amino acids into aldehydes, ammonia, and CO2 through a series of reactions; the net result is ninhydrin
in a partially reduced form hydrindantin:
Ninhydrin then condenses with ammonia and hydrindantin to produce an intensely blue or purple pigment, sometimes called Ruhemann's purple:
The color varies slightly from acid to acid, probably because unreacted acids complex with the pigment. Proline and hydroxyproline give a yellow color. Since all that is required for color development is ammonia and partially reduced ninhydrin, the ammonium salts of weak and strong acids, as well as certain amines, can give a false positive result to the ninhydrin test.
The striking color change is due to the large change in electron confinement on formation of the anion. Note the delocalization of negative charge (how many resonance structures can you draw?). Changes in electron confinement are also associated with acid-base indicator color changes.
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/organic/faq/print-amino-acid-test.shtml