Qualitative analysis: cations

A/L Chemistry · G.C.E. Advanced Level · Grades 12–13

Aim

To identify cations from their reactions with NaOH and NH₃.

Apparatus

• Test tubes and a dropper
• NaOH and NH₃ solutions
• Cation solutions and damp litmus

Method

1. Add a few drops of NaOH and note the precipitate colour.
2. Add excess and see whether it dissolves.
3. Repeat with NH₃.
4. Warm with NaOH and test for NH₃ gas.
5. Identify the unknown cation.

Conclusion

Cu²⁺ pale blue → deep blue in excess NH₃; Fe²⁺ green; Fe³⁺ red-brown; Al³⁺ and Zn²⁺ white, dissolving in excess NaOH (Zn²⁺ also in excess NH₃); NH₄⁺ gives NH₃ on warming.