What indicates a positive result in the biuret test?

A positive result in the biuret test is indicated by a color change to lavender or purple. This test is specifically designed to detect the presence of proteins in a solution. When proteins are present, they react with the biuret reagent, which contains copper sulfate. The copper ions bind to the peptide bonds in the proteins, leading to the formation of a colored complex. The intensity of the color change can often correlate with the concentration of proteins in the sample.

In contrast, other color indicators such as yellow, no change, or dark blue do not signify the presence of proteins. Yellow might indicate other components or reagents not involved in protein detection, while no change signifies the absence of proteins. Dark blue is associated with the presence of copper ions in solution, but not bound to proteins. Thus, the lavender or purple color specifically confirms that proteins are present, marking a successful biuret test result.