The ninhydrin test is a colorimetric test for amines, particularly alpha-amino acids, carboxylic acids and free amino acids on simple or complex proteins. It uses the crystalline compound ninhydrin, which has the chemical name triketohydrindene hydrate, or 1,2,3-indantrione monohydrate.
When ninhydrin reacts with an amino acid, it undergoes a partial reduction process to become hydrindantin. The amino acid is broken down to produce aldehydes, ammonia and carbon dioxide. The ninhydrin then combines with ammonia and hydrindantin to form a vivid blue or purple color, sometimes referred to as Ruhemann's purple. One practical application of the ninhydrin test is in forensic science, where fingerprints on crime scenes are detected by using the test.