Mercury(II) oxide

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Mercury(II) oxide
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UN number 1641
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Template:Longitem Template:Chembox Elements/molecular formula
Molar mass Template:Chem molar mass
Appearance Yellow or red solid
Odor odorless
Density 11.14 g/cm3
Melting point Template:Chembox CalcTemperatures
Solubility insoluble in alcohol, ether, acetone, ammonia
Band gap 2.2 eV[1]
Template:Longitem −44.0·10−6 cm3/mol
Template:Longitem 2.5 (550 nm)[1]
Template:Longitem 70 J·mol−1·K−1[2]
Template:Longitem −90 kJ·mol−1[2]
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Flash point Template:Chembox CalcTemperatures
Template:Longitem Mercury sulfide
Mercury selenide
Mercury telluride
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Cadmium oxide
Template:Longitem Mercury(I) oxide

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Mercury(II) oxide, also called mercuric oxide or simply mercury oxide, is the inorganic compound with the formula HgO. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is very rarely found.

History

An experiment for the preparation of mercuric oxide was first described by 11th century Arab-Spanish alchemist, Maslama al-Majriti, in Rutbat al-hakim.[4] It was historically called red precipitate (as opposed to white precipitate, mercuric amidochloride).

In 1774, Joseph Priestley discovered that oxygen was released by heating mercuric oxide, although he did not identify the gas as oxygen (rather, Priestley called it "dephlogisticated air," as that was the paradigm that he was working under at the time).[5]

Synthesis and reactions

File:Montroydite.jpg
Montroydite structure (red atoms are oxygen)
File:Cinnabar structure.jpg
Cinnabar structure

The red form of HgO can be made by heating Hg in oxygen at roughly 350 °C, or by pyrolysis of Hg(NO3)2.[6] The yellow form can be obtained by precipitation of aqueous Hg2+ with alkali.[6] The difference in color is due to particle size; both forms have the same structure consisting of near linear O-Hg-O units linked in zigzag chains with an Hg-O-Hg angle of 108°.[6]

HgO is soluble in many conventional strong acids through protonation of the anion.[7] The exceptions include acids which form insoluble mercury(II) salts, like mercury(II) iodide in the case of hydroiodic acid. Dissolution is also possible through complexation of the cation; e.g. cyanide ligands form stable water soluble mercury(II) complexes.

Structure

Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite (orthorhombic, 2/m 2/m 2/m, Pnma), and the second is analogous to the sulfide mineral cinnabar (hexagonal, hP6, P3221); both are characterized by Hg-O chains.[8] At pressures above 10 GPa both structures convert to a tetragonal form.[1]

Uses

Mercury oxide is sometimes used in the production of mercury as it decomposes quite easily. When it decomposes, oxygen gas is generated.Script error: No such module "Unsubst".

It is also used as a material for cathodes in mercury batteries.[9]

Health issues

File:HgOlabel.jpg
The label on an HgO powder bottle.

Mercury oxide is a highly toxic substance which can be absorbed into the body by inhalation of its aerosol, through the skin and by ingestion. The substance is irritating to the eyes, the skin and the respiratory tract and may have effects on the kidneys, resulting in kidney impairment. In the food chain important to humans, bioaccumulation takes place, specifically in aquatic organisms. The substance is banned as a pesticide in the EU.[10]

Evaporation at 20 °C is negligible. HgO decomposes on exposure to light or on heating above 500 °C. Heating produces highly toxic mercury fumes and oxygen, which increases the fire hazard. Mercury(II) oxide reacts violently with reducing agents, chlorine, hydrogen peroxide, magnesium (when heated), disulfur dichloride and hydrogen trisulfide. Shock-sensitive compounds are formed with metals and elements such as sulfur and phosphorus.[11]

References

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External links

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