Final answer:
The brown ring test for the presence of nitrate ions relies on the use of concentrated sulfuric acid, alongside an aqueous solution of the salt and ferrous sulfate solution. This test produces a characteristic brown ring that indicates nitrates by the interaction of the reduced nitric oxide with ferrous ions.
Step-by-step explanation:
The correct reactant for the brown ring test for the nitrate (NO3-) ion is an aqueous solution of the salt, ferrous sulfate solution, and concentrated sulfuric acid (conc. H2SO4). The brown ring test is a qualitative analysis procedure used to detect the presence of nitrate ions in a solution. When a solution containing nitrate ions is mixed with a fresh ferrous sulfate solution and concentrated sulfuric acid is carefully added to form a layer, a brown ring is formed at the junction of the two liquids, indicating the presence of nitrate ions.
The formation of the brown ring occurs due to the reduction of the nitrate ion to nitric oxide (NO), which then reacts with the ferrous ion (Fe2+) to form a brown-colored complex. It's essential to use concentrated sulfuric acid in this test because dilute sulfuric acid will not provide the conditions necessary for the formation of the brown ring, as it does not sufficiently lower the pH or provide the appropriate reaction environment.