Tests for cations in solution

Cations are positive ions.

The cations you need to be able to identify are:

• copper

• iron(III)

• silver

• iron(II)

• magnesium

• zinc

• aluminium

• lead

• barium

• sodium

Copper

Solutions containing Cu2+(aq) will be blue/green in colour.

Pour a little of the test solution into a clean test tube.

Add a few drops of dilute sodium hydroxide solution.

A pale blue precipitate indicates the presence of copper ions.

Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s)

To confirm the presence of Cu2+(aq), add about 5 mL of ammonia solution. The pale blue precipitate redissolves to form a clear, royal blue solution.

Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4]2+(aq) + 2OH–

(aq)

Iron(III)

Solutions containing Fe3+(aq) will be orange in colour (or yellow if very dilute).

Pour a little of the test solution into a clean test tube.

Add a few drops of dilute sodium hydroxide solution.

An orange or dark brown precipitate forms.

Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s)

SilverSilver ions also react with sodium hydroxide solution to form a brown precipitate

Iron(III) solutions are usually coloured and the precipitate is dark brown or orange.

Silver solutions are colourless and the precipitate is mud-brown or the colour of milky coffee.

2Ag+(aq) + 2OH–(aq) → Ag2O(s) + H2O(l)

To confirm Fe3+

Potassium thiocyanate, KSCN, can be used to detect the presence of Fe3+.

This test can be used on very dilute solutions or in the presence of other metal ions (especially Fe2+).

To a fresh sample of the test solution, add a few drops of potassium thiocyanate solution. If Fe3+ is present the solution will turn blood-red.

Fe3+(aq) + SCN–(aq) → [FeSCN]2+(aq)

Iron(II)

Solutions of Fe2+ are colourless or pale green.

Pour a little of the test solution into a clean test tube.

Add a few drops of sodium hydroxide solution.

If Fe2+ is present you will see an olive-green precipitate or gel form.

Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)

Magnesium, zinc, aluminium and lead

These four cations all form white precipitates with sodium hydroxide solution. Three form soluble complexes with excess hydroxide, one also forms a soluble complex with ammonia solution.

It is very important when adding sodium hydroxide to colourless solutions that you start with one or two drops only – otherwise you might miss the formation of the precipitate when it redissolves in excess hydroxide.

Magnesium

Pour a little of the test solution into a clean test tube.

Add one or two drops only of sodium hydroxide solution.

If Mg2+ is present a white precipitate forms.

Mg2+(aq) + 2OH–(aq) → Mg(OH)2(s)

When excess sodium hydroxide is added, more precipitate is formed: the precipitate does NOT redissolve.

Zinc

Pour a little of the test solution into a clean test tube.

Add one or two drops only of sodium hydroxide solution.

If Zn2+ is present a white precipitate forms.

Zn2+(aq) + 2OH–(aq) → Zn(OH)2(s)

Add excess sodium hydroxide solution (about 5 mL). The white precipitate slowly redissolves to form a clear, colourless solution.

Zn(OH)2(s) + 2OH–(aq) → [Zn(OH)4]2–(aq)

Al3+ and Pb2+ also form complexes with OH–, but only Zn2+ also forms a complex with ammonia.

To a fresh sample of the test solution add a few drops of sodium hydroxide to form a white precipitate as before.

Add excess ammonia solution (about 5 mL). If Zn2+ is present the white precipitate will slowly redissolve to form a clear, colourless solution.

Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]2+(aq) + 2OH–

(aq)

Aluminium

Pour a little of the test solution into a clean test tube.

Add one or two drops only of sodium hydroxide solution.

If Al3+ is present a white precipitate forms.

Al3+(aq) + 3OH–(aq) → Al(OH)3(s)

Add excess hydroxide (about 5 mL).

The precipitate redissolves to form a clear, colourless solution.

Al(OH)3(s) + OH–(aq) → [Al(OH)4]–(aq)

To a fresh sample of the test solution, add a few drops of sodium hydroxide solution to form a white precipitate as before.

Add excess (5 mL) ammonia solution. The white precipitate does NOT redissolve.

Both Al3+ and Pb2+ form white precipitates with OH– which redissolve in excess OH– but not in excess NH3. However, aluminium sulfate is soluble in water, while lead sulfate is not.

To a fresh sample of the test solution add a few drops of sulfuric acid. If Al3+ is present there will be NO precipitate.

Lead

Pour a little of the test solution into a clean test tube.

Add one or two drops only of sodium hydroxide solution.

A white precipitate forms.

Pb2+(aq) + 2OH–(aq) → Pb(OH)2(s)

Add excess (5 mL) sodium hydroxide solution. If Pb2+ is present the white precipitate redissolves to form a clear, colourless solution.

Pb(OH)2(s) + 2OH–(aq) → [Pb(OH)4]–(aq)

To a fresh sample of the test solution add a few drops of sodium hydroxide solution as before.

Add excess (5 mL) ammonia solution. If Pb2+ is present the white precipitate does NOT redissolve.

To a fresh sample of the test solution add a few drops of dilute sulfuric acid.

If Pb2+ is present a white precipitate will form.

Pb2+(aq) + SO42–(aq) → PbSO4(s)

Barium and sodiumSolutions containing Ba2+ or Na+ will be colourless.

Barium hydroxide is moderately soluble, while sodium hydroxide is very soluble.

Add a few drops of sodium hydroxide. No precipitate suggests barium or sodium. Add more hydroxide (5 mL).

If barium is present there may be a slight cloudiness after excess hydroxide is added.

If sodium is present the solution will remain clear and colourless.

Ba2+(aq) + 2OH–(aq) → Ba(OH)2(s)

Barium salts form a white precipitate with sulfuric acid.

Sodium salts do not form a precipitate with sulfuric acid.

Ba2+(aq) + SO42–(aq) → BaSO4(s)