In Lassaigne's test for detecting nitrogen, the ultimate confirmation is the appearance of a Prussia — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

During Lassaigne's test, cyanide ions ($CN^-$) react with $Fe^{2+}$ to form hexacyanidoferrate(II) ions, $[Fe(CN)_6]^{4-}$. Upon heating with concentrated acid, some $Fe^{2+}$ oxidizes to $Fe^{3+}$, which then reacts with the hexacyanidoferrate(II) complex to precipitate iron(III) hexacyanidoferrate(II), $Fe_4[Fe(CN)_6]_3$, giving the characteristic Prussian blue colour.