Xanthoprotein reaction

The yellow stains on the skin caused by nitric acid are the result of the xanthoprotein reaction. 

Xanthoprotein reactions are given by proteins with aromatic side groups On addition of nitric acid, the product turns yellow and, after further addition of ammonia, orange. The Millon reaction is specific for tyrosine On boiling, e.g., egg-white solution with a solution of mercury in nitric-acid containing nitrous acid, a red-brown precipitate is formed. 

Xanthoprotein reaction a qualitative test for protein, using concentrated nitric acid. The resulting yellow color is due to nitration of aromatic amino acid residues... 

The xanthoprotein test. This is a characteristic reaction of proteins that contain phenyl rings... 

---