RP9: Transition-Metal Identification

1. Principles of Aqueous Transition Metal Complex Ions

In aqueous solution, transition metal salts dissolve to form octahedral hexaaqua complex ions [M(H₂O)₆]ⁿ⁺. Because metal cations polarise the O-H bonds of coordinated water molecules, these hexaaqua ions act as weak acids (Brønsted-Lowry acid-base hydrolysis). Adding basic reagents removes protons stepwise, forming insoluble neutral metal hydroxide complexes:

9620 limits these tests to Fe²⁺, Cu²⁺, Fe³⁺ and Al³⁺ (spec 3.2.6). Co²⁺ appears in 9620 only in ligand substitution with NH₃ and Cl⁻ (3.2.5.2).

2. Diagnostic Qualitative Test Protocol

The OxfordAQA Section 6.1 practical protocol requires testing aqueous metal ion solutions systematically in clean test tubes:

1. Place 10 drops (approx. 1 cm³) of the unknown metal ion solution into three separate clean test tubes labelled 1, 2, and 3.
2. Tube 1 (Sodium Hydroxide): Add dilute 0.4 mol dm⁻³ NaOH dropwise with gentle shaking. Record initial precipitate colour. Then add excess NaOH (approx. 2 to 3 cm³). Observe whether precipitate dissolves or remains insoluble.
3. Tube 2 (Ammonia): Add dilute 2.0 mol dm⁻³ NH₃ dropwise with gentle shaking. Record initial precipitate colour. Then add excess ammonia (approx. 2 to 3 cm³). Observe whether precipitate dissolves (ligand substitution) to give a coloured solution.
4. Tube 3 (Sodium Carbonate): Add 0.5 mol dm⁻³ Na₂CO₃ dropwise. Observe whether a precipitate forms, note its colour, and carefully check for effervescence (fizzing of CO₂ gas).
5. Confirmatory Test (Conc. HCl): In a fume cupboard, add concentrated HCl dropwise to the metal ion solution. Observe ligand substitution forming tetrahedral chloro complexes (e.g. [CuCl₄]²⁻ yellow-green).

3. Precipitates, Excess Reactions, and Characteristic Colours

4. Deprotonation Reactions & Amphoteric Hydroxides

Deprotonation (Hydrolysis) by Base

Both OH⁻ and aqueous NH₃ behave as Brønsted-Lowry bases, pulling protons off the coordinated water molecules:

Because the resulting complex carries no electrical charge, it cannot be effectively hydrated by surrounding water molecules, precipitating out as a solid.

Amphoteric Hydroxide Behaviour

Aluminium(III) hydroxide is amphoteric: it reacts with both acids and excess strong bases. Chromium(III) hydroxide behaves in the same way, but this is beyond the OxfordAQA specification.

• Al(OH)₃(H₂O)₃ + OH⁻: Al(OH)₃(H₂O)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq) + 3H₂O(l) (dissolves to a colourless solution).
• (Beyond the OxfordAQA specification) Cr(OH)₃(H₂O)₃ + 3OH⁻: Cr(OH)₃(H₂O)₃(s) + 3OH⁻(aq) → [Cr(OH)₆]³⁻(aq) + 3H₂O(l) (dissolves to a dark green solution).

Importantly, ammonia (a weak base) does NOT provide a high enough concentration of hydroxide ions to dissolve aluminium hydroxide or chromium hydroxide precipitates.

5. Ligand Substitution Reactions

Excess Ammonia with Copper(II)

Unlike aluminium or iron, copper(II) hydroxide dissolves in excess concentrated ammonia because NH₃ acts as a neutral Lewis base ligand, replacing water molecules via ligand substitution:

The resulting complex ion has an elongated octahedral geometry and a vivid deep blue colour. This reaction provides a definitive qualitative identification of Cu²⁺ ions.

Excess Concentrated Hydrochloric Acid (Chloro Complexes)

Adding concentrated HCl (a source of Cl⁻ ligands) causes complete ligand displacement accompanied by a change in coordination number from 6 to 4 (octahedral to tetrahedral), driven by the larger size of chloride ligands:

6. Diagnostic Reactions with Sodium Carbonate (2+ vs 3+ Ions)

Sodium carbonate solution (Na₂CO₃) distinguishes 2+ from 3+ metal cations reliably based on the acid-base properties of the metal ions:

7. Practice Questions