How can we explain that ninhydrin does not react with Tris (2-Amino-1,3-dihydroxy-2-(hydroxymethyl)propane; used as a buffer in many biological studies) but reacts with ethanolamine (and, of course, also with aminoacids) ? Both Tris and ethanolamine have a free aminogroup ... I have looked at a large number of books and web sites without any specific answer ?