2.1 - Tests for Anions in Aqueous Solutions (Leaving Cert Chemistry): Revision Notes

2.1 - Tests for Anions in Aqueous Solutions

Materials and Apparatus Required

Chemicals

• Sodium carbonate solution
• Sodium hydrogencarbonate solution
• Sodium sulphate solution
• Sodium sulfite solution
• Sodium chloride solution
• Potassium nitrate solution
• Disodium hydrogen phosphate solution
• Magnesium sulphate solution
• Barium chloride solution
• Silver nitrate solution
• Ammonium molybdate reagent
• Hydrochloric acid (dilute)
• Ammonia solution (dilute)
• Limewater
• Concentrated sulfuric acid
• Cold saturated iron(II) sulphate solution

Apparatus

• Test tubes and rack
• Dropping pipettes
• Beakers
• Wash bottle
• Bunsen burner
• Test tube holder
• Stoppers with plastic tubing (for limewater test)
• Labels

Safety Precautions

• Wear safety glasses throughout the experiment.
• Handle concentrated sulfuric acid and hydrochloric acid with care; both are corrosive.
• Ammonia solution and sulfur dioxide ($SO₂$) produced during sulfite tests can irritate the respiratory system—use a fume cupboard.
• Silver nitrate and barium chloride are harmful by ingestion and dangerous to the eyes.
• Magnesium sulfate and limewater can irritate the skin and eyes.

Method

Testing for Carbonate ($CO₃²⁻$) and Hydrogencarbonate ($HCO₃⁻$) Ions:

1. Add 2 cm³ of sodium carbonate solution to one test tube and 2 cm³ of sodium hydrogencarbonate solution to another.
2. Add 2 cm³ of dilute hydrochloric acid to each.
3. Observe and record any effervescence (gas production).
4. Use limewater to confirm if the gas produced is carbon dioxide ($CO₂$).
5. Add magnesium sulphate solution to both tubes and observe whether a precipitate forms.
6. Heat the solutions and note any additional changes.

Testing for Sulphate ($SO₄²⁻$) and Sulfite ($SO₃²⁻$) Ions:

1. Add 2 cm³ of sodium sulfate solution and 2 cm³ of sodium sulfite solution to separate test tubes.
2. Add barium chloride solution to each.
3. A white precipitate indicates either sulphate or sulfite ions.
4. Add dilute hydrochloric acid.
5. If the precipitate dissolves, it indicates sulfite ions; if it remains, sulphate ions are present.

Testing for Chloride ($Cl⁻$) Ions:

1. Add 2 cm³ of sodium chloride solution to a test tube.
2. Add silver nitrate solution.
3. A white precipitate of silver chloride indicates the presence of chloride ions.
4. Add dilute ammonia solution to dissolve the precipitate.

Testing for Nitrate ($NO₃⁻$) Ions:

1. Add 2 cm³ of potassium nitrate solution to a test tube.
2. Add 3 cm³ of cold saturated iron(II) sulfate solution.
3. Slowly add concentrated sulfuric acid down the side of the test tube without mixing.
4. A brown ring at the interface indicates nitrate ions.

Testing for Phosphate ($PO₄³⁻$) Ions:

1. Add 2 cm³ of disodium hydrogen phosphate solution to a test tube.
2. Add ammonium molybdate reagent and heat the solution gently.
3. A yellow precipitate indicates the presence of phosphate ions.
4. Add ammonia solution to confirm that the precipitate will dissolve.

Results

• Carbonate and Hydrogencarbonate: Both produce $CO₂$ gas with hydrochloric acid; carbonate forms a precipitate with magnesium sulphate, and hydrogencarbonate does not unless heated.
• Sulfate and Sulfite: Both form white precipitates with barium chloride; only the sulfite precipitate dissolves in hydrochloric acid.
• Chloride: A white precipitate of silver chloride forms with silver nitrate and dissolves in ammonia.
• Nitrate: Brown ring forms at the interface with iron(II) sulphate and sulfuric acid.
• Phosphate: Yellow precipitate forms with ammonium molybdate, which dissolves in ammonia.

Method and Results Overview Table

Anion	Test	Observation	Confirmatory Test
Carbonate ($CO₃²⁻$)	Add dilute $HCl$, collect gas, test with limewater	Effervescence, $CO₂$ turns limewater milky	Add magnesium sulfate: white precipitate forms
Hydrogencarbonate ($HCO₃⁻$)	Add dilute $HCl$, collect gas, test with limewater	Effervescence, $CO₂$ turns limewater milky	Heat with magnesium sulfate: white precipitate forms
Sulfate ($SO₄²⁻$)	Add barium chloride	White precipitate forms	Add $HCl$: precipitate remains ($BaSO₄$ is insoluble)
Sulfite ($SO₃²⁻$)	Add barium chloride	White precipitate forms	Add $HCl$: precipitate dissolves, releasing $SO₂$gas (distinct smell)
Chloride ($Cl⁻$)	Add silver nitrate	White precipitate of silver chloride ($AgCl$)	Add ammonia solution: precipitate dissolves
Nitrate ($NO₃⁻$)	Add iron(II) sulfate, then concentrated $H₂SO₄$ down the tube wall	Brown ring forms at the boundary between the liquid layers	No further confirmatory test needed
Phosphate ($PO₄³⁻$)	Add ammonium molybdate and heat	Yellow precipitate forms	Add ammonia solution: precipitate dissolves

Example Questions with Answers