A solution contains a mixture of chloride (Cl⁻) and sulfate (SO₄²⁻) ions. Which sequence of reagents will allow the sulfate ion to be tested without interference from the chloride ion?
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Correct answer
A. Add dilute nitric acid, then barium nitrate solution.
Principle or equation
Sulfate ions are identified by adding a barium salt in acidic conditions to form a white precipitate of barium sulfate, which is insoluble in dilute acid. Chloride ions do not interfere because silver chloride is not formed when using barium nitrate, and the acid prevents precipitation of other barium salts.
Why this answer is correct
Step 1: To test for sulfate, add a soluble barium salt (e.g., barium nitrate) in the presence of dilute nitric acid. Step 2: Ba²⁺ + SO₄²⁻ → BaSO₄(s) white precipitate, insoluble in acid. Step 3: Chloride does not react with barium nitrate, so no interference. Step 4: Using barium chloride would introduce chloride ions, which could confuse the test. Silver nitrate would test for chloride, not sulfate. Lead nitrate gives a white precipitate with both sulfate and chloride, so not selective.
Example
To a sample, add 2 mL dilute nitric acid, then 2 mL barium nitrate solution. A white precipitate indicates sulfate.
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