Lassaigne's Test for Nitrogen

Formula / Core Reaction:

$$\text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN}$$

$$6\text{NaCN} + \text{FeSO}_4 \rightarrow \text{Na}_4[\text{Fe}(\text{CN})_6] + \text{Na}_2\text{SO}_4$$

$$\text{Na}_4[\text{Fe}(\text{CN})_6] + \text{Fe}^{3+} \xrightarrow{\text{H}^+} \text{Fe}_4[\text{Fe}(\text{CN})_6]_3 \downarrow$$

Concept: Organic nitrogen is fused with sodium metal to form soluble sodium cyanide ($$\text{NaCN}$$). On adding ferrous sulfate ($$\text{FeSO}_4$$) and ferric ions ($$\text{Fe}^{3+}$$) in an acidic medium, a characteristic Prussian blue complex is formed.

Notation / Key Observation:

• $$\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$$: Iron(III) hexacyanoferrate(II) (Prussian Blue precipitate);
• $$\text{NaCN}$$: Sodium cyanide in Lassaigne's extract;
• $$\text{H}^+$$: Acidification using concentrated $$\text{HCl}$$ or $$\text{H}_2\text{SO}_4$$ to dissolve iron hydroxide precipitates.