Common tests for detecting anions in qualitative analysis are based on characteristic reactions that produce precipitates, color changes, or gas evolution. Below is a list of important anions and their common detection tests:
1. Chloride (Cl⁻)
Test: Silver nitrate test
- Procedure: Add a few drops of dilute nitric acid followed by silver nitrate solution (AgNO₃).
- Observation: A white precipitate of silver chloride (AgCl) forms, which dissolves in ammonia.
2. Bromide (Br⁻)
Test: Silver nitrate test
- Procedure: Add dilute nitric acid and AgNO₃ solution.
- Observation: A pale yellow precipitate of silver bromide (AgBr) forms, partly soluble in ammonia.
3. Iodide (I⁻)
Test: Silver nitrate test
- Procedure: Add dilute nitric acid and AgNO₃.
- Observation: A yellow precipitate of silver iodide (AgI) forms, insoluble in ammonia.
4. Sulfate (SO₄²⁻)
Test: Barium chloride test
- Procedure: Add dilute hydrochloric acid and then barium chloride (BaCl₂) solution.
- Observation: A white precipitate of barium sulfate (BaSO₄) forms, insoluble in acid.
5. Sulfite (SO₃²⁻)
Test: Acid test
- Procedure: Add dilute HCl.
- Observation: Colorless gas (SO₂) with a pungent smell is evolved; it turns acidified potassium dichromate paper green.
6. Carbonate (CO₃²⁻)
Test: Acid test
- Procedure: Add dilute HCl.
- Observation: Effervescence due to carbon dioxide (CO₂) gas, which turns lime water milky.
7. Nitrate (NO₃⁻)
Test: Brown ring test
- Procedure: Add fresh iron(II) sulfate solution and then carefully pour concentrated sulfuric acid along the sides of the test tube.
- Observation: A brown ring forms at the junction of the two layers, confirming nitrate ions.
8. Nitrite (NO₂⁻)
Test: Starch-iodide paper test
- Procedure: Add dilute acid, then test the gas with starch-iodide paper.
- Observation: The paper turns blue, due to liberation of iodine.
9. Phosphate (PO₄³⁻)
Test: Ammonium molybdate test
- Procedure: Add nitric acid and ammonium molybdate solution, then warm gently.
- Observation: A yellow precipitate of ammonium phosphomolybdate forms.
10. Acetate (CH₃COO⁻)
Test: Ester test
- Procedure: Add ethanol and concentrated sulfuric acid, then warm.
- Observation: A sweet fruity smell of ethyl acetate indicates acetate ions.
11. Thiosulfate (S₂O₃²⁻)
Test: Acid test
- Procedure: Add dilute HCl.
- Observation: Sulfur and sulfur dioxide gas are produced; the solution becomes cloudy.
12. Cyanide (CN⁻)
Test: Prussian blue test
- Procedure: Add ferrous sulfate and then acidify.
- Observation: A blue precipitate of Prussian blue forms.
13. Fluoride (F⁻)
Test: Calcium chloride test
- Procedure: Add calcium chloride solution.
- Observation: A white precipitate of calcium fluoride (CaF₂) forms, insoluble in acid.
14. Chromate (CrO₄²⁻)
Test: Lead acetate test
- Procedure: Add lead acetate solution.
- Observation: A yellow precipitate of lead chromate (PbCrO₄) forms.
15. Oxalate (C₂O₄²⁻)
Test: Calcium chloride test
- Procedure: Add CaCl₂ solution, then warm.
- Observation: A white precipitate of calcium oxalate (CaC₂O₄) forms, insoluble in acetic acid.