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Testing for cations

Cation ions are positively charged ions, mostly metals ions. They are ions that would be attracted to a negatively charged cathode, hence cation.

Flame tests

Most people remember doing this practical; probably because you get to burn stuff! Some metal ions will give a very distinctive colour when placed in a hot flame. A loop of wire; usually platinum is placed in some concentrated acid to make sure it is clean and free from any residues that could contaminate the flame and give a false result. Once it is clean hold it in a hot flame for a few seconds until only the blue colour of the flame is visible. Next dip the platinum wire into a solution containing the metal ion you hope to identify, then insert the wire loop back into the hot Bunsen flame and record any colour you observe. The flame colours for some metal ions are shown below:

The different colours produced by metal ions in a flame, flame tests.

The only real problem with flame tests is that if more than one metal ion is present then you can get mixed results where it is hard to tell exactly what metal ion maybe present and also the fact that flame test gives no indication of the amount of the metal ion present. The main reason for this is some of the colours are more intense and over powering than others. The yellow sodium flame is very intense and will mask other colours, as is the red lithium flame colour.

Precipitation reactions

Some transition metal ions form a coloured precipitate when an alkaline solution such as sodium hydroxide is added. The colour of the precipitate formed can help you identify unknown metal ions. A precipitate is an insoluble solid formed when 2 solutions are mixed e.g.

A(aq) + B(aq) → C(s) + D(aq)
Here you need to look at the state symbols; (aq) is for a soluble solution and (s) is for an insoluble solid. Here two solutions A and B are mixed and an insoluble solid C is formed. This insoluble solid could be removed by filtering. Most solutions of metal ions with a 2+ and 3+ charge will form insoluble precipitate when an alkaline solution such as sodium or potassium hydroxide is added to them.

The image below shows 4 boiling tubes each containing a solution of a metal ion with a 2+ or 3+ charge. A few drops of sodium hydroxide was then added to each of the boiling tubes and as you can see from the image below a coloured precipitate is formed in each boiling tube. Equations for each of these reactions is given below: Precipitation reactions are used to identify metal ions.

Most metal ions with a 2+ and 3+ charge will form insoluble precipitate when an alkaline solution such as sodium or potassium hydroxide is added.

Example 1: Precipitates from copper(II) ions (Cu2+)

Copper(ll) sulfate(aq) + sodium hydroxide(aq) → sodium sulfate(aq) + copper hydroxide(s)
CuSO4(aq) + 2NaOH(aq) → Na2SO4(aq) + Cu(OH)2(s)
An ionic equation for this reaction is:
Cu2+(aq) + 2OH-(aq) → Cu2+ (OH-)2(s)
All sodium compounds are soluble so the insoluble solid produced must be copper hydroxide. Copper hydroxide is the blue solid precipitate produced.

Example 2: Precipitates from iron (II) ions (Fe2+)

Word, symbolic and ionic equations for the formation of iron(ll) hydroxide:
Iron (ll) chloride(aq) + sodium hydroxide(aq) → sodium chloride(aq) + iron(ll) hydroxide(s)
FeCl2(aq) + 2NaOH(aq) → 2NaCl(aq) + Fe(OH)2(s)
The solid precipitate of iron hydroxide produced is a green colour. An ionic equation for this reaction can be written, it would also make sense to remove all the spectator ions, that is ions which are unchanged on both the reactant and product sides of the equation. These ions are only there to balance off the charges.
Fe2+(aq) + 2OH-(aq) → Fe2+(OH-)2(s)

Example 3: Word, symbolic and ionic equations for the formation of solid iron(lll) hydroxide:

Iron(lll) chloride(aq) + sodium hydroxide(aq) → sodium chloride(aq) + iron(lll) hydroxide(s)
FeCl3(aq) + 3NaOH(aq) → 3NaCl(aq + Fe(OH)3(s)
Fe3+(aq) + 3OH-(aq) → Fe3+(OH-)3(s)
The solid precipitate of iron (lll) hydroxide produced is a brown colour.

Testing for Al3+, Ca2+ and Mg2+ ions

All the examples above use transition metals but with a little more chemistry it is possible to use this reaction to help identify calcium ions (Ca2+), magnesium ions (Mg2+), and aluminium ions (Al3+).

If a sodium hydroxide solution is added to a solution of these ions then a white (colourless) precipitate of the metal hydroxide is formed in each case.

Ca2+(aq) + 2OH-(aq) → Ca2+(OH-)2(s)
Mg2+(aq) + 2OH-(aq) → Mg2+(OH-)2(s)
and for aluminium:
Al3+(aq) + 3OH-(aq) → Al3+(OH-)3(s)

This might not seem particularly helpful in identifying these three ions if they all produce 3 colourless (white) solid precipitates. However:

This simple test will enable you to test for the presence of Al3+ ions. Now all we need is a simple way to distinguish calcium ions from magnesium ions. This is easily done:

Test for ammonium ions

Ammonium ions (NH4+) can be detected in a substance by testing for the presence of ammonia gas (NH3) which is released when ammonium ions are heated gently with a strong alkali such as sodium or potassium hydroxide.

NH4+(aq) + OH- (aq) → NH3(g) + H2O(l)
The ammonia gas which is released can be detected by using a strip of damp red litmus paper which will turn blue in the presence of the basic ammonia gas. The test is outlined below: testing for ammonium ions, ammonium ions release ammonia gas when heated with a strong alkali, ammonia gas is then released.

Key points

Practice questions

Check your understanding - Questions on testing for cations

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