Qualitative Analysis:Group II Cations

What are the Group 2 Cations?

Pb2+, Hg2+, Cu2+, Bi3+, Sb3+, Cd2+, As3+, Sn4+

(Pb2+, Hg2+, Cu2+, Bi3+, Sb3+, Cd2+, As3+, Sn4+)

…form S2- precipitates that are insoluble in o.3M [H+] solutions

GROUP 2 CATIONS…

…become precipitates in acidic solutions as insoluble sulfide salts

…can be separated from mixture by adding in acidic solution hydrogen sulfide, H2S, obtained from hydrolysis of thioacetamide, CH3CSNH2:

CH3CSNH2 + 2H2O → H2S + CH3CO2- + NH4

Pb2+ + H2S → PbS + 2H+

black

Hg2+ + H2S → HgS + 2H

+

black

Cu2+ + H2S → CuS + 2H

+

black

Cd2+ + H2S → CdS + 2H

+

yellow

2Bi3+ + 3H2S → Bi 2S 3 + 6H

+

dark brown/black

2As3+ + 3H2S → As 2S 3 +

6H +

yellow

3Sb3+ + 3H2S → Sb 2S 3 +

6H +

orange red

Sn4+ + 2H2S → SnS 2 + 4H +

yellow

Group 2 Cations (sulfide salts)

Group 2 Cations are further subdivided based on the reaction of their sulfide precipitate with excess S2-.

React with excess S2-, form soluble complex

sulfides

Remain precipitates in presence of excess S2-

HgSAs2S3

Sb2S3

SnS2

PbSBi 2S 3 CdSCuS

I. Group 2 Cations PrecipitationPROCEDURE1. Place 10 drops of known/unknown

solution in evaporating dish and allow solution to evaporate until a moist residue is left.

2. Cool then add 10 drops H2O.3. Transfer to another test tube and Add

1M aqueous ammonia, NH3(aq), until red litmus paper becomes blue.

4. Add 2 drops of 6M HCl and dilute to 1.5mL.

5. Add 10 drops 5% thioacetamide, CH3CSNH2, solution, which would make the solution’s H+ concentration 0.3M.

6. Heat test tube in water bath for 10 minutes.7. Add 1 mL H2O and heat for 5 minutes.8. Centrifuge. Save the centrifugate for later

analysis.9. Wash precipitate with 10 drops 0.1M HCl

twice. Stir with glass rod, centrifuge and decant.

10. Add 10 drops of wash solution to original centrifuge and discard the rest of the wash solution.

II. Separation of Pb2+, Cu2+, Cd2+, Bi3+

•These 4 ions will remain as solid precipitates in excess S2-.

Procedure1. Add 12 drops of 4M NaOH and 4 drops

of thioacetamide solution to the precipitate from I.

2. Heat in water bath for 5 minutes.3. Centrifuge. Set aside centrifugate for

later and precipitate for III.

III. Identification of LeadProcedure1. Wash centrifugate from II with 1 mL

H20 with 2 drops 6M NH4NO3, which prevents the solution from becoming colloidal. Discard wash solution.

2. Add 15 drops 6M HNO3 to redissolve the sulfides remaining in the centrifugate.

3. Heat in water bath for a few minutes.4. Centrifuge, decant and discard

elemental sulfur formed.

5. Transfer solution to evaporating dish.6. Add 5 drops 4M H2SO4.

7. Evaporate solution under hood until white SO2 fumes appear.

8. Cool and add 1mL H2O.9. Warm solution then stir precipitates.10. Centrifuge and decant. Save centrifuge

for later.11. Wash residue with a few drops of H2O.

Discard wash solution.12. Add 5 drops 1M ammonium acetate and

1 drop 1M acetic acid.

13. Add 1 drop 1M K2CrO4. Scratch the inside wall of test tube with glass rod.

Formation of a yellow precipitate indicates the

presence of Pb2+.

IV. Identification of BismuthProcedure1. Add 15M aqueous ammonia dropwise to

centrifugate from III until it is basic to litmus (5 drops).

2. If the solution turns into a deep blue color, Cu2+ is present in the solution. If a white precipitate forms, then Bi3+ is present in the solution.

3. Centrifuge, decant and save centrifugate. 4. Add a few drops of sodium stannite solution to

precipitate. It can be prepared by placing a drop of 1M SnCl2 in a test tube, adding enough 4M NaOH to dissolve the white precipitate formed, and stirring until precipitate dissolves.

The presence of a black residue indicates the

presence of Bi3+

V. Identification of Copper and Cadmium1.Place 10 drops of centrifugate from IV in

a test tube.2. Add a few drops of 0.1M K4Fe(CN)6.

3. If a pink precipitate forms, there is Cu2+ in the solution. If there is a white precipitate, Cd2+ is probably present.

4. If Cu2+ is present, add 2 drops 1M KCN to 10 drops of the solution from IV. Continue adding KCN dropwise until solution is colorless.

5. Add 2 drops thioacetamide solution.6. Heat in water bath.7. If a yellow or olive-green precipitate

forms, cadmium is present in the solution. If not, then test another sample from IV for Cu2+ but do not add KCN.