An organic compound containing nitrogen is fused with sodium metal. The resulting sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of a Prussian blue colour confirms the presence of nitrogen. What is the exact chemical formula of the complex responsible for this Prussian blue colour?

Step-by-step solution

During Lassaigne's test for nitrogen, the sodium fusion extract contains sodium cyanide ( NaCN ). On boiling with iron(II) sulphate, sodium hexacyanidoferrate(II), Na ₄[ Fe ( CN )₆] , is formed. Upon adding concentrated sulphuric acid, some iron(II) is oxidised to iron(III), which reacts with the sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II), Fe ₄[ Fe ( CN )₆]₃ x H ₂ O . This complex is responsible for the Prussian blue colour. The distractor Fe ₃[ Fe ( CN )₆]₄ has incorrect valencies fo