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
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.029.082
EC Number
  • 231-990-4
UNII
UN number 3287
  • InChI=1S/Hg.4HI.2K/h;4*1H;;/q+2;;;;;2*+1/p-4 ☒N
    Key: OPCMAZHMYZRPID-UHFFFAOYSA-J ☒N
  • [K+].[K+].I[Hg-2](I)(I)I
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
 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

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.[3] In aqueous solution this triiodido complex adds iodide to give the tetrahedral tetraiodo dianion.[4]

Solutions of K2[HgI4] react with Cu(I) salts to give Cu2[HgI4] (copper(I) tetraiodomercurate(II)).[5]

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.[6]

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.[7]

References
  1. Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. p. 4-82. ISBN 978-1-4200-9084-0.
  2. Vogel, Arthur I.; Svehla, G. (1979), Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis (5th ed.), London: Longman, ISBN 0-582-44367-9
  3. Wagenknecht, F.; Juza, R. (1963). "Potassium Triiodomercurate(II)". In Brauer, G. (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 1 (2nd ed.). Academic Press. p. 1100.
  4. Mok, K. F.; McKee, V. (1990). "Structure of a dipotassium tetraiodomercurate(II) salt with dibenzo-18-crown-6". Acta Crystallographica C. 46 (11): 2078–2081. doi:10.1107/S0108270190003742.
  5. Wagenknecht, F.; Juza, R. (1963). "Copper(I) Tetraiodomercurate(II)". In Brauer, G. (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 1 (2nd ed.). Academic Press. p. 1100.
  6. Vogel, Arthur I. (1954), A Textbook of Macro and Semimicro Qualitative Inorganic Analysis (4th ed.), London: Longman, p. 319, ISBN 0-582-44367-9
  7. Vogel, Arthur I.; Svehla, G. (1979), Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis (5th ed.), London: Longman, pp. 293–294, ISBN 0-582-44367-9
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