Mercury(II) iodide

Mercury(II) iodide (α form)

Mercury(II) iodide (β form)

β (left) and α (right) forms
IUPAC name
Mercury(II) iodide
Other names
Mercury diiodide
Mercuric iodide
Red mercury (α form only)
Coccinite (α form only)
3D model (JSmol)
ECHA InfoCard 100.028.976 Edit this at Wikidata
EC Number
  • 231-873-8
  • InChI=1S/Hg.2HI/h;2*1H/q+2;;/p-2 checkY
  • InChI=1/Hg.2HI/h;2*1H/q+2;;/p-2
  • I[Hg]I
Molar mass 454.40 g/mol
Appearance orange-red powder
Odor odorless
Density 6.36 g/cm3
Melting point 259 °C (498 °F; 532 K)
Boiling point 350 °C (662 °F; 623 K)
6 mg/100 mL
Solubility slightly soluble in alcohol, ether, acetone, chloroform, ethyl acetate, CS2, olive oil, castor oil

Soluble in excess KI(Potassium iodide) forming soluble complex K2[Hg2I4 ](Potassium tetraiodomercurate(II)) also known as Nessler's reagent

−128.6·10−6 cm3/mol
D08AK30 (WHO)
GHS labelling:
GHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
H300, H310, H330, H373, H410
P260, P262, P264, P270, P271, P273, P280, P284, P301+P310, P302+P350, P304+P340, P310, P314, P320, P321, P322, P330, P361, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Flash point Non-flammable
Related compounds
Other anions
Mercury(II) fluoride
Mercury(II) chloride
Mercury(II) bromide
Other cations
Zinc iodide
Cadmium iodide
Related compounds
Mercury(I) 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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Mercury(II) iodide is a chemical compound with the molecular formula HgI2. It is typically produced synthetically but can also be found in nature as the extremely rare mineral coccinite. Unlike the related mercury(II) chloride it is hardly soluble in water (<100 ppm).


Mercury(II) iodide is produced by adding an aqueous solution of potassium iodide to an aqueous solution of mercury(II) chloride with stirring; the precipitate is filtered off, washed and dried at 70 °C.

HgCl2 + 2 KI → HgI2 + 2 KCl


Mercury(II) iodide displays thermochromism; when heated above 126 °C (400 K) it undergoes a phase transition, from the red alpha crystalline form to a pale yellow beta form. As the sample cools, it gradually reacquires its original colour. It has often been used for thermochromism demonstrations.[2] A third form, which is orange, is also known; this can be formed by recrystallisation and is also metastable, eventually converting back to the red alpha form.[3] The various forms can exist in a diverse range of crystal structures and as a result mercury(II) iodide possesses a surprisingly complex phase diagram.[4]


Mercury(II) iodide crystals grown in Spacelab

Mercury(II) iodide is used for preparation of Nessler's reagent, used for detection of presence of ammonia.

Mercury(II) iodide is a semiconductor material,[5] used in some x-ray and gamma ray detection and imaging devices operating at room temperatures.[6]

In veterinary medicine, mercury(II) iodide is used in blister ointments in exostoses, bursal enlargement, etc.[citation needed]

It can appear as a precipitate in many reactions.

See also


  1. ^ John Rumble (June 18, 2018). CRC Handbook of Chemistry and Physics (99 ed.). CRC Press. pp. 5–189. ISBN 978-1138561632.
  2. ^ Thermochromism: Mercury(II) Iodide. Retrieved on 2011-06-02.
  3. ^ SCHWARZENBACH, D. (1 January 1969). "The crystal structure and one-dimensional disorder of the orange modification of HgI2". Zeitschrift für Kristallographie - Crystalline Materials. 128 (1–6): 97–114. doi:10.1524/zkri.1969.128.16.97. S2CID 96682743.
  4. ^ Hostettler, Marc; Schwarzenbach, Dieter (February 2005). "Phase diagrams and structures of HgX2 (X = I, Br, Cl, F)". Comptes Rendus Chimie. 8 (2): 147–156. doi:10.1016/j.crci.2004.06.006.
  5. ^ Ayres, F.; Assali, L.V. C.; Machado, W. V. M.; Justo, J. F. (2006). "Role of intrinsic defects in the electronic and optical properties of alpha-HgI2". Appl. Phys. Lett. 88: 011918. doi:10.1063/1.2159573.
  6. ^ Simage, Oy U.S. patent 6,509,203 Semiconductor imaging device and method for producing same, Issue date: Jan 21, 2003