Which procedure and observation confirms the presence of sulfate ions?

• A. Acidify the solution and add barium nitrate; a white precipitate forms.
• B. Add ammonia solution and evaporate.
• C. Add sodium hydroxide and heat.
• D. Add chloride ions and observe fizzing.

Answer: (A)

The test for sulfate ions relies on forming an insoluble compound with a barium ion. When sulfate ions are present, reacting the solution with a source of Ba2+ after acidifying to remove carbonate gives barium sulfate, which appears as a white precipitate. This precipitate is highly insoluble in water and in dilute acids, so it clearly signals the presence of sulfate.

That white precipitate from BaSO4 is diagnostic because, if sulfate isn’t present, no such precipitate forms. The acidification step is important to prevent carbonate ions from reacting with barium to form another insoluble substance, which could be misleading.

The other procedures don’t provide a specific clue for sulfates: ammonia with evaporation doesn’t selectively indicate sulfate, sodium hydroxide and heat would precipitate metal hydroxides from various ions instead of forming a sulfate precipitate, and adding chloride ions with fizzing points to reactions with acids or carbonates, not sulfate ions.