Ariel ChenQualitative Analysis of Cations Lab Report

Known Solution Unknown Solution (#1)Step Procedure Results Conclusion Results Conclusion

1Add HCl to known solution in TT1. Centrifuge. Pour colored solution into TT2.

White ppt forms. Other ions present in solution in TT2

Ag+, Pb2+, and/or Hg2

2+

present

White ppt forms. Other ions present in solution in TT2

Ag+, Pb2+, and/or Hg2

2+

present

2

Add 2 ml of distilled water to the precipitate and place the test tube in a boiling water bath for three minutes. Stir. Centrifuge for a few seconds. Pour the supernatant liquid into another test tube. Immediately add two drops of K2CrO4 to the liquid.

Yellow precipitate

Pb2+ presentYellow precipitate

Pb2+ present

3

Add one ml of NH3 to the white precipitate from Step 2. Stir. Black residue. Centrifuge, then decant the solution into another test tube and discard the black residue.

Black residueHg2

2+

presentNo white ppt from Step 2

Hg22+ and

Ag+ both absent

4

Add HNO3 to the solution from Step 3 until the solution is acid or a permanent white precipitate is formed.

White precipitate

Ag+ present

5

Neutralize the solution from procedure 1 (TT2) by adding NH3 solution until the solution is basic. Check with litmus paper after stirring thoroughly. Add one ml in excess. Add one ml of NH4Cl. Colored ppt. Centrifuge and separate the solution into another test tube. Set the solution aside for Procedure 10. Treat the precipitate by Procedure 6.

Colored pptFe3+ and/or Al3+ present

Colored pptFe3+ and/or Al3+ present

6

Dissolve the precipitate from Procedure 5 by adding 6-M HC1 dropwise. Stir añer each addition. Make basic to litmus with 6-M NaOH. Stir. Then add 1-2 ml in excess. Precipitates. Centrifuge and separate the solution into another test tube. Set the solution aside for Procedure 8. Treat the precipitate by Procedure 7.

Precipitate forms

Precipitate: Fe(OH)3

Precipitate forms

Precipitate: Fe(OH)3

7 Dissolve the precipitate from Procedure 6 by adding dilute 6-M HC1 dropwise. Stir. Add three drops

Deep red color: Fe(SCN)2+

Fe3+ present Deep red color: Fe(SCN)2+

Fe3+ present

Ariel ChenQualitative Analysis of Cations Lab Report

of 0.1-M KSCN solution.

8

Acidify the solution from Procedure 6 by adding dilute HC1 dropwise. Stir after eachaddition and check with litmus. Add drops of NH4Cl and make basic by adding NH3 solution. White/gray ppt. Centrifuge and separate thesolution. Discard the solution and treat the precipitate by Procedure 9.

White/gray ppt

Al(OH)3 White/gray ppt Al(OH)3

9

Dissolve the precipitate from Procedure 8 by adding dilute 6-M HCl dropwise. Stir. Add one ml of the aluminon test reagent. Add 6-M NH3 solution until the solution is just basic. Stir. Check with litmus. Warm the solution in a water bath.

Red ppt (“lake”)

Al3+ presentRed ppt (“lake”)

Al3+ present

10

Add 10 to l5 drops of dimethylglyoxime to the solution from Procedure 5. Warm in a water bath.

Red ppt Ni+ present No ppt Ni+ absent

Unknown Solution #1Ions present: Al3+, Fe3+, Pb2+ Ions absent: Ag+ , Hg2

2+ , Ni+

Discussion1. Cl- is the precipitating reagent for the silver group. A solution of NaCl would also work as well, because it

provides the chloride ion in solution.2. Ag+(aq) + Cl-(aq) →AgCl(s)

AgCl(aq) + 2NH3(aq) → Ag(NH3)2(aq) + Cl-(aq) Ag(NH3)2

+(aq) + Cl-(aq) + 2H+(aq) → AgCl(s) + 2NH4+(aq)

3. Hg ⁺ + e⁻ ←→ HgHg ⁺ ←→ Hg² ⁺ + e⁻

4. AgCl(s)  ←→  Ag+(aq) + Cl-(aq) AgCl(s) + 2 NH3(aq) ←→Ag(NH3)2

+(aq) + Cl-(aq)5. Fe3+(aq) + 3NH3(aq) + 3H2O --> Fe(OH)3(s) + 3NH4

+(aq)Fe3+ (aq) + 6NH3(aq) → Fe(NH3)6

3+(aq)6. Al(OH)3 (s) + OH- (aq) ---- > Al(OH)4

- (aq)Al(OH)3 + 3H2O + 3H+ ---> [Al(H2O)6]3+