Question

In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of: (a) $Na_{4}[Fe(CN)_{6}]$ (b) $Fe_{4}[Fe(CN)_{6}]^{3}$ (c) $Fe_{2}[Fe(CN)_{6}]$ (d) $Fe_{3}[Fe(CN)_{6}]^{4}$

Answer 1

In the Lassaigne's test for nitrogen in an organic compound, the sodium fusion extract is boiled with iron (II) sulphate and then treated with sulphuric acid acidified.

In this process, the sodium cyanide and iron (II) sulphate gets reacted initially and forms sodium hexacyanoferrate (II), finally when it is heated with sulphuric acid. During this heating process, some iron (II) gets oxidised to form iron (III) hexacyanoferrate (II) which is Prussian blue in colour.

The chemical equations involved in the reaction can be represented as  

                   $6{ CN }^{ - }\quad +\quad { Fe }^{ 2+ }\quad \rightarrow \quad { \left[ Fe{ \left( CN \right)  }_{ 6 } \right]  }^{ 4- }$

                   $3{ \left[ Fe{ \left( CN \right)  }_{ 6 } \right]  }^{ 4- }\quad +\quad 4{ Fe }^{ 3+ }\quad \xrightarrow { { H }_{ 2 }O } \quad \begin{matrix} { Fe }_{ 4 }{ \left[ Fe{ \left( CN \right)  }_{ 6 } \right]  }_{ 3 }.x{ H }_{ 2 }O \\ Prussian\quad Blue \end{matrix}$

Hence, the Prussian blue colour is due to the formation of ${ Fe }_{ 4 }{ \left[ Fe{ \left( CN \right)  }_{ 6 } \right]  }_{ 3 }.$

Answer 2

PASS IS I AN BLUE COLOR IS DUE TO THE FORMATION OF

Fe4[Fe(CN)Â6]Â3