Potassium tetraiodomercurate(II)

Potassium tetraiodomercurate(II)
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UN number	3287
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Template:Longitem	Template:Chem2
Molar mass	Template:Chem molar mass
Appearance	yellow crystals
Odor	odorless
Density	4.29 g/cm3
Solubility	soluble in alcohol, ether, acetone
Template:Longitem	Mercury(II) iodide

Potassium tetraiodomercurate(II) is an inorganic compound with the chemical formula Template:Chem2. It consists of potassium cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of ammonia.^[2]

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 Template:Chem2 instead.^[3]

Template:Chem2 is a precursor to analogous copper and silver salts Template:Chem2 (M=Cu, Ag).^[4]

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 (Template:Chem2). At higher concentrations, a brown precipitate derivative of Millon's base (Template:Chem2) may form. The sensitivity as a spot test is about 0.3 μg [[Ammonia|Template:Chem2]] in 2 μL.^[5]

The brown precipitate is not fully characterized and may vary from Template:Chem2 to Template:Chem2.^[6]