A colorless solution contains a mixture of nitrate (NO3-) and sulfate (SO4^2-) ions. Which sequence of reagents will allow the sulfate ion to be tested without interference from the nitrate ion?
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Correct answer
A. Add dilute hydrochloric acid, then barium chloride solution.
Principle or equation
Sulfate ions are identified by forming a white precipitate of barium sulfate (BaSO4) when reacted with barium chloride. To avoid interference, the solution is acidified with dilute hydrochloric acid to remove carbonate and sulfite ions, and nitrate does not interfere.
Why this answer is correct
Adding dilute hydrochloric acid acidifies the solution and removes any carbonate or sulfite ions that could also precipitate with barium. Then adding barium chloride produces a white precipitate of BaSO4 if sulfate is present. Nitrate ions do not react with barium chloride, so they do not interfere.
Example
To test for sulfate, add a few drops of dilute HCl, then add BaCl2 solution. A white precipitate indicates sulfate.
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