Limit Test for Sulfate

• The limit test for sulfate 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 of Limit Test for Sulfate:

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.

Prepare Test Solution:

• Prepare a test solution from the sample, ensuring that any sulfate ions present are in a soluble form.

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.

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.

Observation of Precipitate:

• Look for the formation of a white precipitate of barium sulfate (BaSO₄) in both solutions, indicating sulfate ions.

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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