Qualitative cation mixture and heavy-metal waste
A school club is given a clear, acidic unknown solution labelled 'Mixture 4'. The label states that it contains exactly two cations chosen from Pb2+, Ba2+, Cu2+, and Fe3+, each at about 0.05 M in 0.5 M HNO3. The students have 3 M HCl, 1 M H2SO4, 2 M NH3, 0.1 M KSCN, a nichrome flame wire, a centrifuge, and pH paper. A student adds 3 M HCl dropwise to 2.0 mL of the unknown until no more precipitate forms. A white precipitate appears; the mixture is centrifuged. More HCl gives no further precipitate. The student concludes 'lead' and moves on to the second cation by adding H2SO4 to the supernatant, obtaining a white precipitate. The teacher stops the group: the second white precipitate does not prove barium, and the first conclusion needs confirmation. The club must also dispose of the combined waste, which contains the identified cations and dilute nitric acid.
1. Evaluate the student's conclusion that the first cation is Pb2+. What confirmation test should be done, and what result would support Pb2+?
Read the worked answer
Within the stated list, Pb2+ is the only cation that gives a white chloride precipitate; BaCl2, CuCl2, and FeCl3 are soluble. However, the conclusion is not yet confirmed, and silver contamination from dirty glassware would also give white AgCl.
Confirm lead by dissolving the precipitate in hot water and cooling to see white needles, or by a flame test (pale blue, not decisive on its own). Also add excess HCl and centrifuge carefully: PbCl2 has appreciable solubility in excess chloride via complex formation, so some lead may remain in the supernatant and later give a false sulfate precipitate.
2. Explain why the H2SO4 precipitate does not prove Ba2+ and give a sequence to identify the second cation among Ba2+, Cu2+, and Fe3+.
Read the worked answer
The H2SO4 precipitate could be BaSO4 or PbSO4, so it does not prove barium. First test the original solution or the HCl supernatant with KSCN: a blood-red colour confirms Fe3+. Then add excess NH3: a deep blue solution confirms Cu2+. If both are absent, test for Ba2+ with a flame test (apple-green) and/or treat the sulfate precipitate with ammonium acetate: PbSO4 dissolves, BaSO4 does not.
A clean sequence is HCl precipitation for Pb2+, then KSCN and NH3 tests on the supernatant, then sulfate plus flame test for Ba2+.
3. Propose a safe treatment and disposal route for the combined acidic heavy-metal waste.
Read the worked answer
Do not pour the acidic heavy-metal waste down the sink. Dilute and partially neutralize with sodium carbonate or sodium hydroxide to about pH 8–9, adding the base slowly with stirring. Carbonate/hydroxide precipitation removes Pb2+, Ba2+, Cu2+, and Fe3+; avoid a large excess of NaOH because Pb(OH)2 is amphoteric and can redissolve as plumbite.
Filter or centrifuge, test the supernatant (KSCN for Fe3+, NH3 for Cu2+, sulfate for Ba2+/Pb2+), and if clean, neutralize and dispose according to local rules. Package the solid as heavy-metal waste, label it, and never mix it with sulfide in acid because toxic H2S would be released.