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.