Which set of reagents correctly lists all reagents used in the Biuret method?

• A. Copper sulfate, Rochelle's salt, NaOH, Potassium iodide
• B. Sodium hydroxide, Potassium iodide
• C. Copper sulfate, Sodium chloride, NaOH
• D. Rochelle's salt, Sodium tartrate, Ammonia, Potassium iodide

Answer: (A)

Biuret reaction relies on copper(II) ions forming a colored complex with peptide bonds in a strongly alkaline solution. To make this work, you need a source of Cu2+ (copper sulfate), a way to keep copper in solution and properly complexed (Rochelle salt, which is potassium sodium tartrate), and a high pH environment (provided by sodium hydroxide). In classic formulations, potassium iodide is also included as a stabilizing component to help maintain a clear, intense color.

So, the correct set contains copper sulfate, Rochelle salt, sodium hydroxide, and potassium iodide. The copper provides the Cu2+ for complex formation; Rochelle salt stabilizes and solubilizes the copper complex; sodium hydroxide raises the pH to enable peptide coordination; potassium iodide helps stabilize the reagent system and color.

The other options miss one or more essential elements (for example, lacking the copper source, or the tartrate stabilizer, or the strong base), or replace them with substances that don’t support the same alkaline copper-peptide complex formation and color development.