Qualitative Analysis with NaOH and NH3

Learning Objectives (OCR Spec 5.3.1)

By the end of this lesson you should be able to:

• Write equations for the reaction of Cu2+, Fe2+, Fe3+, Mn2+, Cr3+ aqueous complexes with NaOH(aq)
• Predict the colours of the resulting hydroxide precipitates
• Explain the difference between reaction with dilute NH3 and excess NH3
• Describe the amphoteric behaviour of Cr(OH)3 and Al(OH)3
• Use observations to identify unknown metal ion mixtures
• Write equations showing the conversion of [M(H2O)6]^n+ to M(OH)n(H2O)6-n

The Theory: Why Hydroxide Precipitates Form

When you add OH- to an aqueous solution of a transition metal ion like [Fe(H2O)6]3+, the hydroxide ions abstract protons from the coordinated water molecules:

[Fe(H2O)6]3+ + OH- -> [Fe(H2O)5(OH)]2+ + H2O [Fe(H2O)5(OH)]2+ + OH- -> [Fe(H2O)4(OH)2]+ + H2O [Fe(H2O)4(OH)2]+ + OH- -> Fe(OH)3(H2O)3 + H2O (insoluble)

The neutral species Fe(OH)3(H2O)3 has zero charge and is no longer held in solution - it precipitates as a gelatinous solid. The overall equation is:

[Fe(H2O)6]3+(aq) + 3 OH-(aq) -> Fe(OH)3(H2O)3(s) + 3 H2O(l)

The number of OH- needed equals the charge on the original complex (+2 needs 2 OH-, +3 needs 3 OH-). The charge on the final precipitate is always zero because it has the same number of OH- as the metal's oxidation state.

Colour Summary Table

Memorise this table for the qualitative analysis of unknown metal ions:

Ion (aqueous, octahedral)	Colour of solution	Hydroxide precipitate formula	Precipitate colour	Dissolves in excess NH3?	Dissolves in excess NaOH?
[Cu(H2O)6]2+	Pale blue	Cu(OH)2(H2O)4	Pale blue	Yes - deep blue solution	No
[Fe(H2O)6]2+	Pale green	Fe(OH)2(H2O)4	Green (darkens to brown in air)	No	No
[Fe(H2O)6]3+	Pale yellow / brown	Fe(OH)3(H2O)3	Rusty brown	No	No
[Mn(H2O)6]2+	Very pale pink	Mn(OH)2(H2O)4	Off-white (darkens to brown in air)	No	No
[Cr(H2O)6]3+	Violet / green	Cr(OH)3(H2O)3	Grey-green	Yes - purple solution	Yes - dark green solution

This table answers the majority of OCR qualitative analysis questions.

The Five Key Reactions

1. Copper(II): [Cu(H2O)6]2+

[Cu(H2O)6]2+(aq) + 2 OH-(aq) -> Cu(OH)2(H2O)4(s) + 2 H2O(l)

Observation: pale blue solution turns to pale blue gelatinous precipitate.

With excess ammonia:

Cu(OH)2(H2O)4(s) + 4 NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2 H2O(l) + 2 OH-(aq)

Observation: precipitate dissolves to give a deep royal blue solution.

With excess NaOH: no change, precipitate persists.

2. Iron(II): [Fe(H2O)6]2+

[Fe(H2O)6]2+(aq) + 2 OH-(aq) -> Fe(OH)2(H2O)4(s) + 2 H2O(l)

Observation: pale green solution -> dirty green precipitate.

On standing in air, the precipitate slowly oxidises to iron(III) hydroxide:

4 Fe(OH)2(H2O)4(s) + O2(g) -> 4 Fe(OH)3(H2O)3(s) + H2O(l) (the brown outer layer visible after a few minutes)

With excess NH3: no change. With excess NaOH: no change.

3. Iron(III): [Fe(H2O)6]3+

[Fe(H2O)6]3+(aq) + 3 OH-(aq) -> Fe(OH)3(H2O)3(s) + 3 H2O(l)

Observation: pale yellow / brown solution -> rusty brown precipitate.

With excess NH3: no change, precipitate persists. With excess NaOH: no change, precipitate persists.

Note: [Fe(H2O)6]3+ itself is mildly acidic (because the high charge polarises coordinated water), so the solution has a low pH even before NaOH is added.

4. Manganese(II): [Mn(H2O)6]2+

[Mn(H2O)6]2+(aq) + 2 OH-(aq) -> Mn(OH)2(H2O)4(s) + 2 H2O(l)

Observation: nearly colourless solution -> off-white / cream precipitate. On standing in air the precipitate oxidises to MnO(OH)2 (brown) or eventually MnO2 (black). Mn2+ is so pale that the initial colour is sometimes described as "very pale pink" but is often hard to see.

With excess NH3: no change (unless Mn2+ is present at very high concentration, in which case a small amount may dissolve). With excess NaOH: no change.

5. Chromium(III): [Cr(H2O)6]3+

[Cr(H2O)6]3+(aq) + 3 OH-(aq) -> Cr(OH)3(H2O)3(s) + 3 H2O(l)

Observation: violet (or green) solution -> grey-green gelatinous precipitate.

With excess NH3:

Cr(OH)3(H2O)3(s) + 6 NH3(aq) -> [Cr(NH3)6]3+(aq) + 3 OH-(aq) + 3 H2O(l)

Observation: precipitate dissolves to give a purple solution.

With excess NaOH (chromium hydroxide is amphoteric):

Cr(OH)3(H2O)3(s) + 3 OH-(aq) -> [Cr(OH)6]3-(aq) + 3 H2O(l)

Observation: precipitate dissolves to give a dark green solution.