When H2S is passed through a solution of CuSO4, no precipitate of CuS is obtained until the solution — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II). Step 2: It is prepared by reacting mercury(II) chloride with potassium iodide until the initial red precipitate of HgI2 dissolves in excess KI to form a clear solution: HgI2 + 2KI -> K2[HgI4]. Step 3: The active complex ion present in Nessler's reagent is therefore the tetraiodomercurate(II) anion, [HgI4]2-, matching option (b).