The Nessler’s reagent is an aqueous solution of potassium iodide, mercuric chloride, and potassium hydroxide which is used to determine the presence of ammonia in a specific material. When this reagent detects ammonia in a particular substance, the color of the solution will change to yellow. At higher concentration, it tends to form brown precipitate. Nessler’s test is usually employed in our water systems. Significant amount of ammonia may be detrimental to the aquatic resources such as fishes and crustaceans.
The simplified chemical reaction can be written as this is done:
NH4+ + 2[HgI4]2− + 4OH − → HgO·Hg(NH2)I + 7I− + 3H2O