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.^[2]

Potassium tetraiodomercurate(II)

Names
IUPAC name potassium tetraiodidomercurate(II)
Other names potassium mercuric iodide, Nessler's reagent (principal component)
Identifiers
CAS Number	7783-33-7 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:51568 N
ChemSpider	22948 N
ECHA InfoCard	100.029.082
EC Number	231-990-4
PubChem CID	15980736
UNII	7N83S6IU0F Y
UN number	3287
CompTox Dashboard (EPA)	DTXSID30893684
InChI InChI=1S/Hg.4HI.2K/h;4*1H;;/q+2;;;;;2*+1/p-4 N Key: OPCMAZHMYZRPID-UHFFFAOYSA-J N
SMILES [K+].[K+].I[Hg-2](I)(I)I
Properties
Chemical formula	K2[HgI4][1]
Molar mass	786.406 g·mol−1
Appearance	yellow crystals
Odor	odorless
Density	4.29 g/cm3
Solubility in water	very soluble
Solubility	soluble in alcohol, ether, acetone
Hazards
Safety data sheet (SDS)	External MSDS for Nessler's reagent
Related compounds
Other anions	Mercury(II) iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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.^[3]

Applications

K₂[HgI₄] is a precursor to analogous copper and silver salts M₂[HgI₄] (M=Cu, Ag).^[4]

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.^[5]

NH+4 + 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.^[6]