Question #19dd8

1 Answer
May 25, 2016

The locating agent is ninhydrin.


Ninhydrin is 2,2-dihydroxyindane-1,3-dione.

Its structure is

Ninhydrin is a powerful oxidizing agent.

It causes the oxidative deamination of alpha amino acids to aldehydes, ammonia, and carbon dioxide.

The ammonia then reacts further to form a blue-purple coloured substance.

However, the cyclic amino acid proline reacts differently to form a bright-yellow product.

Hence, when you spray a TLC plate or a paper chromatogram, the colourless amino acids suddenly appear as bluish-purple (or yellow) spots.

The colours variy slightly from acid to acid, probably because the unreacted acids complex with the pigment.