Section 1 of 5Testing for Sulfate Ions
The sulfate test relies on forming a white solid that stays in the test tube.
1
Place a sample of the unknown solution in a test tube and add a few drops of dilute hydrochloric acid to remove any carbonate ions.
2
Add a few drops of barium chloride solution to the acidified sample and watch carefully for a precipitate.
3
A white precipitate of barium sulfate forms if sulfate ions are present in the solution.
4
No precipitate means that sulfate ions are absent or are present only at a very low concentration.
5
Record the observation and state the conclusion clearly, because the test identifies sulfate only by the presence of a precipitate.
Section 2 of 5Ionic Equation and the Precipitate
The ionic equation shows which ions actually react to form the solid.
Ionic equation
- Ionic equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) shows that only the barium and sulfate ions take part in forming the precipitate.
Barium sulfate
- Barium sulfate A white solid that is insoluble in water and in dilute acid, so it stays in the test tube as a precipitate.
Spectator ions
- Spectator ions Chloride ions from barium chloride and hydrogen ions from the acid do not change, so they are left out of the ionic equation.
Carbonate interference
- Carbonate interference Carbonate ions would also form a white precipitate with barium ions,
- which is why acid is added first to remove them as carbon dioxide.
Choice of acid
- Choice of acid Sulfuric acid must not be used to acidify the sample, because it would add sulfate ions and give a false positive result.
| Reagents | Ion detected | Observation |
|---|---|---|
| Dilute HCl, then BaCl₂(aq) | Sulfate, SO₄²⁻ | White precipitate forms |
| Dilute HCl, then BaCl₂(aq) | Carbonate, CO₃²⁻ | Fizzing from carbon dioxide, no precipitate |
Section 3 of 5Comparing Other Ion Tests
Different tests identify different ions, so the right choice depends on what you are looking for.
Comparing Other Ion Tests
Halides
- Halides Silver nitrate solution forms coloured precipitates with halide ions, but it is used to identify halides rather than sulfates.
Metal hydroxides
- Metal hydroxides Sodium hydroxide solution forms precipitates with many metal ions, so it identifies the metal ion rather than the anion.
Carbonates
- Carbonates Adding acid to a carbonate produces fizzing carbon dioxide, which is a gas test rather than a precipitate test.
Limitations
- Limitations The barium chloride test is simple, but a poor choice of acid or reagent can give misleading results.
Section 4 of 5Worked Example
Question Calculate the percentage by mass of sulfur in barium sulfate, BaSO₄.
- Relative formula mass of BaSO₄ = 137 + 32 + (4 × 16) = 233.
- Mass of sulfur in one formula unit = 32.
- Percentage of sulfur = (32 ÷ 233) × 100.
- (32 ÷ 233) × 100 = 13.7%.
Answer 13.7% of the mass of barium sulfate is sulfur.
Section 5 of 5Exam Checklist & Tips
8.3.5 – Key Points for the Exam
- Precipitate An insoluble solid that forms when two solutions are mixed and react.
- Sulfate ion The ion SO₄²⁻, which forms a white precipitate with barium ions.
- Ionic equation An equation that shows only the ions taking part in a reaction.
- Spectator ion An ion that is present on both sides of a reaction and does not change.
- Barium sulfate The insoluble white solid BaSO₄ used as the test product for sulfate.
- False positive A result that suggests a substance is present when it is not.
Common Mistakes
- Using sulfuric acid to acidify the sample, which adds sulfate ions and gives a false positive.
- Omitting the acid step, so carbonate ions could give a precipitate that looks like a positive sulfate result.
- Writing the precipitate as Ba²⁺ rather than BaSO₄, the solid that is actually formed.
- Saying barium chloride reacts with sulfate ions to produce a gas, when the product is a solid.