2 – Detecting the Presence of Anions (LC 2027) (Leaving Cert Chemistry): Revision Notes

2 – Detecting the Presence of Anions

Introduction to anion detection

Anions are negatively charged ions that can be detected in aqueous solutions through qualitative analysis. Qualitative analysis helps us determine which particular substances are present in a solution, rather than measuring how much of each substance is there. These detection methods rely on characteristic reactions that produce observable changes such as colour changes, precipitate formation, or gas evolution.

The key anions you need to know how to detect are:

• Chloride ions (Cl⁻)
• Sulphate ions (SO₄²⁻) and sulfite ions (SO₃²⁻)
• Carbonate ions (CO₃²⁻) and hydrogencarbonate ions (HCO₃⁻)
• Nitrate ions (NO₃⁻)
• Phosphate ions (PO₄³⁻)

Detection of chloride ions (Cl⁻)

The standard test for chloride ions uses silver nitrate solution as the detecting reagent. This test works because silver chloride is insoluble in water and forms a characteristic white precipitate.

Procedure

1. Dissolve the chloride salt in deionised water to create a clear solution
2. Add a few drops of silver nitrate solution to the chloride salt solution
3. Observe the formation of a white precipitate

Observation

The clear solution becomes cloudy due to the formation of a white solid that settles out of solution.

Explanation

When silver nitrate solution mixes with chloride ions, an insoluble white precipitate of silver chloride forms. This is a precipitation reaction where an insoluble substance is produced from two soluble starting materials.

Confirmatory test

To confirm the white precipitate is silver chloride, dilute ammonia solution can be added to the cloudy solution. The cloudiness disappears because silver chloride dissolves in dilute ammonia solution.

Detection of sulphate (SO₄²⁻) and sulfite (SO₃²⁻) ions

Both sulphate and sulfite ions can be detected using barium chloride solution, but they require an additional test to distinguish between them since both form white precipitates with barium.

Procedure

1. Dissolve sulphate and sulfite salts in deionised water in separate test tubes to obtain clear solutions
2. Add a few drops of barium chloride solution to each solution

Observation

Both solutions turn cloudy due to white solid materials settling out of the solutions.

Explanation

The insoluble materials formed are barium sulphate and barium sulfite respectively. Both compounds create white precipitates when barium ions from the barium chloride solution combine with the sulphate or sulfite ions dissolved in water.

Distinguishing between sulphate and sulfite ions

Since both ions produce similar white precipitates with barium chloride, an additional test using dilute hydrochloric acid helps distinguish between them.

Detection of carbonate (CO₃²⁻) and hydrogencarbonate (HCO₃⁻) ions

The detection of carbonate and hydrogencarbonate ions relies on their ability to react with acids to produce carbon dioxide gas. The limewater test confirms the presence of CO₂.

Procedure for both ions

1. Place the carbonate salt in a boiling tube
2. Insert a one-holed rubber stopper into the boiling tube
3. Set up apparatus so gas can be collected and bubbled through limewater
4. Add dilute hydrochloric acid to the carbonate salt in the boiling tube
5. Replace the one-holed rubber stopper

Observation

• A brisk effervescence (fizzing) occurs as acid comes into contact with the carbonate
• Gas bubbles are seen bubbling through the limewater
• The limewater turns a milky colour

Explanation

The fizzing happens because carbonate ions react with acid to produce carbon dioxide gas. When this gas bubbles through limewater (calcium hydroxide solution), it forms calcium carbonate which appears as a milky white suspension. This confirms the gas produced is carbon dioxide.

Distinguishing between carbonate and hydrogencarbonate ions

Since both ions react with acid to produce CO₂, magnesium sulphate solution helps distinguish between them.

Detection of nitrate ions (NO₃⁻)

Since all nitrate salts are soluble in water, precipitation reactions cannot be used to detect nitrate ions. Instead, a special test called the Brown Ring Test is used.

Procedure

1. Dissolve a nitrate salt in deionised water in a test tube
2. Add freshly prepared iron(II) sulphate solution to the nitrate solution in the test tube
3. Using a dropper, carefully add concentrated sulfuric acid down the inside of the test tube, allowing it to form a layer at the bottom
4. Observe the junction between the two layers

Observation

A brown ring forms at the junction of the two layers.

Explanation

The brown ring forms as a result of nitrate ions being present in solution. This serves as the confirmatory test for the presence of the nitrate ion. The brown colour comes from the formation of a complex compound between nitrate ions, iron ions, and sulfuric acid.

Detection of phosphate ions (PO₄³⁻)

Phosphate ions are detected using ammonium molybdate solution in the presence of nitric acid, which creates a distinctive yellow precipitate.

Procedure

1. Dissolve a phosphate salt in deionised water in a test tube
2. Add ammonium molybdate solution (ammonium molybdate is a compound containing molybdenum, element number 42 in the Periodic Table)
3. Using a dropper, add a few drops of concentrated nitric acid to the test tube
4. Place the test tube in warm water

Observation

A yellow precipitate forms.

Explanation

The yellow precipitate is a substance called ammonium phosphomolybdate. This distinctive yellow colour confirms the presence of phosphate ions in solution.

Summary of anion detection tests

Anion	Reagent	Observation	Confirmatory Test
Chloride (Cl⁻)	Silver nitrate	White precipitate	Dissolves in dilute ammonia
Sulphate (SO₄²⁻)	Barium chloride	White precipitate	Insoluble in dilute HCl
Sulfite (SO₃²⁻)	Barium chloride	White precipitate	Soluble in dilute HCl
Carbonate (CO₃²⁻)	Dilute acid + limewater	CO₂ turns limewater milky	Forms precipitate with MgSO₄
Hydrogencarbonate (HCO₃⁻)	Dilute acid + limewater	CO₂ turns limewater milky	No precipitate with MgSO₄
Nitrate (NO₃⁻)	Fe(II) sulphate + conc. H₂SO₄	Brown ring at interface	-
Phosphate (PO₄³⁻)	Ammonium molybdate + HNO₃	Yellow precipitate	-