A ninhydrin test is used to detect the presence of amino acids. During the test, which boils a solution of ninhydrin, if there are amino acids present, the solution turns a blue or purple color. Certain amino acids, such as proline and hydroxyproline, will turn the solution yellow. Ninhydrin spray is often used at crime scenes to make fingerprints visible because they contain trace amounts of amino acids.
A simple ninhydrin solution can be made by putting 25 grams of ninhydrin crystals into a gallon of solvent. Acetone, which may be obtained from any paint store, is the easiest to obtain. The ninhydrin degrades the amino acids into ammonia, aldehydes, and CO2 through the series of reactions. This leaves the ninhydrin in a partially reduced form of hydrindantin. The ninhydrin then condenses with the ammonia and hydrindantin, creating the bluish pigment, called Ruhemann's purple.
Each acid gives a slight variation to the color, and as stated above, hydroxyproline and proline produce a yellow solution. Since the color can be produced with ammonia and hydrindantin by themselves, ammonium salts and certain amines can produce false positives with this test. In addition to fingerprints, this test is often used in ethanol to develop amino acids in paper chromatography.