Qualitative analysis of Nitrite ($$\text{NO}_2^-$$)

Formula / Core Reaction:

$$2\text{NO}_2^- + \text{H}_2\text{SO}_4 \rightarrow \text{SO}_4^{2-} + \text{H}_2\text{O} + \text{NO} \uparrow + \text{NO}_2 \uparrow$$

$$\text{NO}_2^- + \text{I}^- + 2\text{H}^+ \rightarrow \text{NO} + \frac{1}{2}\text{I}_2 + \text{H}_2\text{O}$$

$$\text{I}_2 + \text{Starch} \rightarrow \text{Dark Blue Complex}$$

Concept: Action of dilute $$\text{H}_2\text{SO}_4$$ liberates reddish-brown $$\text{NO}_2$$ gas. Nitrite oxidizes iodide ($$\text{I}^-$$) to free iodine ($$\text{I}_2$$), turning starch-iodide paper intense blue.

Notation / Key Observation:

• $$\text{NO}_2$$: Pungent reddish-brown gas;
• Starch-Iodide test: Turns deep blue due to starch-triiodide complex.