Topic 8 · page 16 of 29

Tests for carbonate, hydrogencarbonate and sulfate ions

Dilute acid releases carbon dioxide from carbonates and hydrogencarbonates, which turns limewater milky. Acidified barium chloride gives a white precipitate of barium sulfate with sulfate ions, and the acid must go in first.

13 min read8.198.19(i)8.19(ii)

Read first: Solubility of Group 2 hydroxides and sulfates

Carbonate and hydrogencarbonate ions 8.198.19(i)

Carbonates, CO₃²⁻, and hydrogencarbonates, HCO₃⁻, both react with dilute acid to give carbon dioxide. The gas is then identified with limewater.

COX3X2−(aq)+2 HX+(aq)→HX2O(l)+COX2(g) \ce{CO3^2-(aq) + 2H+(aq) -> H2O(l) + CO2(g)}
HCOX3X−(aq)+HX+(aq)→HX2O(l)+COX2(g) \ce{HCO3-(aq) + H+(aq) -> H2O(l) + CO2(g)}
A test tube containing a solid and dilute acid, fizzing. A delivery tube leads from the mouth of the tube into a second tube of limewater, which is labelled turns milky.
Add dilute acid to the sample; pass the gas into limewater. A fizz and a milky limewater show carbon dioxide.

MethodTest for carbonate or hydrogencarbonate

  1. Add dilute acid (hydrochloric or nitric) to the solid or solution in a test tube. Look for fizzing.
  2. Pass the gas through limewater, using a delivery tube or by holding a drop of limewater at the mouth of the tube.
  3. If the limewater turns milky (a white precipitate of calcium carbonate), the gas is carbon dioxide, so the sample was a carbonate or hydrogencarbonate.
Ca(OH)X2(aq)+COX2(g)→CaCOX3(s)+HX2O(l) \ce{Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)}

The test cannot tell carbonate from hydrogencarbonate, because both give carbon dioxide. It does tell them from other anions: most acids do not fizz with sulfates, chlorides or nitrates.

Sulfate ions 8.198.19(ii)

The test uses the insolubility of barium sulfate from page 12.

BaX2+(aq)+SOX4X2−(aq)→BaSOX4(s) \ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}
Three test tubes in a row. Tube 1: the sample solution. Tube 2: dilute hydrochloric acid has been added, with no change and an arrow labelled acid first. Tube 3: barium chloride solution added, giving a white precipitate labelled BaSO4 shows sulfate.
Acid first, then barium chloride. A white precipitate that stays shows sulfate.

MethodTest for sulfate

  1. Add dilute hydrochloric acid to the solution in a test tube.
  2. Add barium chloride solution.
  3. A white precipitate of barium sulfate shows sulfate ions.

Why acid first? Barium ions form insoluble precipitates with other anions, too. In neutral solution, carbonate ions give a white precipitate of barium carbonate, which would look the same:

BaX2+(aq)+COX3X2−(aq)→BaCOX3(s) \ce{Ba^2+(aq) + CO3^2-(aq) -> BaCO3(s)}

Adding acid first destroys the carbonate:

BaCOX3(s)+2 HX+(aq)→BaX2+(aq)+HX2O(l)+COX2(g) \ce{BaCO3(s) + 2H+(aq) -> Ba^2+(aq) + H2O(l) + CO2(g)}

so any white precipitate that forms after acidifying is barium sulfate, which does not dissolve in acid.

Check yourself

  1. Describe the test for a carbonate ion and give the ionic equation.
  2. Sodium hydrogencarbonate is added to dilute hydrochloric acid. State what you see and write the ionic equation.
  3. Why is limewater used to test for carbon dioxide, and what is the white solid formed?
  4. Describe the test for sulfate ions and state the observation if sulfate is present.
  5. Explain why dilute hydrochloric acid is added before barium chloride in the sulfate test.
  6. A solution gives a white precipitate with barium chloride but the precipitate disappears when dilute hydrochloric acid is added, with fizzing. Name the anion present, and write the ionic equation for the reaction with the acid.

Answers cover these until you have tried the questions

  1. Add dilute acid and look for fizzing; pass the gas through limewater, which turns milky. COX3X2−(aq)+2 HX+(aq)→HX2O(l)+COX2(g)\ce{CO3^2-(aq) + 2H+(aq) -> H2O(l) + CO2(g)}
  2. It fizzes (effervescence), and a gas is given off. HCOX3X−(aq)+HX+(aq)→HX2O(l)+COX2(g)\ce{HCO3-(aq) + H+(aq) -> H2O(l) + CO2(g)}
  3. Carbon dioxide reacts with the calcium hydroxide in limewater to form insoluble calcium carbonate, a white solid, so the limewater goes milky. Ca(OH)X2(aq)+COX2(g)→CaCOX3(s)+HX2O(l)\ce{Ca(OH)2(aq) + CO2(g) -> CaCO3(s) + H2O(l)}
  4. Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate forms.
  5. To remove carbonate ions, which would also give a white precipitate (BaCO₃) with barium ions. The acid destroys carbonate (it fizzes), so only barium sulfate, which does not dissolve in acid, forms a precipitate.
  6. Carbonate (CO₃²⁻): BaCO₃ is a white precipitate that reacts with acid. BaCOX3(s)+2 HX+(aq)→BaX2+(aq)+HX2O(l)+COX2(g)\ce{BaCO3(s) + 2H+(aq) -> Ba^2+(aq) + H2O(l) + CO2(g)}