Potassium tetraiodomercurate(II)

Potassium tetraiodomercurate(II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate(II) anion. It is mainly used as Nessler's reagent, a 0.09 mol/L solution of potassium tetraiodomercurate(II) (K2[HgI4]) in 2.5 mol/L potassium hydroxide, used to detect ammonia.
Preparation and structure.
Crystallizing from a concentrated aqueous solution of mercuric iodide with potassium iodide is the monohydrate KHgI3.H2O, which is pale orange. In aqueous solution this triodido complex adds iodide to give the tetrahedral tetraiodo dianion.
Solutions of K2HgI4 react with Cu(I) salts to give Cu2HgI4.
Nessler's reagent.
Named after Julius Neßler (Nessler), an alkaline solution of K2HgI4 is called Nessler's reagent. This pale solution becomes deeper yellow in the presence of ammonia. At higher concentrations, a brown precipitate may form. The sensitivity as a spot test is about 0.3 μg NH3 in 2 μL.
The formula for the brown precipitate, a derivative of Millon's base, is given as 3HgO·Hg(NH3)2I2 and as NH2·Hg2I3.