Berthelots reagent is an alkaline solution of phenol and hypochlorite, used in analytical chemistry.  It is named after its inventor, Marcellin Berthelot. Ammonia reacts with Berthelots reagent to form a blue product which is used in a colorimetric method for determining ammonia. The reagent can also be used for determining urea.  In this case the enzyme urease is used to catalyze the hydrolysis of urea into carbon dioxide and ammonia.  The ammonia is then determined with Berthelot's reagent.