Question

Design a separation scheme to separate cations, Al3+, Cu2+, Ni2+, Ag+.

Answer 1

First, ions are removed in groups from the initial aqueous solution. After each group has been separated, then testing is conducted for the individual ions in each group. Here is a common grouping of cations

Group I: Ag⁺, Hg₂²⁺, Pb²⁺
Precipitated in 1 M HCl

Group II: Al3+, (Cd2+), Co2+, Cr3+, Fe2+ and Fe3+, Mn2+, Ni2+, Zn2+
Precipitated in 0.1 M H2S solution at pH 9

4. Ag⁺

Add 15 drops of 6M NH3•H2O to the centrifuge test tube with 15 drops of Group I solution.

[Cu(NH3)4] 2+(aq) + 4H+ (aq) → Cu2+(aq) + 4NH4 + (aq)

[Ag(NH3)2] + (aq) + 2H+ (aq) → Ag+ (aq) + 2NH4 + (aq)

10 drops of 6 M HCl to the supernatant in part B and stir the solution.

Ag+ (aq) + Cl– (aq) → AgCl(s)

white precipitate shows that Ag+ is most likely present. Centrifuge, decant, and save the supernatant . If copper ions are present, the solution may appear light blue due to the presence of [Cu(H2O)6] ²⁺.

Add 20 drops of distilled water to the white precipitate. Stir and centrifuge the solution. Discard the water. Add 5 drops of 6M NH3·H2O to the rinse water. If the rinse water turns blue then Cu2+ present, you must rinse the precipitate repeatedly until the rinse water doesn’t turn blue upon the addition of ammonia. The rinsing is to remove copper(II) ions, which cause the blue color.

Add 20 drops of 6M NH3•H2O To the remaining precipitate. Stir the solution. If the precipitate dissolves, then you have confirmed that Ag+ is present.

AgCl(s) + 2NH3(aq) → [Ag(NH3)2] + (aq) + Cl– (aq)

D. Confirmation of Cu2+ There are two tests to confirm the presence of copper(II) ions.

The first confirmation test: To one-half of the supernatant you saved from part C, add 10 drops of 6 M NH3·H2O. A dark blue solution confirms the presence of Cu2+.

2. The addition of ammonia to a Cu2+ solution causes the formation of the dark blue complex ion[Cu(NH3)4] 2+. When potassium hexacyanoferrate(II) is added to a Cu2+ solution, a reddish precipitate of copper(II) hexacyanoferrate(II) is formed.

Cu2+(aq) + 4NH3(aq) → [Cu(NH3)4] 2+(aq)

dark blue 2Cu2+(aq) + [Fe(CN)6] 4–(aq) → Cu2[Fe(CN)6](s) reddish-purple

1 and 3 Al3+ , Ni2+

Place 20 drops of the Group II (known or unknown) solution into a clean centrifuge test tube and add 20 drops of 3M NaOH. Heat in a boiling water bath for 10 min.

The equations

Ni2+(aq) + 2OH– (aq) → Ni(OH)2(s)

Al3+(aq) + 4OH– (aq) → [Al(OH)4] – (aq) (aluminate ion)

Save the precipitate and the supernatant. Label the supernatant Aluminum and nickel.

The Confirmation of Ni2+

4 drops of dimethylglyoxime (DMG) to the tube labeled "Nickel" and stir. There is no need to centrifuge, as the nature of the precipitate, is more obvious before centrifuging. A strawberry-colored ppt confirms the presence of Ni2+. The equation

[Ni(NH3)6] 2+(aq) + 2H2DMG → [Ni(HDMG)2](s) + 2NH4 + (aq) + 4NH3(aq) pink-red

conformation of Al3+

1. Add 10 drops of 6 M HNO3 to the solution from part A above labeled "Al & Ni" and stir. This will breakup the hydroxide complexes.

2. Now add 10 drops of 6M NH3•H2O to the solution and stir. Check the solution with universal indicator paper to be sure it is basic (dark blue).

3. If the solution is acidic, add 10 more drops of 6M NH3 ·H2O. Stir and re-check the acidity of the solution. Continue adding 6M NH3 ·H2O until the solution is basic. You should get a clear gelatinous precipitate. It may be hard to see until the tube is centrifuged. Even then, some people have difficulty seeing the precipitate.

4. Centrifuge and decant the supernatant into a clean tube. Label this tube "Zinc."

5. Add 5 drops of 6M HNO3 to the precipitate that supposedly contains aluminum(III) ions and stir until it dissolves. If it looks like a precipitate is still present, test the solution for acidity with pH paper. If the solution is not acidic, add 6M HNO3 one drop at a time, with stirring, until the solution is acidic.

6. Now add 4 drops of aluminon reagent and 10 drops of 6 M NH3•H2O. Stir and centrifuge. A cherrycolored precipitate confirms the presence of Al3+.

Al3+(aq) + 3NH3(g) + 3H2O(l) + aluminon → [Al(OH)3•aluminon](s) + 3NH4 + (aq) reddish-pink