QA - Test for cations

Introduction

This tutorial provides a step-by-step guide on how to identify various cations in solution using aqueous sodium hydroxide and aqueous ammonia. Understanding these tests is crucial for chemistry students, particularly in distinguishing between cations such as ammonium, calcium, copper(II), iron(II), iron(III), lead(II), and zinc.

Step 1: Prepare Your Materials

Before starting the tests, ensure you have the following materials ready:

• Aqueous sodium hydroxide (NaOH)
• Aqueous ammonia (NH4OH)
• Test samples containing the cations (aluminium, ammonium, calcium, copper(II), iron(II), iron(III), lead(II), and zinc)
• Test tubes or beakers
• Pipettes or droppers
• Safety goggles and gloves for personal protection

Step 2: Conducting the Sodium Hydroxide Test

Use aqueous sodium hydroxide to identify specific cations. Follow these steps for accurate results:

1. Add a few drops of NaOH to a test sample in a test tube.
2. Observe the reactions and look for precipitate formation:
   • Aluminium: White precipitate (Al(OH)3) that dissolves in excess NaOH.
   • Calcium: White precipitate (Ca(OH)2) that does not dissolve.
   • Copper(II): Blue precipitate (Cu(OH)2).
   • Iron(II): Green precipitate (Fe(OH)2) that may turn brown on exposure to air.
   • Iron(III): Reddish-brown precipitate (Fe(OH)3).
   • Lead(II): White precipitate (Pb(OH)2) that dissolves in excess NaOH.
   • Zinc: White precipitate (Zn(OH)2) that dissolves in excess NaOH.

Step 3: Conducting the Ammonia Test

Next, use aqueous ammonia to further identify cations that may not have been confirmed in the previous step. Follow these steps:

1. Add a few drops of NH4OH to the same test sample in a different test tube.
2. Observe the reactions and note any precipitate formation:
   • Copper(II): Blue precipitate (Cu(OH)2) that may dissolve to form a deep blue solution in excess ammonia.
   • Zinc: White precipitate (Zn(OH)2) that dissolves in excess ammonia to form a colorless solution.
   • Lead(II): White precipitate (Pb(OH)2) may also dissolve in excess ammonia.

Step 4: Confirming Ammonium Ion Presence

To confirm the presence of ammonium ions, perform the following:

1. Add a few drops of NaOH to a test sample suspected of containing ammonium ions.
2. Heat the mixture gently.
3. Test for ammonia gas:
   • Use litmus paper; the release of ammonia will turn red litmus paper blue, confirming the presence of ammonium ions.

Conclusion

In this tutorial, we covered how to utilize aqueous sodium hydroxide and aqueous ammonia to identify various cations. The key steps involve preparing your materials, conducting tests with both reagents, and confirming the presence of ammonium ions. Understanding these methods is essential for practical chemistry applications and helps build a strong foundation for further studies in the field. Be sure to practice these tests in a safe environment, following all safety protocols.