Question

Which anion of the simple salt can be confirmed by Brown ring test?

• A. \(NO_2^-\)
• B. \(NO_3^-\)
• C. \(Br^-\)
• D. \(SO_4^{2-}\)

Hint:

The brown ring is composed of the nitrosoferrous sulfate complex, which forms only in the presence of nitrate ions.

Answer 1

The Correct Option is B

Step 1: Recall what the Brown ring test detects.
The brown ring test is the classic laboratory confirmation for one particular anion. Let us recall how it is done and which ion gives the brown colour.
Step 2: Describe the test procedure.
We add freshly made ferrous sulfate solution to the salt solution, then gently pour concentrated sulfuric acid down the side of the tube so it forms a layer at the bottom.
Step 3: Observe what forms.
A brown ring appears at the junction of the two layers. This ring is the nitroso iron complex $[Fe(H_2O)_5(NO)]^{2+}$.
Step 4: Write the underlying chemistry.
The acid and $Fe^{2+}$ reduce the anion to nitric oxide: \[ NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow NO + 3Fe^{3+} + 2H_2O \] and the $NO$ then binds to iron to give the brown complex.
Step 5: Match to the options.
Producing $NO$ and the brown complex is the signature of the nitrate ion, $NO_3^-$.
Step 6: Conclude.
The anion confirmed by the brown ring test is the nitrate ion.
\[ \boxed{NO_3^-} \]