Which combination of reagents is used in the Kjeldahl digestion step for protein analysis?

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

Answer: (A)

In the Kjeldahl method, proteins are digested by breaking down the organic matrix and converting nitrogen into ammonium sulfate. This requires a strong oxidizing environment provided by concentrated sulfuric acid. A catalyst is added to speed up the digestion, with copper sulfate being the classic choice; it accelerates the oxidation reactions and helps reach the high temperatures needed for full digestion. Sometimes a salt like potassium sulfate is also used to raise the boiling point and improve efficiency, but the key pair for the digestion step is the acid plus the catalyst.

The other reagent combinations don’t fit this step because they don’t provide the essential powerful oxidizing medium plus a catalytic boost used in Kjeldahl digestion.