What color change indicates a positive test for protein using Biuret solution?

A positive test for protein using Biuret solution is indicated by a color change from blue to purple. This occurs because the Biuret reagent, which contains copper ions, interacts with the peptide bonds present in proteins. When proteins are present, the copper ions form a complex with these peptide bonds, resulting in a visible color change.

The original blue color of the Biuret solution is due to the copper ions. When proteins are present, the formation of this complex causes the color to shift to purple, which signifies a positive test. The specific shade of purple can vary depending on the concentration of protein, but the essential characteristic is the transition from blue to purple.

Other colors mentioned would not indicate the presence of proteins: blue to yellow, blue to green, or blue to pink do not indicate a successful reaction with proteins when using Biuret reagent. Only the shift to purple demonstrates that proteins are indeed present in the sample. This method is widely used in biochemical laboratories to assess protein concentration in various solutions.