The brown ring test is used for the detection of :

1. CO₃^2-
2. Cl^-
3. NO₃^-
4. SO₃^2-

NO₃^-

Reason — The brown ring test is a qualitative test used to detect the presence of nitrate ions (NO₃^-) in a solution. A brown ring forms at the junction of the two liquids if nitrate ions are present.