Question

The blue colouration obtained from the Lassaigne's test of nitrogen is due to the formation of

• A. \( \text{Fe}_4[\text{Fe(CN)}_6]_3 \)
• B. \( \text{Fe}_2[\text{Fe(CN)}_6]_5 \)
• C. \( \text{K}_2[\text{Fe(CN)}_6]_5 \)
• D. \( \text{K}_4[\text{Fe(CN)}_6]_3 \)

Hint:

In Lassaigne's test, the blue coloration indicates the presence of nitrogen, forming Prussian blue when iron (III) reacts with ferrocyanide.

Answer 1

The Correct Option is A

The blue colour observed in Lassaigne's test for nitrogen is attributed to the formation of the complex compound called Prussian blue. This compound forms during the test when organic compounds containing nitrogen are converted into sodium cyanide (NaCN) through fusion with sodium. The cyanide ion then reacts with ferrous sulfate (\( \text{FeSO}_4 \)) to eventually form the blue-colored complex. The sequence of reactions is as follows:

1. Initially, the organic compound is fused with sodium to form sodium cyanide:
   \( \text{Na} + \text{C} + \text{N} \rightarrow \text{NaCN} \)
2. Sodium cyanide reacts with ferrous sulfate to produce sodium hexacyanoferrate(II):
   \( 6\text{NaCN} + \text{FeSO}_4 \rightarrow \text{Na}_4[\text{Fe(CN)}_6] + \text{Na}_2\text{SO}_4 \)
3. Upon addition of acid (\( \text{HCl} \)) and further reaction with ferric sulfate (\( \text{Fe}_2(\text{SO}_4)_3 \)), the Prussian blue complex is formed:
   \( 3\text{Na}_4[\text{Fe(CN)}_6] + 4\text{Fe}_2(\text{SO}_4)_3 \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3 + 6\text{Na}_2\text{SO}_4 \)

The correct answer is thus \( \text{Fe}_4[\text{Fe(CN)}_6]_3 \), which is the chemical formula for Prussian blue, the blue coloration seen as a result of the test.

Answer 2

The blue coloration obtained from the Lassaigne's test of nitrogen is due to the formation of Prussian blue.

During this test, organic compounds containing nitrogen are fused with sodium metal, forming sodium cyanide (NaCN). The subsequent reaction between sodium cyanide, ferrous sulfate, and ferric ions results in the formation of Prussian blue, a complex with the formula:

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

The steps to reach this conclusion involve:

1. Fusion of organic compound with sodium to form sodium cyanide.
2. Addition of ferrous sulfate results in the formation of sodium hexacyanoferrate(II).
3. Addition of ferric ions to the sodium hexacyanoferrate(II) produces the blue complex, Prussian blue.

Thus, the correct answer is \(\text{Fe}_4[\text{Fe(CN)}_6]_3\).