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

Reaction Equations:

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

$$[\text{Fe(H}_2\text{O)}_6]^{2+} + \text{NO} \to \underbrace{[\text{Fe(H}_2\text{O)}_5\text{(NO)}]^{2+}}_{\text{Brown Ring Complex}} + \text{H}_2\text{O}$$

Concept: A brown ring forms at the junction of two liquids due to complex formation.

Oxidation State & Paramagnetism: In $$[\text{Fe(H}_2\text{O)}_5\text{(NO)}]^{2+}$$, Iron exists in +1 oxidation state ($$\text{Fe}^+$$) and $$\text{NO}$$ exists as nitrosyl cation ($$\text{NO}^+$$). It has 3 unpaired electrons ($$\mu = 3.87\text{ BM}$$).