The reaction of K_3[Fe(CN)_6] with freshly prepared FeSO_4 solution produces a dark blue precipitate called Turnbull's blue. Reaction of K_4[Fe(CN)_6] with the FeSO_4 solution in complete absence of air produces a white precipitate X, which turns blue in air. Mixing the FeSO_4 solution with NaNO_3 , followed by a slow addition of concentrated H_2SO_4 through the side of the test tube produces a brown ring. Among the following, the brown ring is due to the formation of

Salt Analysis PYQ — JEE Advanced 2021 · Shift 2 · Q16 with Solution

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Correct answer: D

Identify the test being described

The mixture of $FeSO_4$ solution with $NaNO_3$ , followed by careful addition of concentrated $H_2SO_4$ , is the brown ring test for nitrate ion.
Species formed in the brown ring test

In acidic medium, nitrate ion oxidizes $Fe^{2+}$ and gets reduced to nitric oxide, $NO$ .

A simplified step is: $NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow NO + 3Fe^{3+} + 2H_2O$
Formation of the brown ring complex

The nitric oxide formed then coordinates with ferrous ion: $[Fe(H_2O)_6]^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+} + H_2O$

This nitrosyl complex is responsible for the characteristic brown ring.
Match with the options

The brown ring complex is: $[Fe(NO)(H_2O)_5]^{2+}$

So the correct option is D.
Check against stored answer

Stored correct answer: D

This matches the derived answer.