Identification of Cations
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
Testing Cations with Sodium Hydroxide and Ammonia
A cation is a positively charged ion. Most metal cations are identified by adding aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃) separately, a few drops at a time and then in excess, and observing the colour of any precipitate and whether it dissolves in excess.
| Cation | + NaOH, dropwise | + NaOH, excess | + NH₃(aq), dropwise | + NH₃(aq), excess |
|---|---|---|---|---|
| Aluminium, Al³⁺ | White ppt | Dissolves - colourless solution | White ppt | Insoluble - stays white ppt |
| Calcium, Ca²⁺ | White ppt | Insoluble - stays white ppt | No precipitate | No precipitate |
| Chromium(III), Cr³⁺ | Grey-green ppt | Insoluble - stays grey-green ppt | Grey-green ppt | Insoluble - stays grey-green ppt |
| Copper(II), Cu²⁺ | Light blue ppt | Insoluble - stays light blue ppt | Light blue ppt | Dissolves - dark/deep blue solution |
| Iron(II), Fe²⁺ | Green ppt | Insoluble - stays green ppt | Green ppt | Insoluble - stays green ppt |
| Iron(III), Fe³⁺ | Red-brown ppt | Insoluble - stays red-brown ppt | Red-brown ppt | Insoluble - stays red-brown ppt |
| Zinc, Zn²⁺ | White ppt | Dissolves - colourless solution | White ppt | Dissolves - colourless solution |
The ammonium ion, NH₄⁺, is different: it gives no precipitate with either reagent. Instead, warming solid ammonium compounds with aqueous sodium hydroxide releases ammonia gas, identified by its pungent smell and by turning damp red litmus paper blue.
Distinguishing Aluminium from Zinc
Aluminium and zinc hydroxides both form white precipitates that dissolve in excess sodium hydroxide, so NaOH alone cannot tell them apart. Aqueous ammonia is needed to distinguish them: the aluminium precipitate stays insoluble in excess ammonia, but the zinc precipitate dissolves in excess ammonia just as it does in excess NaOH.
Worked Example
- Question (based on a real exam question): aqueous solution X gives a white precipitate with both NaOH and aqueous ammonia (dropwise), and the precipitate dissolves to a colourless solution in excess of both reagents. Which cation is present - Al³⁺, Ca²⁺, Cr³⁺, or Zn²⁺?
- Answer: Zn²⁺. Al³⁺ also dissolves in excess NaOH, but its precipitate would stay insoluble in excess ammonia. Ca²⁺ would give no precipitate with ammonia at all, and Cr³⁺ would give a grey-green (not white) precipitate. Only zinc dissolves in excess of both reagents.
Flame Tests
Some metal cations can be identified by the colour they give a Bunsen flame. A clean nichrome or platinum wire is dipped in dilute hydrochloric acid then into the sample, and held in the edge of a roaring (blue) flame.
| Cation | Flame Colour |
|---|---|
| Lithium, Li⁺ | Red |
| Sodium, Na⁺ | Yellow |
| Potassium, K⁺ | Lilac |
| Calcium, Ca²⁺ | Brick red (orange-red) |
| Barium, Ba²⁺ | Light green |
| Copper(II), Cu²⁺ | Blue-green |
Worked Example
- Question (based on a real exam question): a student carries out a flame test on aqueous barium chloride. (i) What type of Bunsen flame should be used? (ii) What is the result of the test?
- Answer: (i) a blue (roaring) flame; (ii) a light green flame colour, identifying Ba²⁺.
Common Mistakes
- ❌ Assuming sodium hydroxide alone can distinguish every cation
- ✓ Al³⁺ and Zn²⁺ behave identically with NaOH (both dissolve in excess) - aqueous ammonia must also be used to tell them apart
- ❌ Mixing up the precipitate colours for Fe²⁺ and Fe³⁺
- ✓ iron(II) gives a green precipitate; iron(III) gives a red-brown precipitate
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