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Limit test for Sulphate is a qualitative test that detects whether the sulfate ion (SO₄²⁻) concentration in a sample is below a predefined acceptable limit.
It is based on the precipitation reaction between barium ions (Ba²⁺) and sulfate ions (SO₄²⁻), forming an insoluble white precipitate of barium sulfate (BaSO₄).
Chemical Reaction
Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)
Procedure
1) Prepare Reference Standard Solution
Make a reference solution with a known concentration of sulfate ions (usually from sodium or potassium sulfate) representing the acceptable sulfate limit.
2) Prepare Test Solution
Prepare a test solution from the sample, ensuring that any sulfate ions present are in a soluble form.
3) Add Barium Chloride (BaCl₂)
Add equal volumes of barium chloride solution to both the test and reference solutions. BaCl₂ provides barium ions for the reaction, and its concentration should slightly exceed the sulfate limit.
4) Acidification with Dilute HCl
Acidify both solutions with dilute hydrochloric acid to prevent the formation of other barium salts that could interfere with the test.
5) Observation of Precipitate
Look for the formation of a white precipitate of barium sulfate (BaSO₄) in both solutions, indicating sulfate ions.
6) Comparison of Precipitate
Compare the turbidity or intensity of the precipitate in the test solution with the reference standard.
If the test solution is less turbid or has a weaker precipitate, the sulfate concentration is below the acceptable limit.
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