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Identification of ions

Flame test

Flame tests can be used to identify some metal ions.

Different metal ions produce different flame colours when they are heated.

This is the basis of a flame test.

This table shows the flame test colours for five common metal cations:

Ion present Flame test colour
Lithium, Li+ Crimson
Sodium, Na+ Yellow
Potassium, K+ Lilac
Calcium, Ca2+ Orange-red
Copper, Cu2+ Green

If a sample contains a mixture of ions, some flame colours can be masked. Flame colours may not be clearly visible.

Alums are salts. Alums contain aluminium ions. flame tests can be used to distinguish between these three alums. These three alums could be identified from the results of flame tests colour.

yellow = sodium (alum)

lilac = potassium (alum)

colourless = ammonium (alum)

Flame Emission Spectroscopy

Flame emission spectroscopy is an instrumental method used to analyse metal ions in solutions.

The sample is put into a flame and the light given out is passed through a spectroscope.

When substances are heated they often emit energy in the form of light.

The output is an emission spectrum in which different ions produce lines in different parts of the spectrum.

Each metal ion produces a unique emission spectrum. Flame emission spectroscopy also works for mixtures of ions.