Nesslers reagent, named after Julius Neßler (Nessler), is a 0.09 mol/L solution of potassium tetraiodomercurate(II) in 2.5 mol/L potassium hydroxide. This pale solution becomes deeper yellow in the presence of ammonia (). At higher concentrations, a brown precipitate derivative of Millons base () may form. The sensitivity as a spot test is about 0.3 μg Ammonia| in 2 μL.
The brown precipitate is not fully characterized and may vary from  to .