View source for Potassium tetraiodomercurate(II)

{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 439640770
| Name = 
| ImageFile = Potassium tetraiodomercurate(II).png
| ImageName = 
| ImageFile1 = Potassium tetraiodomercurate(II)-3D-vdW.png
| ImageSize1 = 
| IUPACName = potassium tetraiodidomercurate(II)
| OtherNames = potassium mercuric iodide, <br> Nessler's reagent (principal component)
| SystematicName = 
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 7783-33-7
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 7N83S6IU0F
| EINECS = 231-990-4
| RTECS = 
| UNNumber = 3287
| PubChem = 15980736
| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
| ChemSpiderID = 22948
| ChEBI_Ref = {{ebicite|changed|EBI}}
| ChEBI = 51568
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChI = 1S/Hg.4HI.2K/h;4*1H;;/q+2;;;;;2*+1/p-4
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = OPCMAZHMYZRPID-UHFFFAOYSA-J
| SMILES = [K+].[K+].I[Hg-2](I)(I)I
  }}
| Section2 = {{Chembox Properties
| Formula = {{chem2|K2[HgI4]}}<ref name=crc>{{CRC90|page=4-82}}</ref>
| K=2|Hg=1|I=4
| Appearance = yellow crystals
| Odor = odorless
| Density = 4.29 g/cm<sup>3</sup>
| Solubility = very soluble
| SolubleOther = soluble in [[ethanol|alcohol]], [[diethyl ether|ether]], [[acetone]]
| BoilingPtC = 
| pKa = 
}}
| Section3 = 
| Section4 = 
| Section5 = 
| Section6 = 
| Section7 = {{Chembox Hazards
| ExternalSDS = [http://msds.chem.ox.ac.uk/NE/Nesslers_reagent.html External MSDS for Nessler's reagent]
| HPhrases = 
| PPhrases = 
| GHS_ref = 
| NFPA-H = 
| NFPA-F = 
| NFPA-R = 
| NFPA-S =
| FlashPt = 
  }}
| Section8 = {{Chembox Related
| OtherAnions = [[Mercury(II) iodide]]
| OtherCations = 
  }}
}}
'''Potassium tetraiodomercurate(II)''' is an [[inorganic compound]] with the [[chemical formula]] {{chem2|K2[HgI4]|auto=1}}. It consists of [[potassium]] cations and tetraiodomercurate(II) anions. It is the active agent in Nessler's reagent, used for detection of [[ammonia]].<ref>{{VogelQualitative}}
</ref>

==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 [[water of crystallization|monohydrate]] {{chem2|K[Hg(H2O)I3]}} instead.<ref>{{cite book |last1=Wagenknecht |first1=F. |title=Handbook of Preparative Inorganic Chemistry |last2=Juza |first2=R. |publisher=Academic Press |year=1963 |editor-last=Brauer |editor-first=G. |edition=2nd |volume=1 |page=1100 |chapter=Potassium Triiodomercurate(II) |chapter-url=https://archive.org/details/Handbook_of_Preparative_Inorganic_Chemistry_1_2_Brauer/page/n1155/ |via=the [[Internet Archive]]}}</ref>  

== Applications ==
{{chem2|K2[HgI4]}} is a precursor to analogous [[copper]] and [[silver]] salts {{chem2|M2[HgI4]}}&nbsp;(M=Cu,&nbsp;Ag).<ref>{{cite book |last1=Wagenknecht |first1=F. |url= |title=Handbook of Preparative Inorganic Chemistry |last2=Juza |first2=R. |publisher=Academic Press |year=1963 |editor-last=Brauer |editor-first=G. |edition=2nd |volume=1 |page=1100 |chapter=Copper(I) Tetraiodomercurate(II) |chapter-url=https://archive.org/details/Handbook_of_Preparative_Inorganic_Chemistry_1_2_Brauer/page/n1155/ |via=the [[Internet Archive]]}}</ref>

===Nessler's reagent===
'''Nessler's reagent''', named after [[Julius Neßler]] (Nessler), is a 0.09&nbsp;mol/L solution of potassium tetraiodomercurate(II) in 2.5&nbsp;mol/L [[potassium hydroxide]]. This pale solution becomes deeper yellow in the presence of [[ammonia]] ({{chem2|NH3}}). At higher concentrations, a brown [[precipitate]] derivative of [[Millon's base]] ({{chem2|HgO*Hg(NH2)Cl}}) may form. The sensitivity as a [[spot test]] is about 0.3&nbsp;μg&nbsp;[[Ammonia|{{chem2|NH3}}]] in 2&nbsp;μL.<ref name="Vogel4th-319">{{VogelQualitative4th|page=319}}</ref>

:{{chem2|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 {{chem2|HgO*Hg(NH2)I}} to {{chem2|3HgO*Hg(NH3)2I2}}.<ref>{{VogelQualitative5th|pages=293–294}}</ref>

==References==
<references/>

==External links==
*[https://web.archive.org/web/20051015082344/http://www-cie.iarc.fr/htdocs/monographs/vol58/mono58-3.htm IARC Monograph: "Mercury and Mercury Compounds"]
*[https://web.archive.org/web/20060425193836/http://www.npi.gov.au/database/substance-info/profiles/53.html National Pollutant Inventory - Mercury and compounds fact sheet]

{{Mercury compounds}}
{{potassium compounds}}

[[Category:Iodo complexes]]
[[Category:Potassium compounds]]
[[Category:Mercury(II) compounds]]
[[Category:IARC Group 3 carcinogens]]
[[Category:Coordination complexes]]
[[Category:Chemical tests]]
[[Category:Mercurates]]