Schiff base
Note that to generate the ninhydrin chromophore (2-(1,3-dioxoindan-2-yl)iminoindane-1,3-dione), the amine is condensed with a molecule of ninhydrin to give a Schiff base. Thus only ammonia and primary amines can proceed past this step. At this step, there must be an alpha hydrogen present to form the Schiff base. Therefore, amines bound to tertiary carbons do not react further and thus are not detected. The reaction of ninhydrin with secondary amines gives an iminium salt, which is also coloured, and this is generally yellow–orange in color.