Mercury(II) iodide

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 ==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. ==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. 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==