Which of the following ions is responsible for the brown colour in the ring test for a nitrate? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: The brown ring test for nitrate ions relies on the reduction of nitrate to nitric oxide (NO) by iron(II) under acidic conditions. Step 2: The generated NO coordinates with hydrated Fe2+ ions in the aqueous phase to form a stable, colored complex cation. Step 3: The formula of this brown coordination complex is [Fe(H2O)5(NO)]2+, matching option (a).