During the qualitative analysis of an organic compound, a student adds freshly prepared FeSO ₄ solution to the Lassaigne's extract, heats the mixture, and then acidifies it with concentrated H ₂ SO ₄ . A complex with the formula Fe ₄[ Fe ( CN )₆]₃ x H ₂ O is formed. What is the characteristic colour of this complex and which element's presence in the organic compound does it confirm?

Step-by-step solution

The given complex is Fe ₄[ Fe ( CN )₆]₃ x H ₂ O , which is iron(III) hexacyanoferrate(II). This complex has a characteristic Prussian blue colour. It is formed during Lassaigne's test when the organic compound contains nitrogen. The nitrogen and carbon from the organic compound fuse with sodium to form sodium cyanide ( NaCN ). Upon treatment with FeSO ₄ and concentrated H ₂ SO ₄ , the cyanide ions form the Prussian blue complex, confirming the presence of nitrogen. Answer: Prussian blue, Nitrogen