Nitrate ($$\text{NO}_3^-$$) - Brown Ring Test

$$\text{NO}_3^- + 3\text{Fe}^{2+} + 4\text{H}^+ \rightarrow \text{NO} \uparrow + 3\text{Fe}^{3+} + 2\text{H}_2\text{O}$$

$$[\text{Fe}(\text{H}_2\text{O})_6]^{2+} + \text{NO} \rightarrow [\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+} + \text{H}_2\text{O}$$

Concept: Nitrates are reduced to nitric oxide ($$\text{NO}$$) by ferrous ions ($$\text{Fe}^{2+}$$) in acidic conditions. Unreacted $$[\text{Fe}(\text{H}_2\text{O})_6]^{2+}$$ binds $$\text{NO}$$ to yield a brown coordination compound at the liquid junction.

Notation / Key Observation:

• $$[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+}$$: Pentaaquanitrosyliron(II) complex (Brown ring);
• Note: Oxidation state of iron in this complex is +1 ($$\text{Fe}^+$$ and $$\text{NO}^+$$).