Question 31 of 34beginner🧠 RememberMCQ1 mark

In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:

Exercise: EXERCISES | Q: 8.37
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Explanation

Option (B) is correct. In Lassaigne's test, the sodium fusion extract is boiled with iron(II) sulphate, forming sodium hexacyanidoferrate(II). Upon acidification with concentrated sulphuric acid, some iron(II) ions are oxidised to iron(III) ions. These iron(III) ions react with sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II) (ferriferrocyanide), which has the chemical formula Fe4[Fe(CN)6]3\text{Fe}_4[\text{Fe}(\text{CN})_6]_3 and is Prussian blue in colour.

Solution Steps

  1. Step 1: In Lassaigne's test, sodium cyanide reacts with iron(II) sulphate to form sodium hexacyanidoferrate(II).

  2. Step 2: On heating with concentrated sulphuric acid, some iron(II) ions are oxidised to iron(III) ions.

  3. Step 3: These iron(III) ions react with sodium hexacyanidoferrate(II) to produce iron(III) hexacyanidoferrate(II), Fe4[Fe(CN)6]3\text{Fe}_4[\text{Fe}(\text{CN})_6]_3, which gives the Prussian blue colour.

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