The reaction of $\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$ with freshly prepared $\mathrm{FeSO}_{4}$ solution produces a dark blue precipitate called Turnbull's blue. Reaction of $\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$ with the $\mathrm{FeSO}_{4}$ solution in complete absence of air produces a white precipitate $\mathbf{X}$, which turns blue in air. Mixing the $\mathrm{FeSO}_{4}$ solution with $\mathrm{NaNO}_{3}$, followed by a slow addition of concentrated $\mathrm{H}_{2} \mathrm{SO}_{4}$ through the side of the test tube produces a brown ring. Among the following, the brown ring is due to the formation of

(D) $\left[\mathrm{Fe}(\mathrm{NO})\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2+}$

(A) $\left[\mathrm{Fe}(\mathrm{NO})_{2}\left(\mathrm{SO}_{4}\right)_{2}\right]^{2-}$

(B) $\left[\mathrm{Fe}(\mathrm{NO})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{3+}$

(C) $\left[\mathrm{Fe}(\mathrm{NO})_{4}\left(\mathrm{SO}_{4}\right)_{2}\right]$

JEE Advanced · 2021 · 03 Oct 2021 · Paper 2 · Practical and Analytical Chemistry

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_{2} SO_{4}

through the side of the test tube produces a brown ring.

Among the following, the brown ring is due to the formation of

Answer: (D)

The brown ring test for nitrate involves the formation of a brown-colored complex. When

FeSO_{4}

solution is mixed with

NaNO_{3}

and concentrated

H_{2} SO_{4}

is added, nitrate is reduced to NO, which coordinates with Fe²⁺ to form the brown ring complex

[Fe (H_{2} O)_{5} (NO)]^{2 +}

[Fe (NO) (H_{2} O)_{5}]^{2 +}

. This complex is responsible for the characteristic brown color at the junction of the two layers.

JEE Advanced · 2021 · 03 Oct 2021 · Paper 2 · Single correct option