Nessler's reagent is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide. Its chemical formula is K2HgI4. It is used to detect the presence of ammonia. It turns into a pale brown colour in the presence of ammonia.
Nessler’s Reagent Reaction:
2Nessler′sreagent⏟K2HgI4+NH3+3KOH→IodideofMillon′sbase(brownprecipitate)⏟HgO.Hg(NH2)I+7KI+2H2O