Question
How do you systematically identify common anions — carbonate, sulphate, chloride, and nitrate — using chemical tests?
(JEE Main and CBSE 12 — anion analysis is tested alongside cation analysis)
Solution — Step by Step
Add dilute H2SO4 to the salt and observe:
- Brisk effervescence with CO2 (turns lime water milky) → Carbonate (CO3 2-) or Bicarbonate (HCO3-)
- Colourless gas with rotten egg smell (turns lead acetate paper black) → Sulphide (S2-)
- Colourless pungent gas (turns moist litmus red) → Sulphite (SO3 2-)
- Brown fumes → Nitrite (NO2-)
| Anion | Test | Observation |
|---|---|---|
| CO3 2- | Dil. HCl + lime water | Effervescence, lime water turns milky |
| SO4 2- | BaCl2 solution + dil. HCl | White precipitate (BaSO4) insoluble in HCl |
| Cl- | AgNO3 solution + dil. HNO3 | White precipitate (AgCl) soluble in NH4OH |
| Br- | AgNO3 solution | Pale yellow precipitate (AgBr), partially soluble in NH4OH |
| I- | AgNO3 solution | Yellow precipitate (AgI), insoluble in NH4OH |
| NO3- | Brown ring test (FeSO4 + conc. H2SO4) | Brown ring at junction of two layers |
| PO4 3- | Ammonium molybdate + conc. HNO3 | Yellow precipitate on warming |
For nitrate detection:
- Dissolve the salt in water
- Add freshly prepared FeSO4 solution
- Carefully pour concentrated H2SO4 along the side of the test tube (it should form a separate layer)
- A brown ring forms at the junction:
graph TD
A["Unknown Anion"] --> B{"Add dil. H2SO4"}
B -->|"Effervescence + CO2"| C["Carbonate"]
B -->|"Rotten egg smell"| D["Sulphide"]
B -->|"No gas"| E{"Add BaCl2"}
E -->|"White ppt insoluble in HCl"| F["Sulphate"]
E -->|"No ppt"| G{"Add AgNO3"}
G -->|"White ppt, soluble in NH4OH"| H["Chloride"]
G -->|"No ppt"| I{"Brown ring test"}
I -->|"Brown ring"| J["Nitrate"]Why This Works
Anion identification relies on forming characteristic insoluble precipitates or coloured compounds. BaSO4 is one of the most insoluble salts known — adding BaCl2 to a sulphate solution gives an instant white precipitate that resists acid dissolution. AgCl is also insoluble but dissolves in ammonia (forming the soluble diamine silver complex [Ag(NH3)2]+).
The brown ring test exploits the ability of NO (nitric oxide, produced by reducing nitrate with Fe2+) to form a coloured complex with Fe2+. This complex is unstable, so the test must be read quickly.
Alternative Method
For JEE Main, the solubility in NH4OH distinguishes halide precipitates:
- AgCl — soluble in NH4OH (forms [Ag(NH3)2]+)
- AgBr — partially soluble in NH4OH
- AgI — insoluble in NH4OH
This solubility trend is tested as a matching question. Remember: as the halide gets heavier, the AgX becomes less soluble in ammonia.
Common Mistake
The most common error in the sulphate test: students add BaCl2 without first acidifying with dilute HCl. Without acid, carbonates, sulphites, and phosphates can also form white precipitates with Ba2+ — giving false positives for sulphate. Adding HCl dissolves these other precipitates (BaCO3, BaSO3 dissolve in acid) while BaSO4 remains insoluble.
Also, in the brown ring test, the concentrated H2SO4 must be added slowly along the wall to form a separate layer. If mixed directly, the test fails because the FeSO4 gets oxidised by the concentrated acid.