Mercury(II) iodide

Mercury(II) iodide

File:Mercury(II) iodide Crystal in VCGS furnace.jpg

Mercury(II) iodide crystal in a VCGS furnace

Mercury(II) iodide, also known as mercuric iodide, is a chemical compound with the formula HgI_. It is a red or orange solid that is insoluble in water. Mercury(II) iodide is known for its polymorphism, existing in two distinct crystalline forms: the red alpha form and the yellow beta form.

Structure and Properties

Mercury(II) iodide exhibits interesting structural properties. The red form, which is stable at room temperature, has a tetragonal crystal structure. Upon heating to about 126 °C, it undergoes a phase transition to the yellow form, which has an orthorhombic structure. This transition is reversible, and cooling the yellow form will revert it back to the red form.

The compound is known for its high density and low solubility in water. It is also photosensitive, meaning it can decompose upon exposure to light, especially ultraviolet light.

Synthesis

Mercury(II) iodide can be synthesized by the direct reaction of elemental mercury with iodine. The reaction is typically carried out in an aqueous solution, where mercury is dissolved in a small amount of nitric acid to form mercury(II) nitrate, which then reacts with potassium iodide to precipitate mercury(II) iodide:

${\text{Hg(NO}}_3{\text{)}}_2+2\text{KI}\to {\text{HgI}}_2+2{\text{KNO}}_3$

Applications

Mercury(II) iodide has several applications, primarily in the field of medicine and chemistry. It is used in the preparation of Nessler's reagent, which is employed in the qualitative analysis of ammonia. Additionally, due to its high density and atomic number, mercury(II) iodide is used in X-ray and gamma-ray detection.

Safety and Handling

Mercury(II) iodide is toxic and should be handled with care. It can cause mercury poisoning if ingested or inhaled. Proper safety equipment, such as gloves and goggles, should be used when handling this compound. Disposal of mercury(II) iodide must be conducted in accordance with local environmental regulations to prevent contamination.

Related pages

• Mercury (element)
• Iodine
• Polymorphism (materials science)
• Phase transition