Potassium tetraiodomercurate(II)

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Potassium tetraiodomercurate(II)
IUPAC name
potassium tetraiodidomercurate(II)
Other names
potassium mercuric iodide,
Nessler's reagent (principal component)
3D model (JSmol)
ECHA InfoCard 100.029.082
EC Number
  • 231-990-4
UN number 3287
Appearance yellow crystals
Odor odorless
Density 4.29 g/cm3
very soluble
Solubility soluble in alcohol, ether, acetone
Safety data sheet 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

Potassium tetraiodomercurate(II) is an inorganic compound consisting of potassium cations and the tetraiodomercurate(II) anion. It is mainly used as Nessler's reagent, a 0.09 mol/L solution of potassium tetraiodomercurate(II) (K2[HgI4]) in 2.5 mol/L potassium hydroxide, used to detect ammonia.[2]

Preparation and structure[edit]

Crystallizing from a concentrated aqueous solution of mercuric iodide with potassium iodide is the monohydrate KHgI3.H2O, which is pale orange.[3] In aqueous solution this triodido complex adds iodide to give the tetrahedral tetraiodo dianion.[4]

Solutions of K2HgI4 react with Cu(I) salts to give Cu2HgI4.[5]

Nessler's reagent[edit]

Named after Julius Neßler (Nessler), an alkaline solution of K2HgI4 is called Nessler's reagent. This pale solution becomes deeper yellow in the presence of ammonia. At higher concentrations, a brown precipitate may form. The sensitivity as a spot test is about 0.3 μg NH3 in 2 μL.[citation needed]

NH4+ + 2 [HgI4]2− + 4 OH(iodide of Millon's base)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.[clarification needed][6]


  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. ^ F. Wagenknecht, R. Juza, "Potassium Triiodomercurate(II)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. 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: 2078. doi:10.1107/S0108270190003742.
  5. ^ F. Wagenknecht, R. Juza, "Copper(I) Tetraiodomercurate(II)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1100.
  6. ^ Svehla, G. (1979). Vogel's Textbook of Macro and semimicro qualitative inorganic analysis (5th ed.). London: Longman Group. pp. 293–294. ISBN 0-582-44367-9.

External links[edit]