Nessler's Reagent is an alkaline solution of Potassium tetraiodomercurate(II) ($K_2[Hg{I}_4]$).

• It is used as a highly sensitive test for Ammonia ($N{H}_3$) or Ammonium ions ($N{H}_4^{+}$).
• Reaction: Ammonia reacts with the reagent to produce a brown-colored precipitate of Iodide of Millon's base ($N{H}_2-Hg-O-Hg-I$).
• This test is so sensitive that it can detect even trace amounts of ammonia in drinking water.