Mercury(II) iodide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the molecular formula Hg I₂. It is typically produced synthetically but can also be found in nature as the extremely rare mineral

coccinite Coccinite is a rare mercury iodide mineral with chemical formula of HgI2, mercury(II) iodide. It was first discovered in Casas Viejas, Mexico; it has also been reported from Broken Hill, New South Wales, and from a uranium mine in Thuringia and ...

. Unlike the related

mercury(II) chloride Mercury(II) chloride (mercury bichloride, mercury dichloride, mercuric chloride), historically also sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2, used as a laboratory reagent. ...

it is hardly soluble in water (<100 ppm).

Production

Mercury(II) iodide is produced by adding an aqueous solution of

potassium iodide Potassium iodide is a chemical compound, medication, and dietary supplement. It is a medication used for treating hyperthyroidism, in radiation emergencies, and for protecting the thyroid gland when certain types of radiopharmaceuticals are u ...

to an aqueous solution of

with stirring; the precipitate is filtered off, washed and dried at 70 °C. : HgCl₂ + 2 KI → HgI₂ + 2 KCl

Properties

Mercury(II) iodide displays

thermochromism Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an example of this property used in a consumer product although thermochromism also has more practical uses, such as baby bottles, which c ...

; when heated above 127 °C (400 K) it undergoes a

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

, 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

demonstrations. A third form, which is orange, is also known; this can be formed by recrystallisation and is also

metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...

, eventually converting back to the red alpha form. 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.

Uses

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

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

. Mercury(II) iodide is a

semiconductor material A semiconductor is a material with electrical conductivity between that of a Electrical conductor, conductor and an Insulator (electricity), insulator. Its conductivity can be modified by adding impurities ("doping (semiconductor), doping") to ...

, used in some

x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

and

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

detection and imaging devices operating at room temperatures.Simage, Oy Semiconductor imaging device and method for producing same, Issue date: Jan 21, 2003 In

veterinary medicine Veterinary medicine is the branch of medicine that deals with the prevention, management, medical diagnosis, diagnosis, and treatment of disease, disorder, and injury in non-human animals. The scope of veterinary medicine is wide, covering all a ...

, mercury(II) iodide is used in

blister A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled ...

ointment A topical medication is a medication that is applied to a particular place on or in the body. Most often topical medication means application to body surfaces such as the skin or mucous membranes to treat ailments via a large range of classes ...

s in

exostoses An exostosis, also known as a osteochondroma, is a benign chondrogenic lesions derived from aberrant cartilage from the perichondral ring. Exostoses can cause chronic pain ranging from mild to moderate, but rarely severe, depending on the shape, s ...

, bursal enlargement, etc. It can appear as a precipitate in many reactions.

References

{{Antiseptics and disinfectants Iodides Metal halides Mercury(II) compounds Semiconductor materials

Production

Properties

Uses

See also

References