What is mercury oxide?

The mercuric oxide (I) or ferric oxide, whose chemical formula is represented as Hg 2 O, is a compound in solid phase, considered as toxic and unstable from the chemical point of view, becoming mercury in elemental form and oxide mercury (II).

There are only two chemical species that mercury can form when it combines with oxygen, because this metal has only two oxidation states (Hg + and Hg 2+ ): mercury (I) oxide and mercury (II) oxide. Mercury (II) oxide is in a solid state of aggregation, being obtained in two relatively stable crystalline forms.

This compound is also known simply as mercury oxide, so only this species will be discussed hereinafter. A very common reaction that occurs with this substance is that, when subjected to heating, its decomposition occurs, producing mercury and gaseous oxygen in an endothermic process.

Chemical structure

Under atmospheric pressure conditions, this species occurs in only two crystalline forms: one called cinnabar and the other known as montrodite, which is very rarely found. Both forms become tetragonal above 10 GPa of pressure.

The structure of cinnabar is based on primitive hexagonal cells (hP6) with trigonal symmetry, whose helical axis is oriented to the left (P3 2 21); on the other hand, the structure of montrodite is orthorhombic, based on a primitive lattice that forms sliding planes perpendicular to the three axes (Pnma).

In contrast, two forms of mercury oxide can be visually distinguished, because one is red and the other is yellow. This distinction in color occurs thanks to the dimensions of the particle, because the two shapes have the same structure.

The red form of mercury oxide can be produced by heating metallic mercury in the presence of oxygen at a temperature around 350 ° C, or by pyrolysis of mercury (II) nitrate (Hg (NO 3 ) 2 ).

Similarly, to produce the yellow form of this oxide, the precipitation of the Hg 2+ ion in aqueous form with a base can be used.

Mercury oxide properties

– It has a melting point of approximately 500 ° C (equivalent to 773 K), above which it undergoes decomposition, and a molar mass or molecular weight of 216.59 g / mol.

– It is in a solid state of aggregation in different colors: orange, red or yellow, according to the degree of dispersion.

– It is an oxide of inorganic nature, whose ratio with oxygen is 1: 1, which makes it a binary species.

– It is considered insoluble in ammonia, acetone, ether and alcohol, as well as in other solvents of an organic nature.

– Its solubility in water is very low, being approximately 0.0053 g / 100ml at standard temperature (25 ° C) and increasing with increasing temperature.

– It is considered soluble in most acids; however, the yellow form shows greater reactivity and greater dissolving capacity.

– When mercury oxide is exposed to air, it decomposes, while its red form does so when exposed to light sources.

– When subjected to heating to the temperature at which it decomposes, it releases highly toxic mercury gases.

– Only when heated to 300-350 ° C can mercury combine with oxygen at a profitable rate.

Uses / applications

It is used as a precursor to obtain elemental mercury, because it undergoes decomposition processes quite easily; in turn, when it decomposes, it produces oxygen in its gaseous form.

Similarly, this oxide of inorganic nature is used as a standard titration or titration agent for anionic species, since a compound is generated that has greater stability than its initial form.

In this sense, mercury oxide undergoes dissolution when it is found in concentrated solutions of basic species, producing compounds called hydroxocomplexes.

These compounds are complexes with the structure M x (OH) y , where M represents a metallic atom and the subscripts x and y represent the number of times that species is found in the molecule. They are extremely useful in chemical research.

In addition, mercury (II) oxide can be used in laboratories for the production of different salts of the metal; for example, mercury (II) acetate, which is used in organic synthesis processes.

This compound, when mixed with graphite, is also used as a cathodic electrode material in the production of mercury batteries and mercury-zinc oxide electrical cells.


– This substance, which shows basic characteristics in a very weak way, is a very useful reagent for various applications such as those mentioned previously, but at the same time it presents significant risks for humans when exposed to it.

– Mercury oxide has high toxicity, being able to be absorbed through the respiratory tract, since it releases irritating gases when it is in the form of an aerosol, in addition to being extremely toxic if it is ingested or if it is absorbed through the skin when entering direct contact with it.

– This compound causes eye irritation and can cause kidney damage that later leads to kidney failure problems.

– When it is consumed in one way or another by aquatic species, this chemical substance bioaccumulates in them and affects the organism of human beings who regularly consume them.

– The heating of the mercury oxide originates mercury vapors that have high toxicity in addition to gaseous oxygen, thus increasing the risk of flammability; that is to say, to produce fires and to improve combustion in them.

– This inorganic oxide has a powerful oxidizing behavior, for which it produces violent reactions when it comes into contact with reducing agents and certain chemical substances such as sulfur chloride (Cl 2 S 2 ), hydrogen peroxide (H 2 O 2 ), chlorine and magnesium (only when heated).

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