In the Biuret reagent, which base is used?

• A. Sodium hydroxide
• B. Potassium hydroxide
• C. Ammonia
• D. Hydrochloric acid

Answer: (A)

The Biuret test relies on forming a copper(II)–peptide complex when the solution is strongly alkaline. The base provides the high pH needed to deprotonate peptide nitrogens so they can coordinate with Cu2+ ions from the reagent, producing the characteristic violet color of the Biuret complex. Sodium hydroxide is the standard base used because it delivers a strong, stable alkaline environment in the Biuret reagent, enabling consistent formation of the Cu2+–peptide complex. Potassium hydroxide could work similarly but is less commonly used in prepared reagents; hydrochloric acid would prevent complex formation by acidifying the solution, and ammonia, though basic, does not provide the same strong, controlled alkaline conditions in the typical formulation.