Chemical Complex Responsible for the Brown Ring Test

The brown ring test is a classic qualitative inorganic analysis test used to detect the presence of nitrate ($\text{NO}_3^-$) ions in a solution.

1. Chemical Reactions Involved: When freshly prepared ferrous sulfate ($\text{FeSO}_4$) solution is added to an aqueous solution containing nitrate ions in the presence of concentrated sulfuric acid ($\text{H}_2\text{SO}_4$), nitrate ions are reduced to nitric oxide ($\text{NO}$) by $\text{Fe}^{2+}$ ions: $\text{NO}_3^- + 3\text{Fe}^{2+} + 4\text{H}^+ \longrightarrow \text{NO} + 3\text{Fe}^{3+} + 2\text{H}_2\text{O}$

2. Formation of the Brown Ring Complex: The generated nitric oxide ($\text{NO}$) ligand coordinates with the remaining unoxidized aqueous iron(II) complex, $[\text{Fe}(\text{H}_2\text{O})_6]^{2+}$, displacing one water molecule to form a brown-colored coordination complex at the junction of the two liquid layers: $[\text{Fe}(\text{H}_2\text{O})_6]^{2+} + \text{NO} \longrightarrow [\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+} + \text{H}_2\text{O}$

3. Oxidation State and Bonding: In the complex ion $[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+}$, an electron transfers from the $\text{NO}$ molecule to $\text{Fe}^{2+}$, resulting in iron in the $+1$ oxidation state ($\text{Fe}^+$) and the nitric oxide ligand present as a nitrosonium ion ($\text{NO}^+$).

Therefore, the brown ring is due to the formation of the complex ion $[\text{Fe}(\text{NO})(\text{H}_2\text{O})_5]^{2+}$.

Correct Option: (D)