Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula . It consists of potassium cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of ammonia.
The compound crystallizes from a heated solution of mercuric iodide, potassium iodide, and precisely 2% water in acetone. Attempted synthesis in concentrated aqueous solution will give the pale orange monohydrate instead.[1]
is a precursor to analogous copper and silver salts (M=Cu, Ag).[2]
Nessler's 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 Millon's base may form. The sensitivity as a spot test is about 0.3 μg in 2 μL.
The brown precipitate is not fully characterized and may vary from to .