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