The biuret test detects peptide bonds, and when they are present in an alkaline solution, the coordination complexes associated with a copper ion are violet in color. The protein concentration affects the intensity of the color, and the color will be more intense with more proteins.
To perform the test, potassium or sodium hydroxide at a 1 percent strong base go onto the sample along with about three to four drops of aqueous copper sulfate. A protein is present if the solution turns purple. Scientists can determine five to 160 milligrams per milliliter. For a measurable color shift, at least three amino acids need to be present in a peptide chain.