(ANALYSIS OF THE GROUP II METALS)

Pb, Bi, Cu, Cd, Hg, As, Sb, Sn


PROCEDURE 1: Precipitation of Group II: Pb+2, Bi+3, Cu+2, Cd+2, Hg+2, As+3, Sb+3, Sn+2, and Sn+4. Add 5 drops of 4M nitric acid (HNO3) to 10 drops of the known or unknown (or to the centrifugate from the group I separation) and evaporate the solution to a moist residue. Cool and add 10 drops of water. Add 1M ammonia hydroxide (NH4OH) until the solution is just basic to litmus. Add 1M hydrochloric acid (HCl) until the solution is just acidic to litmus. Now add 2 drops of 6M HCl and dilute the solution to 1.5 ml. Add 10 drops of 5% thioacetamide solution (CH3CSNH2). (The solution should now be 0.3M with respect to the hydrogen ion if the directions have been followed carefully). Heat the solution contained in a test tube in a hot water bath for at least 5 minutes. Add 1 ml. of H2O and heat the mixture for another 5 minutes. Separate the precipitate and reserve the centrifugate for the analysis of Groups III-V. Wash the precipitate with 10 drops of 1M HCl twice. Add the first 10 drops of wash solution to the original centrifugate and discard the rest.

PROCEDURE 2: Precipitate from (1): PbS, Bi2S3, CuS, CdS, HgS, As2S3,Sb2S3, and SnS2. Add a mixture of 12 drops of 4M sodium hydroxide (NaOH) and 4 drops of 5% thioacetamide solution to the precipitate. Heat the mixture in a hot water bath for 5 minutes. Separate the mixture and reserve the solution for procedure 6. The residue consists of the sulfides of the ions of Division A (Pb, Bi, Cu, and Cd).

PROCEDURE 3: Residue from (2): PbS, CuS, CdS, Bi2S3. Wash the residue with 1 ml. of water to which 2 drops of 1M ammonium nitrate (NH4NO3) have been added. Discard the wash solution. Add 15 drops of 6M HNO3 to the residue and heat the mixture in a hot water bath to dissolve the sulfides. Separate and discard any sulfur which is formed by the oxidation of sulfide ions and transfer the solution to a casserole or evaporating dish. Add 5 drops of 4M sulfuric acid (H2SO4) and evaporate the solution under the hood to white SO3 fumes (important). Cool and add 1 ml. of H2O to the mixture. Warm the mixture and stir up the precipitate. Separate the mixture. Reserve the solution for procedure 4. Wash the residue lead sulfate (PbSO4) with a few drops of water and discard the wash solution. Extract the residue with a mixture of 5 drops of 1M ammonium acetate (NH4C2H3O2) and 1 drop of 1M acetic acid (CH3COOH). Add 1 drop of 1M K2CrO4 to the extract. The formation of a yellow precipitate confirms the presence of lead as lead chromate, PbCrO4.

PROCEDURE 4: Solution from (3): Bi+3, Cu+2, and Cd+2. Add 15M aq. ammonia dropwise to the solution until it is basic to litmus (about 5 drops). The development of a deep blue color in the solution indicates the presence of tetraaminecopper (II) ions, Cu(NH4)4. The formation of a white precipitate indicates the presence of Bi as bismuth hydroxide, Bi(OH)3. Separate the mixture and reserve the solution for procedure 5. Add several drops of freshly prepared sodium stannite solution to the precipitate. The immediate formation of a black residue confirms the presence of bismuth. (Preparation of Sodium Stannite: In a separate test tube place 1 drop of 1M stannous chloride (SnCl2) and add sufficient 4M NaOH to dissolve the white precipitate which first forms - stir well).

PROCEDURE 5: Solution from (4): Cu(NH4)4+2 blue and Cd(NH4)4+2 colorless. If the solution is colorless a trace of copper may still be present. Place 10 drops of the solution in a test tube and add several drops of 0.1M potassium ferrocyanide (K4Fe(CN)6). The formation of a pink precipitate Cu2Fe(CN)6 indicates the presence of a small concentration of copper. The formation of a white precipitate indicates the absence of Cu and the probable presence of Cd as Cd2Fe(CN)6. If copper II ions are present add 2 drops of 1M potassium cyanide (KCN) and 2 drops of thioacetamide solution to the remainder of the solution from procedure 4. Heat the mixture in a hot water bath. The formation of a yellow precipitate CdS confirms the presence of cadmium. If Cu is found to be absent, test for Cd as described before but leave out the KCN.

PROCEDURE 6: Solution from (2): Thiosalts of Division B ions. HgS2-2, AsS3-3, SbS3-3, and SnS2-2. Add 1M HCl until the solution is just acidic to litmus. Heat the mixture in a hot water bath for several minutes. Separate the mixture and discard the solution. The presence of HgS, As2S3, Sb2S3, and SnS2 may be found in the precipitate.

PROCEDURE 7: Precipitate from (6): HgS, As2S3, Sb2S3, and SnS2. Add 1 ml. of 6M HCl to the precipitate and stir the mixture. Heat the mixture in a hot water bath and then separate the residue. Add 15 drops of 6M HCl to the residue and reserve the combined centrifugates for procedure 11. Treat the residue according to procedure 8.

PROCEDURE 8: Residue from (7): HgS and As2S3. Add 12 drops of 4M aq. ammonia and 6 drops of 3% hydrogen peroxide (H2O2) to the residue. Stir the mixture and heat it in a hot water bath 5-6 minutes. Separate the residue HgS and reserve the solution for procedure 10.

PROCEDURE 9: Residue from (8): HgS. To the black residue add 6 drops of 5% sodium hypochlorite (NaClO) and 2 drops of 6M HCl. Stir the mixture. Add 1 ml. of water and separate the sulfur from the solution. The formation of a white, gray, or black precipitate confirms the presence of mercury.

PROCEDURE 10: Solution from (8): AsO4-3. Add 2 drops of 15M aq. ammonia and 5 drops of Magnesia mixture to the solution. The formation of a white precipitate MgNH4AsO4 frequently slow in forming, indicates arsenic.

PROCEDURE 11: Solution from (7): SnCl6-2 and SbCl4-. Boil the solution until all the hydrogen sulfide (H2S) (rotten egg gas) has been expelled. Add a volume of water equal to that of the solution and add 2 drops of 6M HCl. Place a piece of aluminum wire about 1/4 inches long in the solution and heat it for a few minutes. If Sb is present, black flakes of the metal will appear. Separate the mixture. Treat the black flakes with 3 drops of 4M HNO3 and several drops of 1M oxalic acid (H2C2O4). Add 2 drops of thioacetamide to the solution and place the test tube in hot water. The formation of a red-orange precipitate of Sb2S3 confirms the presence of antimony. To the solution obtained add a few drops of 0.2M HgCl2. A white, gray or black precipitate confirms the presence of tin.


Created by James R. Fromm (jfromm@3rd1000.com)