A biochemical test to detect proteins in solution, named after the substance biuret (H2NCONHCONH2), which is formed when urea is heated. Sodium hydroxide is mixed with the test solution and drops of 1% copper(II) sulphate solution are then added slowly. A positive result is indicated by a violet ring, caused by the reaction of peptide bonds in the proteins or peptides. Such a result will not occur in the presence of free amino acids.