a color reaction used for the qualitative and quantitative identification of amino acids, imino acids, and amines. Upon heating in an alkaline medium with substances containing primary amino groups (— NH₂), ninhydrin (triketohydrindene hydrate, C₉H₆O₄) yields a product with a stable deep blue-violet color, with maximum absorption at about 570 nanometers. Since absorption at this wavelength is a linear function of the number of free amino groups, the ninhydrin reaction is the primary means of quantitative identification of amino groups in colorimetry and spectrophotometry. The ninhydrin reaction is also used to detect secondary amino groups (> NH) in imino acids (proline and hydroxyproline); a bright yellow product is obtained as a result of the test.

The sensitivity of the ninhydrin reaction is up to 0.01 percent. Modern automatic amino-acid analysis is carried out by combining the ion-exchange separation of amino acids and their quantitative identification using the ninhydrin reaction. The reaction is also used to detect each type of amino acid present in quantities no smaller than 2–5 micrograms when techniques of paper chromatography are used to separate mixtures of amino acids.