The reagents in common use are: Millon's reagent, a solution of mercuric nitrate containing nitrous acid, this gives a violet-red coloration; nitric acid, which gives a yellow colour, turning to gold when treated with ammonia (xanthoproteic reaction); fuming sulphuric acid, which gives violet solutions; and caustic potash and copper sulphate, which, on warming, gives a red to violet coloration (biuret reaction).
They give the biuret and xanthoproteic reactions, and form salts with both acids and bases.
Characteristic yellow staining of the skin round the mouth from the formation of xanthoproteic acid serves to distinguish it from poisoning by other acids.
How would you define xanthoproteic? Add your definition here.