Exists in two allotropic forms: red tetragonal allotropic modification (alpha form) and the yellow rhombic modification (beta form).
The red iodide has a density 6.36 g/cm3 at 25°C; transforms to yellow form at 127°C; also converts to yellow form at –180°C; slightly soluble in water (100 mg/L at 25°C); moderately soluble in acetone and alcohol.
The yellow iodide has a density of 6.09 g/cm3 at 27°C; melts at 259°C; vaporizes at 354°C; practically insoluble in water; low-to-moderately soluble in alcohol, benzene and other organic solvents.
Mercury(II) iodide is used as an analytical reagent, in Nesslers reagent for the analysis of ammonia. The compound also is used in ointments for the treatment of skin diseases.
Mercury(II) iodide is precipitated in its yellow form by adding a stoichiometric amount of potassium iodide to an aqueous solution of mercury(II) salt (e.g., HgCl2):
Hg2+ + 2I¯ → HgI2
The yellow precipitate rapidly turns red and dissolves in solution when excess potassium iodide is added.
Also, mercury (II) iodide is formed when mercury is rubbed with iodine moistened with ethanol.
Mercury(II) iodide reacts with excess iodide ion forming complex tetraiodomercurate(II) ion, [HgI4]2¯:
HgI2 + 2I¯ → [HgI4]2¯
In caustic soda or caustic potash solution, mercury(II) iodide forms complex salts, Na2HgI4 and K2HgI4, respectively. Alkaline solution of this complex in excess potassium hydroxide is known as Nessler’s reagent, used to analyze ammonia. The reaction of Nessler’s reagent with ammonia may be written as:
2[HgI4]2¯ + NH3 + 3OH¯ → I—Hg—O—Hg—NH3 +7I¯ + 2H2O
Similar complexes of silver, copper and other metals are known. Some of them change colors on heating and are used in heat-sensitive paints and applied to machine parts made out of brass or iron:
2Cu2+ + Hg2+ + 6I¯ → Cu2HgI4 + I2
(red)
2Ag+ + Hg2+ + 6I¯ → Ag2HgI4 + I2
(yellow)
In general, mercury(II) iodide forms neutral complexes of compositions M2(HgI4) and M(HgI3) with alkali and alkaline metal iodides.
When heated with dilute caustic potash solution, a yellow brown powder of composition HgI2•3HgO is obtained.
Mercuric iodide is a heavy, scarlet red, odorless, crystalline solid. It may be shipped as a red solution. It turns to a yellow powder at 127℃ and red upon cooling. Molecular weight= 454.40;Boiling point=(sublimes) 354℃; Freezing/Melting point=259℃. Hazard Identification (based on NFPA-704 M Rating System): Health 3, Flammability 0, Reactivity 0. Slightly soluble in water.
Mecuric iodide is a heavy, scarlet red, odorless, crystalline solid. It may be shipped as a red solution.
It turns to a yellow powder @ 127℃ and red upon cooling
Mercury(II) iodide is is used for preparation of Nessler's reagent, used for detection of presence of ammonia. It is a semiconductor material, used in some x-ray and gamma ray detection and imaging devices operating at room temperatures. In veterinary medicine, it is used in blister ointments in exostoses, bursal enlargement, etc.
In animal chemistry for preparation of Nessler's Reagent (alkaline mercuric potassium iodide solution).
ChEBI: Mercury diiodide is a mercury coordination entity composed of mercury and iodine with formula HgI2.
A scarlet-red odorless tasteless powder. Sensitive to light. Insoluble in water and sinks in water. At elevated temperatures turns yellow but turns back to red upon cooling. Poison.
MERCURY(II) IODIDE is a mild reducing agent. Reacts with sodium azide to form mercury(II) azide, which is shock, friction, and heat sensitive. Incompatible with acetylene, ammonia, chlorine dioxide, azides, chlorine trifluoride, calcium (because of amalgam formation), sodium carbide, lithium, rubidium, copper .
Highly toxic by ingestion, inhalation, and
skin absorption; strong irritant.
All forms of exposure to MERCURY(II) IODIDE are hazardous. Acute systemic mercurialism may be fatal within a few minutes; death by uremic poisoning is usually delayed 5-12 days. Acute poisoning has resulted from inhaling dust concentrations of 1.2-8.5 mg/m 3 of air; symptoms include tightness and pain in chest, coughing, and difficulty in breathing. Ingestion causes necrosis, pain, vomiting, and severe purging. Contact with eyes causes ulceration of conjunctiv a and cornea. Contact with skin causes irritation and possible dermatitis; systemic poisoning can occur by absorption through skin.
Special Hazards of Combustion Products: Fumes from fire may contain toxic mercury vapor.
A poison. When heated
to decomposition it emits very toxic fumes
of Hg and I-. See also MERCURY(I1)
IODIDE.
Mercuric iodide is used in medicine
and in analytical chemistry.
If this chemical gets into the eyes, remove any contact lenses at once and irrigate immediately for at least 15 min, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts the skin, remove contaminated clothing and wash immediately with soap and water. Seek medical attention immediately. If this chemical has been inhaled, remove from exposure, begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR if heart action has stopped. Transfer promptly to a medical facility. When this chemical has been swallowed, get medical attention. Give large quantities of water and induce vomiting. Do not make an unconscious person vomit. Medical observation is recommended for 2448 h after breathing overexposure, as pulmonary edema may be delayed. As first aid for pulmonary edema, a doctor or authorized paramedic may consider administering a corticosteroid spray. Antidotes and special procedures for medical personnel: The drug NAP (n-acetyl penicillamine) has been used to treat mercury poisoning, with mixed success. Note to physician: For severe poisoning BAL [British AntiLewisite, dimercaprol, dithiopropanol (C3H8OS2)] has been used to treat toxic symptoms of certain heavy metals poisoning including mercury. Although BAL is reported to have a large margin of safety, caution must be exercised, because toxic effects may be caused by excessive dosage. Most can be prevented by premedication with 1-ephedrine sulfate (CAS: 134-72-5).
Color Code—Blue: Health Hazard/Poison: Store in a secure poison location. Prior to working with this chemical you should be trained on its proper handling and storage. Mercuric iodide must be stored to avoid contact with chlorine trifluoride, sodium, and potassium, since violent reactions occur. Store in tightly closed containers in a cool, well-ventilated area away from light, acids, and heat. Protect containers from physical damage.
This compound requires a shipping label of “POISONOUS/TOXIC MATERIALS” (solution). It falls in Hazard Class 6.1 and Packing Group II.
Crystallise it from MeOH or EtOH and wash it repeatedly with distilled water (solubility is 0.006% at ~25o). It has also been mixed thoroughly with excess 0.001M iodine solution, filtered, washed with cold distilled water, rinsed with EtOH and Et2O, and dried in air. It changes colour reversibly to yellow at ~130o. [Friend Nature 109 341 1922.] POISONOUS.
Violent reaction with active metals;
potassium, sodium, acids, chlorine trifluoride. Inorganic
mercury compounds are incompatible with acetylene,
ammonia, chlorine dioxide; azides, calcium (amalgam formation), sodium carbide, lithium, rubidium, copper.
Mercury iodide is a mild reducing agent. Keep away from
oxidizers. Reacts with sodium azide to form mercury(II)
azide, which is shock-, friction-, and heat-sensitive.
Incompatible with acetylene, ammonia, chlorine dioxide,
azides, chlorine trifluoride, calcium (because of amalgam
formation), sodium carbide, lithium, rubidium, copper
(NIOSH, 1997)
