Qualitative analysis: identifying ions in solution
The qualitative analysis standard tests whether you can select and sequence tests to identify unknown cations and anions in solution, describe observations precisely, and justify the order of the tests. You are expected to know solubility rules, flame tests and precipitation reactions from memory and to eliminate ions systematically rather than guess.
Cation tests
- Flame test on a nichrome wire cleaned in concentrated HCl: Na⁺ yellow-orange, K⁺ lilac (view through cobalt glass if Na⁺ is present), Cu²⁺ blue-green, Ca²⁺ brick-red, Li⁺ crimson, Ba²⁺ apple green.
- Add dilute NaOH dropwise, then in excess. Cu²⁺: blue precipitate, dissolving in excess ammonia to a deep blue solution. Fe²⁺: dirty green. Fe³⁺: red-brown. Al³⁺: white, soluble in excess NaOH but not in excess ammonia. Zn²⁺: white, soluble in both excess NaOH and excess ammonia. Mg²⁺: white, insoluble in excess.
Anion tests
- Acidify with dilute HNO₃, then add AgNO₃: Cl⁻ white precipitate (soluble in ammonia), Br⁻ cream, I⁻ yellow.
- Add BaCl₂ to a sample acidified with dilute HNO₃: a white precipitate of BaSO₄ confirms SO₄²⁻. Acidification prevents a false positive from CO₃²⁻.
- Add dilute acid: effervescence that turns limewater milky confirms CO₃²⁻ or HCO₃⁻.
Worked example
A blue solution gives a blue precipitate with a few drops of NaOH. The precipitate dissolves in excess ammonia to give a deep blue solution, so Cu²⁺ is present. A fresh sample acidified with dilute HNO₃ gives a white precipitate with BaCl₂ that does not dissolve in acid, so SO₄²⁻ is present. The salt is copper(II) sulfate.
Traps
- Adding NaOH straight in excess, so a precipitate that dissolves is never seen.
- Using AgNO₃ without acidifying, so Ag₂CO₃ is mistaken for AgCl.
- Recording 'precipitate formed' without a colour, or calling a coloured solution 'clear' as if it were colourless.
- Relying on a flame test alone when two cations give similar colours.