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Ion Analysis

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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)

About the Author

by: Guest
Total views: 460
Word Count: 257
Date: Thu, 9 Aug 2007 Time: 12:00 AM
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