A small amount of the compound is dissolved in 1 mL of water. If a blue color is not formed immediately, then heat the test tube in a hot water bath for 5 min. Positive Test A moderate to deep blue color of a liquid or a dark blue solid is a positive test. Some a -amino acids are not very soluble in cold water. However, these amino acids are soluble in hot water and will give a positive test when the solution is heated.
Aliphatic amines yield a blue precipitate.
Anilines give a brown or green color, but other aromatic amines produce a blue-purple color.