Biuret Reagent is an aqueous solution of potassium sodium tartrate treated with cupric sulfate and sodium hydroxide. In the presence of peptide bonds (protein), this blue solution will change color to pink-purple.

What color does Biuret solution turn when positive?

Biuret Reagent

The biuret test uses an alkaline mixture, or reagent, composed of potassium hydroxide and copper sulfate. The normal color of biuret reagent is blue. The reagent turns violet in the presence of peptide bonds -- the chemical bonds that hold amino acids together.

How does Biuret solution work?

The biuret method

Copper salts in alkaline solution form a purple complex with substances containing two or more peptide bonds. The absorbance produced is proportional to the number of peptide bonds that are reacting and therefore to the number of protein molecules present in the reaction system.