The reaction of \({\mathrm{K}}_{3}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_{6}\right]\) with freshly prepared \({\mat…

The reaction of \({\mathrm{K}}_{3}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_{6}\right]\) with freshly prepared \({\mathrm{FeSO}}_{4}\) solution produces a dark blue precipitate called Turnbull's blue, Reaction of \({\mathrm{K}}_{4}\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_{6}\right]\) 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 \({\mathrm{H}}_{2}{\mathrm{SO}}_{4}\) through the side of the test tube produces a brown ring. Precipitate X is