for each ion:
  • Ca2+ White precipitate - Ca(OH)2 (s)
  • Cu2+ Blue precipitate - Cu(OH)2 (s)
  • Fe2+ Green precipitate - Fe(OH)2 (s)
  • Fe3+ Brown precipitate - Fe(OH)3 (s)
To detect the Hydrogen ion (H+), which is not a metal, but still has a positive charge, Universal indicator can be used, which will turn red. (because anything with the hydrogen ion in it is an acid)


Testing for Anions

Anions are negatively charged (-) ions and there are many different tests for them. The following table shows the tests and positive results:

IonTestPositive Result
Hydroxide (OH-)Add an indicator (red litmus)
or add an acid
Litmus turns blue
or test tube becomes warm
Sulphate (SO4-2)Add Barium Chloride solutionWhite solid
Carbonate (CO3-2)Add an acidGas given off which should be CO2 (test with limewater)
Chloride (Cl-)Add acidified solution of AgNO3 (aq)White precipitate
Bromide (Br -)Add acidified solution of AgNO3 (aq)Cream precipitate
Iodide (I-)Add acidified solution of AgNO3 (aq)Yellow precipitate

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