When ammonia is added to a copper(II) sulfate solution, the pale blue Cu²⁺(aq) ion is progressively replaced by the deep blue tetraamminecopper(II) complex [Cu(NH₃)₄]²⁺. The intensity of this colour change can be measured precisely using a colorimeter, and the relationship between ammonia concentration and absorbance reveals the stepwise nature of complex ion formation. This makes it a visually dramatic and analytically rich IB Chemistry HL IA.

This practical is suitable for IB Diploma Chemistry HL.

Background Theory

Copper(II) ions form a series of stepwise complexes with ammonia ligands:

Cu²⁺ + NH₃ ⇌ [Cu(NH₃)]²⁺ ⇌ [Cu(NH₃)₂]²⁺ ⇌ [Cu(NH₃)₃]²⁺ ⇌ [Cu(NH₃)₄]²⁺

Each step is characterised by a successive stability constant K₁, K₂, K₃, K₄. The overall stability constant β₄ = K₁ × K₂ × K₃ × K₄ ≈ 10¹² for this system, meaning the fully complexed form strongly dominates at high [NH₃].

The pale blue Cu²⁺(aq) absorbs at ~800 nm, while the deep blue [Cu(NH₃)₄]²⁺ absorbs strongly at ~600 nm. By setting the colorimeter to 600 nm (red filter), absorbance increases as more complex is formed. Plotting absorbance vs [NH₃] gives a curve that rises steeply then plateaus — the plateau indicates complete complexation.

Variables

• Independent variable (IV): Concentration of ammonia solution (mol dm⁻³) — e.g. 0, 0.10, 0.25, 0.50, 0.75, 1.00, 1.50, 2.00, 3.00, 4.00 mol dm⁻³
• Dependent variable (DV): Absorbance at 600 nm (or red filter) measured by colorimeter
• Controlled variables (CV): Concentration and volume of CuSO₄ solution, total volume of solution (kept constant by adding distilled water), temperature, same colorimeter cuvette and filter

Equipment

• Copper(II) sulfate solution, CuSO₄ (0.10 mol dm⁻³)
• Ammonia solution, NH₃(aq) — stock at 4.0 mol dm⁻³, diluted to prepare working concentrations
• Distilled water
• Colorimeter with red filter (~600 nm) and cuvettes
• Pipettes (1 cm³, 5 cm³, 10 cm³)
• Volumetric flasks or measuring cylinders
• Small beakers or test tubes for mixing

Safety

⚠️ Ammonia solution is corrosive and gives off irritating fumes — work in a well-ventilated area or fume cupboard. Wear gloves and eye protection. Copper sulfate is harmful if ingested. Dispose of all solutions into the appropriate waste disposal bottles provided.

Method

1. Calibrate the colorimeter to zero absorbance using a cuvette of distilled water as a blank.
2. Pipette 5.0 cm³ of CuSO₄ solution into each of 10 small beakers.
3. Add ammonia solution in increasing volumes (keeping total volume constant at 10.0 cm³ by topping up with distilled water). The ammonia volume added determines [NH₃] in the final mixture — calculate this for each sample.
4. Mix each solution thoroughly and allow to stand for 2 minutes for equilibration.
5. Transfer each sample to a cuvette and measure absorbance at 600 nm (red filter).
6. Record absorbance for each [NH₃] value.
7. Repeat the full set of measurements three times using freshly prepared solutions and calculate mean absorbance.

Results Table

[NH₃] in mixture (mol dm⁻³)	Absorbance 1	Absorbance 2	Absorbance 3	Mean Absorbance
0
0.10
0.25
0.50
0.75
1.00
1.50
2.00
3.00
4.00

Analysis

1. Plot mean absorbance (y-axis) against [NH₃] (x-axis). You should observe a curve that rises steeply at low [NH₃] and then levels off (plateaus) as all Cu²⁺ is converted to [Cu(NH₃)₄]²⁺.

2. Identify the [NH₃] at which the absorbance plateaus. This is the saturation point where further ammonia produces no additional complexation.

3. Using Beer-Lambert law (A = εlc), if the path length l and molar absorptivity ε are known, calculate the concentration of [Cu(NH₃)₄]²⁺ at the plateau and confirm it equals the initial [CuSO₄].

4. The shape of the curve at low [NH₃] reflects the stepwise formation of intermediate complexes. Consider how the curve might look different if only one step existed.

Discussion Points

• Why does the solution turn deep blue rather than remaining pale blue as ammonia is added?
• Why is a red filter used rather than a blue one?
• Why does the absorbance plateau at high [NH₃]? What does this tell you about the equilibrium position?
• How does the large overall stability constant β₄ explain the sharp colour change observed?
• What would happen to the absorbance if you then added excess dilute HCl? Explain in terms of complex ion equilibria.

IA Guidance

This is a visually compelling and theoretically deep IA that connects coordination chemistry, Beer-Lambert law, and equilibrium constants. To score highly:

• Research Design: Justify your [NH₃] range by estimating the [NH₃] needed for complete complexation from β₄. Explain why total volume must be kept constant and how you will achieve this.
• Data Analysis: Include error bars. Attempt to fit a mathematical model to the rising portion of the curve — if absorbance ∝ [complex], and [complex] follows a Langmuir-type saturation function, you can linearise using a double-reciprocal (Lineweaver-Burk style) plot.
• Conclusion: Compare the [NH₃] at half-saturation to the expected value from K₁. Discuss what this reveals about the first complexation step.
• Evaluation: Discuss the assumption that ammonia does not significantly change the total volume, and the effect of NH₃ volatility on concentration accuracy. Suggest using a sealed system or freshly prepared solutions to minimise this error.

• Practical Chemistry