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