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. 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).

Synthesis and reactions

thumb|left|Montroydite structure (red atoms are oxygen)

thumb|left|[[Cinnabar structure]]

The red form of HgO can be made by heating Hg in oxygen at roughly 350&nbsp;°C, or by pyrolysis of Hg(NO<sub>3</sub>)<sub>2</sub>. The yellow form can be obtained by precipitation of aqueous Hg<sup>2+</sup> with alkali. 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. At pressures above 10 GPa both structures convert to a tetragonal form.

Health issues

thumb|left|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.

Evaporation at 20&nbsp;°C is negligible. HgO decomposes on exposure to light or on heating above 500&nbsp;°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.

References

  • National Pollutant Inventory – Mercury and compounds fact sheet
  • Information at Webelements.