Reaction of concentrated nitric acid with some substituted aromatic rings gives a yellow color (Xanthos = yellow in Greek).
Procedure
Set up test tubes as follows: In one tube put 2 drops of tryptophan solution, in another 2 drops of unknown compound, in another 2 drops of glycine solution, and in another about 5 mg tryosine powder. Then with great care, add 1.0 mL concentrated nitric acid (HNO3) to each test tube. Hold the tubes so that they do not point at you or anyone else. Heat the tubes gently in a water bath until they boil. Cool the tubes slowly and add the 4% NaOH, drop by drop, until the solutions are alkaline.
Did any color change take place before the alkali was added? What was the final color of each of the solutions?
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