JEE Challenger
More from Organic Chemistry - Some Basic Principles and Techniques
ChemEasyAdv 2007 • Paper 2

Composition of Prussian Blue in Lassaigne's Test

Comprehension Passage

Redox reactions play a pivotal role in chemistry and biology. The values of standard redox potential (E∘E^\circ) of two half-cell reactions decide which way the reaction is expected to proceed. A simple example is a Daniell cell in which zinc goes into solution and copper gets deposited.

Given below are a set of half-cell reactions (acidic medium) along with their E∘E^\circ (V\text{V} with respect to normal hydrogen electrode) values:

I2+2e−→2I−E∘=0.54 VFe3++e−→Fe2+E∘=0.77 VO2(g)+4H++4e−→2H2OE∘=1.23 VCl2+2e−→2Cl−E∘=1.36 VMn3++e−→Mn2+E∘=1.50 V\begin{aligned} \text{I}_2 + 2e^- &\rightarrow 2\text{I}^- && E^\circ = 0.54\text{ V} \\ \text{Fe}^{3+} + e^- &\rightarrow \text{Fe}^{2+} && E^\circ = 0.77\text{ V} \\ \text{O}_2(g) + 4\text{H}^+ + 4e^- &\rightarrow 2\text{H}_2\text{O} && E^\circ = 1.23\text{ V} \\ \text{Cl}_2 + 2e^- &\rightarrow 2\text{Cl}^- && E^\circ = 1.36\text{ V} \\ \text{Mn}^{3+} + e^- &\rightarrow \text{Mn}^{2+} && E^\circ = 1.50\text{ V} \end{aligned}

Sodium fusion extract, obtained from aniline, on treatment with iron(II) sulphate and H2SO4\text{H}_2\text{SO}_4 in presence of air gives a Prussian blue precipitate. The blue colour is due to the formation of

Options

A

Fe4[Fe(CN)6]3\text{Fe}_4[\text{Fe(CN)}_6]_3

Correct
B

Fe3[Fe(CN)6]2\text{Fe}_3[\text{Fe(CN)}_6]_2

C

Fe4[Fe(CN)6]2\text{Fe}_4[\text{Fe(CN)}_6]_2

D

Fe3[Fe(CN)6]3\text{Fe}_3[\text{Fe(CN)}_6]_3

Topics & Concepts

Organic Chemistry - Some Basic Principles and TechniquesLassaigne TestPrussian BlueQualitative Analysis of Organic Compounds

Step-by-Step Solution

In Lassaigne's test for nitrogen detection:

  1. Sodium fusion converts nitrogen and carbon from aniline into sodium cyanide:
    Na+C+N→ΔNaCN\text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN}

  2. The extract is boiled with FeSO4\text{FeSO}_4, forming sodium ferrocyanide:
    Fe2++6CN−→[Fe(CN)6]4−\text{Fe}^{2+} + 6\text{CN}^- \rightarrow [\text{Fe(CN)}_6]^{4-}

  3. In the presence of air and acid (H2SO4\text{H}_2\text{SO}_4), some Fe2+\text{Fe}^{2+} is oxidized to Fe3+\text{Fe}^{3+}, which reacts with hexacyanoferrate(II) to form ferric ferrocyanide, Prussian blue:
    4Fe3++3[Fe(CN)6]4−→Fe4[Fe(CN)6]3↓  (Prussian blue)4\text{Fe}^{3+} + 3[\text{Fe(CN)}_6]^{4-} \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3\downarrow\; (\text{Prussian blue})

Correct Answer: A

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