Systematic Analysis of Cations (2)

Systematic Analysis of Cations

Testing for Cations

• Perform tests using salt solution to identify cations present.

Confirmatory Test for Group I: Pb2+

• Procedure: Mix salt solution with potassium chromate solution.

• Observation: Curdy yellow precipitate.

• Inference: This precipitate is soluble in boiling water and crystallizes on cooling, confirming the presence of Pb2+.

Group I Testing Observations

1. Dilute HCl added to salt solution:

   • Observation: White precipitate.

   • Inference: Group I is present, possible Pb2+.

2. Dilute HCl and H2S:

   • Observation: Black precipitate.

   • Inference: Group II present, possible Cu2+.

3. NH4Cl + NH4OH (basic medium):

   • Observations:

     • White precipitate: Possible Al3+.

     • Dirty green precipitate: Possible Fe2+.

     • Reddish-brown precipitate: Possible Fe3+.

4. NH4Cl + NH4OH + H2S (basic medium):

   • Observations:

     • Dirty white precipitate: Possible Zn2+.

     • Black precipitate: Possible Ni2+.

5. NH4Cl + NH4OH + (NH4)2CO3 (alkaline medium):

   • Observation: White precipitate.

   • Inference: Group V present, possible Ca2+.

6. If none of the above groups are present:

   • Inference: Group VI may be present, possibly Na+, K+ or NH4+, Mg2+.

Confirmatory Tests for Group II

Confirmatory Test for Cu2+

• Procedure: Add concentrated ammonia solution to salt solution in excess.

• Observation: Pale blue or bluish-green precipitate.

• Inference: Soluble in excess ammonia forming a deep blue solution, confirming Cu2+.

Confirmatory Tests for Group III

Confirmatory Test for Al3+

• Procedure: Add a drop of salt solution and a drop of dilute ethanoic acid on a watch glass, then add alizarin followed by ammonia until alkaline.

• Observation: Brick red lake forms.

• Inference: Confirms Al3+.

Confirmatory Test for Fe2+

• Procedure: Salt solution + dilute H2SO4 + potassium permanganate solution.

• Observation: Potassium permanganate loses its color.

• Inference: Confirms Fe2+.

Confirmatory Test for Fe3+

• Procedure: Salt solution + potassium permanganate solution.

• Observation: Color of potassium permanganate remains.

• Inference: Confirms Fe3+.

Confirmatory Tests for Group IV

Confirmatory Test for Zn2+

• Procedure: Salt solution + NaOH solution.

• Observation: White precipitate dissolves in excess NaOH and reappears upon heating with acetic acid and H2S.

• Inference: Confirms Zn2+.

Confirmatory Test for Ni2+

• Procedure: Salt solution + NaOH solution.

• Observation: Light green precipitate dissolves in ammonia giving a blue solution.

• Inference: Confirms Ni2+.

Confirmatory Tests for Group V and VI

Confirmatory Test for Ca2+

• Procedure: Salt solution + acetic acid + ammonium oxalate.

• Observation: White precipitate.

• Inference: Confirms Ca2+.

Confirmatory Test for Na+

• Procedure: Salt solution + KOH + potassium pyroantimonate. Scratch the sides of the test tube.

• Observation: White crystalline precipitate.

• Inference: Confirms Na+.

Confirmatory Test for K+

• Procedure: Salt solution + picric acid. Scratch the sides of the test tube.

• Observation: Yellow crystalline precipitate.

• Inference: Confirms K+.

Confirmatory Test for NH4+

• Procedure: Salt solution + Nessler’s reagent.

• Observation: Brown precipitate or brown-yellow coloration.

• Inference: Confirms NH4+.

Confirmatory Test for Mg2+

• Procedure: Salt solution + ammonium chloride + ammonium hydroxide + ammonium dihydrogen phosphate.

• Observation: White precipitate insoluble in excess of ammonium dihydrogen phosphate.

• Inference: Confirms Mg2+.