Cations are identified by the colour of the hydroxide precipitate with sodium hydroxide and ammonia, e.g. Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) brown, then confirmed with reagents such as potassium iodide for Pb2+ or the brown ring test for the nitrate ion.
Qualitative analysis identifies the ions in an unknown salt, and the cation is usually found by adding two alkalis in turn, sodium hydroxide solution and ammonia solution, and reading the colour of the metal hydroxide precipitate and whether it dissolves in excess. A confirmatory test then pins down ions that look alike. These reactions, and their ionic equations, are examined every year in 4541.
Precipitates with sodium hydroxide
Adding sodium hydroxide to the salt solution precipitates the metal hydroxide. The colour identifies many cations:
- Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s), green precipitate
- Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s), brown precipitate
- Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s), blue precipitate
- Pb2+(aq) + 2OH−(aq) → Pb(OH)2(s), white, dissolves in excess
- Al3+(aq) + 3OH−(aq) → Al(OH)3(s), white, dissolves in excess
- Zn2+(aq) + 2OH−(aq) → Zn(OH)2(s), white, dissolves in excess
- Mg2+(aq) + 2OH−(aq) → Mg(OH)2(s), white, insoluble in excess
- Ca2+(aq) + 2OH−(aq) → Ca(OH)2(s), white, insoluble in excess
The white precipitates of aluminium, lead(II) and zinc all dissolve in excess sodium hydroxide, so this step alone cannot separate them.
Precipitates with ammonia solution
Repeating the test with ammonia solution separates the look-alikes:
- Zinc hydroxide dissolves in excess ammonia to give a colourless solution, but aluminium and lead(II) hydroxides do not.
- Copper(II) hydroxide dissolves in excess ammonia to give a deep blue solution, a useful confirmation of the copper(II) ion.
So the pair of tests, sodium hydroxide then ammonia, narrows the cation down before a confirmatory test finishes the job.
Confirmatory tests
- Iron(II), Fe2+: add potassium hexacyanoferrate(III) solution, a dark blue precipitate confirms Fe2+.
- Iron(III), Fe3+: add potassium thiocyanate solution, a blood-red colouration confirms Fe3+.
- Lead(II), Pb2+: add potassium iodide solution, a yellow precipitate confirms Pb2+: Pb2+(aq) + 2I−(aq) → PbI2(s).
- Ammonium, NH4+: warm with sodium hydroxide, ammonia gas is given off, turning moist red litmus blue.
The brown ring test
The brown ring test confirms the nitrate ion, NO3−, and is often examined alongside the cation tests. Add freshly prepared iron(II) sulfate solution to the test solution, then carefully pour concentrated sulfuric acid down the inside of the tilted tube so it forms a separate lower layer. A brown ring appears at the junction of the two liquids, confirming the nitrate ion. Handle the concentrated acid with care, adding it slowly down the side.
Common mistakes to avoid
Give the charge on each ion correctly, Fe2+ versus Fe3+, Pb2+, and balance the hydroxide so the ionic equation is right. State both the colour of the precipitate and whether it dissolves in excess; for the three white precipitates, that solubility is the whole point. Do not stop at the precipitate when a confirmatory test is asked for. For the brown ring test, add the concentrated sulfuric acid slowly and down the side, and describe the ring at the junction.
How it appears in the SPM exam
In 4541/2 you may identify a cation from precipitate colours and solubility, write the ionic equation for a hydroxide precipitate, or give the confirmatory reagent and observation. In 4541/1, objective items test precipitate colours and confirmatory results. In 4541/3, a qualitative-analysis practical asks you to add the reagents, record the observations, and deduce the ion. Work through sodium hydroxide, then ammonia, then the confirmatory test, and cation identification becomes a reliable routine.
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