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.[2]
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IUPAC name
potassium tetraiodidomercurate(II)
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Other names
potassium mercuric iodide,
Nessler's reagent (principal component) | |
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3D model (JSmol)
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ChEBI |
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ECHA InfoCard | 100.029.082 |
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PubChem CID
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UNII |
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UN number | 3287 |
CompTox Dashboard (EPA)
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Properties | |
K2[HgI4][1] | |
Molar mass | 786.406 g·mol−1 |
Appearance | yellow crystals |
Odor | odorless |
Density | 4.29 g/cm3 |
very soluble | |
Solubility | soluble in alcohol, ether, acetone |
Hazards | |
Safety data sheet (SDS) | External MSDS for Nessler's reagent |
Related compounds | |
Other anions
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Mercury(II) iodide |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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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.[3]
K2[HgI4] is a precursor to analogous copper and silver salts M2[HgI4] (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 (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.[5]
The brown precipitate is not fully characterized and may vary from HgO·Hg(NH2)I to 3HgO·Hg(NH3)2I2.[6]