Qualitative Analysis – Identifying Cations

In this section, students are required to describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the following aqueous cations: aluminium, ammonium, calcium, copper(II), iron(II), iron(III), lead(II) and zinc (formulae of complex ions are not required).

Cation Tests and Observations

QA Cation.jpg

What exactly happened?

Reaction with aqueous sodium hydroxide

Outcome 1: No precipitate formed (NH4+)

Ammonium salts will react with aqueous sodium hydroxide to form a soluble salt, water and ammonia gas.

Example: Ammonium chloride + Sodium Hydroxide → Sodium Chloride (soluble) + Water + Ammonia gas

NH4Cl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) + NH3 (g)

On heating, ammonia gas is given off. Gas turns moist red litmus paper blue.

Why should the litmus paper be moist?

Ammonia, being an alkaline gas, produces hydroxide ions (OH) in the presence of water.

H2O (l) + NH3 (g) → NH4(aq) + OH–  (aq)


Outcome 2: Precipitate formed (Zn2+, Al2+,Pb2+,Fe2+,Fe3+,Cu2+,Ca2+)

When sodium hydroxide is added to other metal cations, an insoluble hydroxide is formed.

Example 1:
Lead(II) chloride + Sodium Hydroxide → Lead(II) Hydroxide (insoluble) + Sodium Chloride (soluble)

PbCl2 (aq) + 2 NaOH (aq) → Pb(OH)(s)+ 2 NaCl (aq)

White precipitate of lead(II) hydroxide is formed.

Example 2:
Iron(II) nitrate + Sodium Hydroxide → Iron(II) Hydroxide (insoluble) + Sodium Nitrate (soluble)

Fe(NO3)2 (aq) + 2 NaOH (aq) → Fe(OH)(s)+ 2NaNO3 (aq)

Green precipitate of iron (II) hydroxide is formed.

Reaction with aqueous ammonia

Outcome 1: No precipitate formed (Ca2+)

No precipitate is formed as there is little amount of hydroxide ions present to form precipitate with calcium hydroxide, which is partially soluble.

Note: Aqueous ammonia is a weak alkali, hence only low concentration of hydroxide ions is present.

Outcome 2: Precipitate formed (Zn2+, Al2+,Pb2+,Fe2+,Fe3+,Cu2+)

When aqueous ammonia is added to other metal cations, an insoluble hydroxide is formed.

Example: Copper(II) chloride + Aqueous Ammonia → Copper(II) Hydroxide (insoluble) + Ammonium Chloride (soluble)

CuCl2 (aq) + 2 NH4OH (aq) → Cu(OH)(s)+ 2 NH4Cl (aq)

Light blue precipitate of copper (II) hydroxide is formed.

Example: Zinc nitrate + Aqueous Ammonia → Zinc Hydroxide (insoluble) + Ammonium Nitrate (soluble)

Zn(NO3)2 (aq) + 2 NH4OH (aq) → Zn(OH)(s)+ 2 NH4NO3 (aq)

White precipitate of zinc (II) hydroxide is formed.

Question

Lead(II) ions and aluminum ions give the same results in both aqueous sodium hydroxide and aqueous ammonia. How do we distinguish lead(II) ions from aluminium ions?

Add any solution containing chloride (or sulfate) ions. Only lead(II) ions give white precipitate with chloride ions (or sulfate ions).

Learn more about Qualitative Analysis – Identifying Anions

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