Tests for anions and ammonium ionsEdexcel International A Level Chemistry: Revision notes
Section 1
Carbonate and hydrogencarbonate ions
Add dilute acid to the sample. Carbonates and hydrogencarbonates give effervescence of carbon dioxide, which turns limewater milky (cloudy).
- CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
- HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l)
The limewater test: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, where the white solid CaCO₃ makes the limewater cloudy.
Worked example: 0.53 g of Na₂CO₃ (M = 106.0 g mol⁻¹) is 5.0 × 10⁻³ mol, so it gives 5.0 × 10⁻³ × 24.0 = 0.12 dm³ of CO₂ at room temperature and pressure.
Dilute acid shows carbonate or hydrogencarbonate. Do not claim it distinguishes between the two.
Section 2
Sulfate ions
Add dilute hydrochloric acid and then barium chloride solution. A white precipitate of barium sulfate shows sulfate ions.
- Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
The acid is added first to react with any carbonate ions (and sulfite ions), which would also form a white precipitate with barium ions. BaCO₃ dissolves in acid but BaSO₄ does not.
Do not use sulfuric acid to acidify, as it supplies sulfate ions and gives a false positive.
Writing 'add sulfuric acid to acidify' loses the mark, because it already contains sulfate ions.
Section 3
Ammonium ions
Add sodium hydroxide solution and warm gently. Ammonium ions are deprotonated to give ammonia gas.
- NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Ammonia is identified in two ways:
- it turns damp red litmus paper blue (it is an alkaline gas)
- it forms white smoke of ammonium chloride with a glass rod dipped in concentrated hydrochloric acid: NH₃(g) + HCl(g) → NH₄Cl(s)
Ammonia has a pungent smell, so the test is done in a fume cupboard or with a small amount.
Section 4
Putting the tests in order
When a sample could contain several ions, the order of the tests matters.
- Dilute hydrochloric acid first: effervescence and limewater milky shows carbonate or hydrogencarbonate. The acid also removes the carbonate.
- Acidified barium chloride next: a white precipitate shows sulfate.
- Sodium hydroxide and warming separately: ammonia shows ammonium.
Always state the reagent, the observation and the ionic equation. Use damp litmus paper, because dry litmus paper does not show the colour change.
In a six-mark 'identify the solutions' question, give each test with the reagent, the observation and the conclusion.
Section 5
Calculations with the tests
The tests can be used quantitatively, because the precipitate or gas formed is in a fixed ratio to the ion.
Worked example: 1.32 g of (NH₄)₂SO₄ (M = 132.1 g mol⁻¹) is 0.0100 mol. Ba²⁺ + SO₄²⁻ → BaSO₄ is 1 : 1, so 0.0100 mol of BaSO₄ forms. M(BaSO₄) = 233.4 g mol⁻¹, so the mass is 0.0100 × 233.4 = 2.33 g.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Tests for anions and ammonium ions
- A student adds dilute hydrochloric acid to a white powder and sees vigorous effervescence. She passes the gas given off through limewater, which turns milky. The powder is known to be a sodium salt.The powder is sodium carbonate. A 0.53 g sample reacts with excess dilute hydrochloric acid. Calculate the maximum volume of carbon dioxide produced, in cm³, at room temperature and pressure. (Na₂CO₃: M = 106.0 g mol⁻¹; molar volume of a gas at room temperature and pressure = 24.0 dm³ mol⁻¹)2 marks
- A technician at a swimming pool tests a sample of pool water for dissolved sulfate. He adds dilute hydrochloric acid and then barium chloride solution, and a white precipitate forms.Explain why dilute sulfuric acid should not be used instead of dilute hydrochloric acid to acidify the sample.2 marks
- A soil-testing laboratory analyses a solid fertiliser that is thought to be ammonium sulfate, (NH₄)₂SO₄. Relative atomic masses: H 1.0, N 14.0, O 16.0, S 32.1, Ba 137.3.Describe how the laboratory could show that the fertiliser contains ammonium ions. Give the ionic equation for the reaction and the observation that identifies the gas.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).