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| Names | |
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IUPAC name
potassium tetraiodidomercurate(II)
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| Other names
potassium mercuric iodide,
Nessler's reagent (principal component) | |
| Identifiers | |
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3D model (JSmol)
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| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.029.082 |
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PubChem CID
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| UNII | |
| UN number | 3287 |
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CompTox Dashboard (EPA)
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| Properties | |
| K2[HgI4] | |
| 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 | |
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Other anions
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Mercury(II) iodide |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 and structure
Crystallizing from a concentrated aqueous solution of mercuric iodide with potassium iodide is the monohydrate K2[HgI4]·H2O, which is pale orange. In aqueous solution this triiodido complex adds iodide to give the tetrahedral tetraiodo dianion.
Solutions of K2[HgI4] react with Cu(I) salts to give Cu2[HgI4] (copper(I) tetraiodomercurate(II)).
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 of iodide of Millon's base (HgO·Hg(NH2)I) may form. The sensitivity as a spot test is about 0.3 μg NH3 in 2 μL.
- NH+4 + 2 [HgI4]2− + 4 OH− → HgO·Hg(NH2)I↓ + 7 I− + 3 H2O
The formula for the brown precipitate, a derivative of Millon's base, is given as 3HgO·Hg(NH3)2I2 and as NH2·Hg2I3.