In the COD procedure, Mercuric Sulfate is added to act as a catalyst.

• A. To act as a catalyst
• B. To color the solution
• C. To raise the pH
• D. To precipitate solids

Answer: (A)

Mercuric sulfate acts as a catalyst in the COD digestion, speeding up the oxidation of organic matter by potassium dichromate in the acidic medium. This ensures complete digestion within the standard digestion time. It also helps suppress interference from chloride ions by forming complexes with them, preventing chloride from being oxidized and skewing the COD result. It does not color the solution, raise the pH, or precipitate solids, so those roles aren’t applicable here.