Mercury(II) iodide is the inorganic compound with the molecular formula HgI2. The compound has attracted interest because it exhibits thermochromism, i.e. its color changes markedly with temperature. 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 poorly soluble in water (<100 ppm).
Production Mercury(II) iodide precipitates when an aqueous solution of potassium iodide is added to an aqueous solution of mercury(II) chloride.
HgCl2 + 2 KI → HgI2 + 2 KCl
Properties Mercury(II) iodide displays thermochromism; when heated above 127 °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. 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. 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.
Structure Mercuric iodide is a linear monomer in solution. Upon crystallization, it forms various polymorphs.
Uses
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, used in some x-ray and gamma ray 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, mercury(II) iodide is used in blister ointments in exostoses, bursal enlargement, etc.
It can appear as a precipitate in many reactions.
See also
Mercury(I) iodide, Hg2I2
References