In Kjeldahl nitrogen determination, which two reagents are essential for the digestion step?

• A. Sulfuric acid + Copper sulfate
• B. Nitric acid + Silver nitrate
• C. Hydrochloric acid + Zinc
• D. Sodium hydroxide + Ammonium sulfate

Answer: (A)

During Kjeldahl digestion, the goal is to convert all organic nitrogen into ammonium in a strong acidic environment. Concentrated sulfuric acid provides the oxidative, degradative environment that breaks down the sample and forms ammonium mainly as ammonium sulfate. A copper-based catalyst, typically copper sulfate, speeds up the digestion process and helps ensure complete conversion by promoting oxidation and reducing foaming or charring. The other reagent pairings don’t create this necessary acidic, catalytic digestion context: nitric acid isn’t used for the Kjeldahl digestion step, and silver nitrate or zinc with hydrochloric acid pertain to other chemical analyses or reductive/different digestion schemes. Sodium hydroxide with ammonium sulfate would not establish the required strong acidic digestion either.