Xanthoproteic reaction

The xanthoproteic reaction is a method that can be used to detect a presence of protein soluble in a solution, using concentrated nitric acid.

The test gives a positive result in amino acids carrying aromatic groups, especially in the presence of tyrosine.

Slowly add sodium hydroxide (NaOH, 40 % w/v in water) solution until the mixture becomes alkaline and a colour change is observed.

If the colour changes from yellow to orange, this indicates the presence of an aromatic amino acid.

When human skin or nails are exposed to nitric acid, they turn yellow after some time, indicating the presence of protein.