THE ANALYSIS OF THE GROUP III METALS

(Co, Ni, Mn, Fe, Al, Cr, Zn)


PROCEDURE 1. Precipitation of Group III. (a) If the solution to be analyzed is a known or unknown for Group III only, take 10 drops of the solution, add 1 drop of 6M HCl, dilute to 1 ml. and add 5 drops of 5% thioacetamide solution. Heat the solution in a hot H2O bath for at least 5 minutes. (b) If the solution to be analyzed is that from the Group II separation, add 5 drops of 5% thioacetamide solution and heat the mixture in a hot water bath for at least 5 minutes. To the solution resulting from either procedure (a) or (b) above, add 5 drops of 15M aq. ammonia and stir up the precipitate. Heat the mixture for 5 minutes in the hot water bath. Separate the precipitate and wash it with a few drops of H2O. Reserve the solution for the analysis of Groups IV and V.

PROCEDURE 2. Precipitate from (1): CoS, NiS, FeS, MnS, Al(OH)3, Cr(OH)3, ZnS. Add 10 drops of 1M HCl to the precipitate and stir the mixture immediately because prolonged contact with the acid causes some dissolution of CoS and NiS. Wash the sulfides which remain (CoS & NiS) with 4 drops of 1M HCl. Reserve the combined centrifugates for procedure (4).

PROCEDURE 3. Residue from (2): CoS and NiS. Add 3 drops of 12 M HCl and 1 drop of 14M HNO3 to the residue and heat the mixture in a hot H2O bath. Separate any sulfur that forms and boil the solution to remove any excess nitric acid or oxides of nitrogen. Add sufficient 4M aq. ammonia to the solution to make it slightly basic to litmus. Dilute the solution to1 ml. and divide it into three parts.

3a. Test for Nickel. Add one drop of dimethylglyoxime to one part of the solution. The formation of a pink or red precipitate confirms the presence of Nickel.

3b. Test for Cobalt. Acidify a second portion of the solution with 1M HCl and add several crystals of NH4CNS to it. Now add an equal volume of acetone and agitate the mixture. The development of a blue color proves the presence of cobalt. If the solution becomes red upon the addition of ammonium thiocyanide (NH4CNS), Iron (III) ions are present. Add 1 drop of 1M sodium fluoride (NaF) to the solution. Now, if the solution is bluish-green to green, the presence of cobalt is confirmed.

3c. Test for Cobalt. Acidify the third portion of the solution from (3) with 4M acetic acid (CH3COOH), and add several large crystals of potassium nitrite (KNO2). Warm the mixture. The formation of a yellow precipitate confirms the presence of cobalt.

PROCEDURE 4. Solution from (2): Mn++, Fe++, Al+++, Cr+++, Zn++. Transfer the solution to a casserole, add 1 ml. of 4 M HNO3, and evaporate the solution to a moist residue. Take up the residue in 1 ml. of H2O and transfer the solution to a test tube. Add 10 drops of 4M NaOH beyond the amount of this reagent that is required to initiate precipitation. Now add 6 drops of 3% hydrogen peroxide (H2O2) to the mixture and heat it in a hot H2O bath for 5 minutes. Separate the mixture and wash the residue with 10 drops of water to which has been added 1 drop of 4M NaOH. Save the combined centrifugates for procedure (6).

PROCEDURE 5. Residue from (4): Mn(OH)3, MnO2, Fe(OH)3. Treat the residue with 1 ml. of 4M HNO3 and 2 drops of 1M sodium nitrite (NaNO2). Stir the mixture and heat it in a hot water bath. Separate any residue that remains. Heat the solution to boiling, then cool it, and divide it into two parts.

5a. Test for Iron. Dilute one part of the solution to 1 ml. and add 2 or 3 crystals of NH4CNS. If iron is present, a red color will develop in the solution.

5b. Test for Manganese. To the 2nd part of the solution from procedure 5 add a small quantity of solid sodium bismuthate (NaBiO3) and a few drops of 4M HNO3. The formation of a pink or purple color confirms the presence of manganese.

PROCEDURE 6. Solution from (4): Al(OH)4-, CrO4-2, Zn(OH)4-2. Add 4M acetic acid to the solution until it is acid to litmus and then add two or three drops of the acid in excess. Now add 4M aq. ammonia until the solution is distinctly alkaline to litmus. If a white glutinous precipitate forms, it is probably aluminum hydroxide. Separate the precipitate and reserve the solution for procedure (7). Confirm the presence of aluminum by dissolving the precipitate in 4M CH3COOH and adding 2 drops of Aluminum Reagent and enough 1M ammonium carbonate (NH4)2CO3 to make the solution basic. The formation of a reddish-colored precipitate confirms the presence of aluminum.

PROCEDURE 7. Solution from (6): CrO4-2, Zn(OH)4-2. Divide the solution into two parts.

7a. Test for Chromium. To one part of the solution add 1M acetic acid until the solution is acid to litmus. Then add 2 drops of 0.1M Pb(C2H3O2)2. The formation of a yellow precipitate confirms the presence of chromium.

7b. Test for Zinc. To the other part add 5 drops of 5% thioacetamide and heat in a hot water bath. The formation of a white precipitate, ZnS, which is soluble in 4M HCl, indicates the presence of zinc. Heat the solution to expel H2S and then neutralize it with 4 M aq. ammonia. Add 10 drops of 1M HCl and 5 drops 0.1M K4Fe(CN)6. The formation of a white precipitate K2ZnFe(CN)5, proves the presence of zinc.


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