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Cation qualitative analysis: identify an unknown

With hydrochloric acid, hot water, ammonia, sodium hydroxide and a few confirmatory reagents, separate and confirm seven cations one by one and work out what an unknown contains.

Difficulty ●●● ≈ 35 min

Aim Identify which of Ag⁺, Pb²⁺, Hg₂²⁺, Cu²⁺, Fe³⁺, Al³⁺ and Zn²⁺ an unknown contains, and explain every step with solubility products, formation constants and amphoterism.
Seven cations · every step computed from Ksp and formation constants

Cation qualitative-analysis bench

The chloride group (group I)

Hydroxides and ammine complexes

Unknown
Test-tube rack (solutions: computed from spectra; solids: documented colours)

Lab notebook

Start with step 1: add 6 M HCl to the unknown.

Which ions do you think the unknown contains?
This is a computed qualitative-analysis scheme. Whether each step precipitates, and how much, follows from Ksp values, formation constants and acid–base equilibria (the site’s precipitation and complex-ion kernel, Martell & Smith constants): HCl, for example, brings down only about 97 % of the Pb²⁺ (PbCl₂ is fairly soluble and forms chloro complexes), which is why the scheme removes the rest with H₂SO₄; Cu(OH)₂ dissolves completely to the deep-blue ammine only in an NH₃/NH₄⁺ buffer. A few steps are documented reaction outcomes: Hg₂Cl₂ disproportionating to Hg + HgNH₂Cl (grey-black) in ammonia, the aluminon lake, and the hot-water solubility of PbCl₂. Precipitate colours are observed colours; solution colours are computed from absorption bands through CIE colorimetry. Unknowns are generated from their number by a deterministic hash (each ion present with about 45 % probability, 0.06–0.12 M). In a real lab, handle Na₂S in a fume cupboard and mind the toxicity of mercury, lead and chromate.

Procedure

  1. Choose an unknown number. Step 1: add 6 M HCl and centrifuge. Precipitate P1 holds the group I chlorides; keep solution S1 for later.
  2. Steps 2–5 work on P1: extract PbCl₂ with hot water and test it with K₂CrO₄; treat the residue with ammonia (mercury(I) turns grey-black, AgCl dissolves); acidify the ammonia extract to test for silver.
  3. Steps 6–7 work on S1: remove leftover Pb²⁺ with H₂SO₄, then add excess NaOH to split amphoteric Al and Zn from Fe and Cu.
  4. Steps 8–12: dissolve the precipitate in acid and add ammonia (copper turns deep blue, iron stays solid and is confirmed with KSCN); acidify the solution and add ammonia (Al(OH)₃ precipitates and is confirmed with aluminon; zinc stays dissolved and is confirmed with Na₂S).
  5. Review every positive and negative observation in the notebook, tick the ions you think are present, and press “Check my answer”.

What to notice

  • Hydrochloric acid brings down only about 97 % of the lead: PbCl₂ is fairly soluble and forms chloro complexes — which is why the scheme sweeps up the rest with sulfuric acid.
  • Step 3 is yellow even without lead (the colour of chromate itself); the positive result is a yellow precipitate, not a yellow solution.
  • The deep blue of step 8 and the blood red of step 9 are both computed from absorption spectra; the more metal, the deeper the colour.
  • Try several numbers: unknowns with a single ion are easiest; with five or six ions, watch for colours masking one another.

In a real lab

  • Wash every precipitate with a little distilled water after centrifuging, or ions from one group are carried into the next and give false positives.
  • Na₂S releases highly toxic H₂S with acid and must be used in a fume cupboard; mercury, lead and chromate waste must be collected separately, never poured down the drain.
  • Gelatinous precipitates such as Al(OH)₃ centrifuge poorly; a few minutes in a hot water bath helps them coagulate.
  • Run a known positive and a blank alongside each test, to be sure the reagents work and nothing is contaminated.

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