Question
Paragraph: 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. Question: Among the following, the brown ring is due to the formation of