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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.