Tuesday, 9 September 2014

Colourless Cations

Most of the cations we need to identify are colourless: Ag+, Mg2+, Pb2+, Ba2+, Zn2+, Al3+, Na+ and NH4+.

This makes them a bit tougher to identify in an unknown solution.

Step One

The easiest way to start is to add a few drops of NaOH:
Muddy Brown Precipitate = Ag+
No Precipiatate = Na+ or NH4+  (you will not be asked to tell these two ions apart in an assessment)
White Precipitate = any of the others

Step Two

If a white precipitate has formed, add another 2-3 mL of NaOH:
Precipitate Remains = Ba2+ or Mg2+
Precipitate Disappears (redissolves) =  Pb2+, Zn2+or Al3+

Barium or Magnesium?

Collect a new sample and add about 1 mL of sulfuric acid. The sulfate ions will form a white precipitate with barium ions, but not with magnesium:
White Precipitate =  Ba2+
No Precipitate =  Mg2+

Lead, Zinc or Aluminium?

Collect a new sample and add a few drops of potassium iodide (KI):
Yellow Precipitate =  Pb2+
No Precipitate =  Al3+or Zn2+

Collect a new sample if it is not  Pb2+
Add excess/concentrated ammonia. Both will form a white precipitate with the hydroxide ions in ammonia (it is a base), but zinc will redissolve, forming a complex ion with the ammonia molecules. Aluminium will not make this complex.
No Precipitate =  Zn2+
White Precipitate =  Al3+

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