Sulfate IonsOxford AQA IGCSE Chemistry: Revision notes
Section 1
How are sulfate ions tested for?
Sulfate ions in solution can be detected using barium chloride solution. When barium chloride is added to a solution containing sulfate ions, a white precipitate forms. This precipitate is barium sulfate, which is insoluble in water.
Section 2
Why is dilute hydrochloric acid added first?
The test is carried out in the presence of dilute hydrochloric acid. This step is important because it removes a possible source of error: without the acid, other ions (such as carbonate ions) present in the sample could also form a white precipitate with barium ions, giving a false positive result. Dilute hydrochloric acid reacts with and removes carbonate ions before the barium chloride is added, so any white precipitate that then forms can be confidently attributed to sulfate ions.
Forgetting the dilute hydrochloric acid step is a common error - without it, a positive result cannot reliably be attributed to sulfate ions alone, since carbonate ions would interfere.
Section 3
How is the test carried out?
- Add a few drops of dilute hydrochloric acid to the unknown solution.
- Add barium chloride solution to the mixture.
- Observe whether a white precipitate forms.
- A white precipitate confirms the presence of sulfate ions.
In an exam answer, always name both reagents (dilute hydrochloric acid and barium chloride solution) and the result (white precipitate) for full marks - naming only barium chloride is an incomplete answer.
Section 4
How does this test compare with other precipitate-based ion tests?
Several ion tests in this topic work by forming a precipitate with a specific reagent, but the exact reagent and colour differ for each ion.
| Ion tested | Reagent | Positive result |
|---|---|---|
| Sulfate ions | Barium chloride (+ dilute HCl) | White precipitate |
| Chloride ions | Silver nitrate (+ dilute HNO3) | White precipitate |
| Bromide ions | Silver nitrate (+ dilute HNO3) | Cream precipitate |
| Iodide ions | Silver nitrate (+ dilute HNO3) | Yellow precipitate |
Because both the sulfate and chloride tests can give a white precipitate, it is essential to remember which reagent goes with which ion - barium chloride for sulfate, silver nitrate for halides.
Must Know
- Sulfate ions are tested for using barium chloride solution in the presence of dilute hydrochloric acid.
- A positive result is a white precipitate (barium sulfate).
- The dilute hydrochloric acid removes carbonate ions first, so they cannot cause a false positive.
- Both reagent and acid must be mentioned for a full-mark answer.
- Do not confuse this test with the silver nitrate test used for halide ions, which also gives precipitates but with different reagents and colours.
That's the notes covered.
Carry on to the next subtopic.