In the Berthelot method for ammonia determination, which color is produced that is detected colorimetrically?

• A. Indophenol blue
• B. Methylene blue
• C. Prussian blue
• D. Phenolphthalein

Answer: (A)

In the Berthelot method, ammonia in the sample reacts with phenol and hypochlorite under alkaline conditions to form indophenol blue. The intensity of this blue color is measured colorimetrically and is directly related to the amount of ammonia present. This makes indophenol blue the indicator dye produced by the reaction, which is why the blue color is detected to quantify ammonia. Other colors listed come from different chemistries: methylene blue isn’t produced in this reaction, Prussian blue arises from iron-cyanide chemistry, and phenolphthalein is just an acid–base indicator, not the dye formed in the Berthelot reaction.