Overview
Mercury(II) iodide, commonly called mercuric iodide, is the inorganic compound with the formula HgI2. It consists of mercury in the +2 oxidation state and iodide anions; further background on the mercury ion and iodide ion clarifies its composition. The compound is best known for its striking red crystalline form at room temperature and for a thermally induced yellow form at elevated temperature.
Physical and chemical characteristics
HgI2 shows polymorphism: a red alpha form stable at ordinary temperatures and a yellow beta form that appears after heating. This reversible color change is often described as thermochromism. It is essentially insoluble in water but dissolves in solutions containing excess iodide by forming complex anions. Chemically, it precipitates when iodide ions meet soluble mercury(II) salts via the reaction Hg2+ + 2 I- → HgI2(s). Detailed notes on its oxidation state and bonding explain why it behaves as a covalent/ionic hybrid.
Preparation and reactions
Synthesis is typically accomplished by mixing aqueous solutions of a mercury(II) salt and potassium iodide, which yields a bright red precipitate that can be purified by recrystallization. In excess iodide the solid red form converts to soluble tetraiodomercurate(II) species. On heating it transforms to the yellow phase and can decompose if strongly overheated, releasing toxic mercury vapors.
Uses and importance
Historically it saw limited medicinal and pigment use, but such applications declined because of toxicity. Today HgI2 is valued as a semiconductor material for room-temperature X-ray and gamma-ray detectors and in specialized optical and electronic devices. Its light-absorption properties have motivated research into thin-film and sensor applications; further technical context can be found at related resources.
Safety and notable facts
Like other mercury compounds, mercury(II) iodide is highly toxic. Exposure risks include inhalation of dust or vapors and skin absorption; proper handling, containment and disposal are essential. Because of its toxicity and environmental persistence, many former uses have been discontinued in favor of safer alternatives. For laboratory work consult material safety guidance and authoritative sources before use.