The ninhydrin reaction is a chromogenic analytical reaction between ninhydrin and compounds bearing a free primary amino group, especially α-amino acids. An α-amino acid condenses with the tricarbonyl form of ninhydrin, then undergoes decarboxylation and oxidative deamination to yield carbon dioxide, an aldehyde containing one fewer carbon atom, and an aminated indandione intermediate ^[1]. Condensation of that intermediate with another ninhydrin-derived species produces the blue-violet chromophore Ruhemann’s purple ^[2]. Its visible absorbance is commonly measured near 570 nm and is proportional to accessible primary amino groups under controlled reagent, pH, heating, and timing conditions ^[3]. Secondary amino acids such as proline give a different yellow chromophore and response. The concept is therefore a derivatization-based color reaction, not a nonspecific protein stain: analytical identity depends on ninhydrin chemistry with nucleophilic nitrogen and on spectrophotometric or visual detection of the resulting chromophore.