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)
7783-33-7 YesY
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PubChem 24542
UN number 3287
Molar mass 786.41 g·mol−1
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 the 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.[1]

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

Solutions of K2HgI4 react with Cu(I) salts to give Cu2HgI4.[4]

Nessler's reagent[edit]

Named after Julius 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 + 4OH → HgO·Hg(NH2)I ↓ + 7I + 3H2O

The formula for the brown precipitate is given as 3HgO.Hg(NH3)2I2 and as NH2.Hg2I3[5]

Nessler's reagent is generally prepared by combining potassium iodide and mercury(II) chloride.[6] Nessler's reagent may be used with Nessler tubes.

Thoulet's solution[edit]

A concentrated solution of K2HgI4 is sometimes referred to as Thoulet solution. Like Clerici solution, Thoulet's solution is highly dense and is used to assess the density of solids.[2]


  1. ^ 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 
  2. ^ a b 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.
  3. ^ 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. 
  4. ^ 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.
  5. ^ 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. 
  6. ^ "Microbiological Investigations". Fisheries and Aquaculture Department, FAO. Retrieved April 25, 2014. 

External links[edit]