Tests for Metal Ions (Sodium Hydroxide)

Purpose of Sodium Hydroxide Tests

Sodium hydroxide (NaOH) solution is used to identify certain metal ions in aqueous solutions by forming precipitates. When aqueous sodium hydroxide is added to a solution containing metal ions, insoluble metal hydroxides often form as precipitates with characteristic colours. These colours and the behaviour of the precipitates with excess sodium hydroxide help confirm the identity of the metal ions present.

Key points:

• Use aqueous sodium hydroxide to test for metal ions.
• Observe the colour of the precipitate formed.
• Check if the precipitate dissolves or remains when excess sodium hydroxide is added.
• Different metal ions produce distinct precipitates, allowing identification.

Common Metal Ion Reactions with Sodium Hydroxide

When aqueous sodium hydroxide is added to solutions containing metal ions, the following precipitates form immediately:

Metal Ion	Colour of Precipitate	Solubility in Excess NaOH
Calcium (Ca2+)	White	Insoluble (precipitate remains)
Copper(II) (Cu2+)	Blue	Insoluble initially (precipitate forms immediately), but dissolves in excess NaOH
Iron(II) (Fe2+)	Green	Insoluble (precipitate remains)
Iron(III) (Fe3+)	Brown	Insoluble (precipitate remains)

These precipitates are metal hydroxides formed by the reaction:

$$\text{Metal ion (aq)} + \text{OH}^- (aq) \rightarrow \text{Metal hydroxide (s)}$$

For example, copper(II) ions react as:

$$\text{Cu}^{2+} (aq) + 2\text{OH}^- (aq) \rightarrow \text{Cu(OH)}_2 (s)$$

This forms a blue precipitate of copper(II) hydroxide.

For instance, if you add sodium hydroxide to a solution containing iron(III) ions, a brown precipitate of iron(III) hydroxide forms, which remains insoluble in excess sodium hydroxide.

Effect of Excess Sodium Hydroxide

After the initial precipitate forms, adding excess sodium hydroxide can help differentiate some metal ions further:

• Calcium (Ca²⁺): The white precipitate remains insoluble in excess sodium hydroxide.
• Copper(II) (Cu²⁺): The blue precipitate dissolves in excess sodium hydroxide to form a deep blue solution of the tetrahydroxocuprate(II) complex ion $[\text{Cu(OH)}_4]^{2-}$. This complex forms due to coordination of hydroxide ions around the copper ion.
• Iron(II) (Fe²⁺): The green precipitate remains insoluble in excess sodium hydroxide.
• Iron(III) (Fe³⁺): The brown precipitate remains insoluble in excess sodium hydroxide.

This behaviour is useful to confirm the presence of copper(II) ions, as only copper(II) hydroxide dissolves in excess NaOH to give a deep blue solution.

For example, copper(II) hydroxide reacts with excess sodium hydroxide as:

$$\text{Cu(OH)}_2 (s) + 2\text{OH}^- (aq) \rightarrow [\text{Cu(OH)}_4]^{2-} (aq)$$

Learning Example: Identifying Metal Ions by Sodium Hydroxide Test

If you add sodium hydroxide to a solution and observe a green precipitate that does not dissolve in excess sodium hydroxide, what metal ion is present?

Answer: The green precipitate indicates iron(II) ions (Fe²⁺). Since the precipitate remains insoluble in excess NaOH, this confirms iron(II) hydroxide.

Summary Table of Sodium Hydroxide Tests for Metal Ions

Metal Ion	Precipitate Colour with NaOH	Solubility in Excess NaOH	Notes
Calcium (Ca2+)	White	Insoluble	Precipitate remains; no colour change
Copper(II) (Cu2+)	Blue	Dissolves to deep blue solution	Distinctive deep blue complex ion forms
Iron(II) (Fe2+)	Green	Insoluble	Precipitate remains green
Iron(III) (Fe3+)	Brown	Insoluble	Precipitate remains brown