Potassium tetraiodomercurate(II)

Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula K2[HgI4]. It consists of potassium cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of ammonia.

Preparation
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 K[Hg(H2O)I3] instead.

Applications
K2[HgI4] is a precursor to analogous copper and silver salts M2[HgI4] (M=Cu, Ag).

Nessler's reagent
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 (NH3). At higher concentrations, a brown precipitate derivative of Millon's base (HgO*Hg(NH2)Cl) may form. The sensitivity as a spot test is about 0.3 μg NH3 in 2 μL.


 * NH4+ + 2 [HgI4](2−) + 4 OH− → HgO*Hg(NH2)I↓ + 7 I− + 3 H2O

The brown precipitate is not fully characterized and may vary from HgO*Hg(NH2)I to 3HgO*Hg(NH3)2I2.