Ninhydrin deaminates amino acids liberating ammonia and
gets reduced to hydrindantin. The liberated ammonia condenses with hydrindantin to form a violet coloured compound diketohydrindylidenediketohydrindamine (DYDA) at pH 5.0. Potassium cyanide prevents the oxidation of the reduced hydrindantin. The intensity of violet colour is directly proportional to the amount of amino acid (Yemm & Cocking, 1955)
