Ion Analysis : Testing for Anions/Cations

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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)

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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:

Ion	Test	Positive 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 solution	White solid
Carbonate (CO3-2)	Add an acid	Gas 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