MERCURY(II) OXIDE

576 MERCURY(II) OXIDE

Occurrence and Uses

Mercury(II) oxide is found natively in the mineral montroydite. The oxide is used primarily to prepare other mercury salts. The red form of the oxide is used as a depolarizer in a certain type of dry batteries. Other applications are paints and pigments; as a reagent in several wet analyses; and to catalyze in organic reactions. The oxide also is used in ointments as a topical antiseptic.

Physical Properties

Mercury(II) oxide exists in two modifications, red and yellow, differing in particle size—finer particles under 5µm appear red, while particle size greater than 8µm appear reddish. The yellow form converts to red oxide on heating, which reverts back to yellow oxide on cooling. At 400°C, the red oxide becomes black which changes back to red again on cooling. The oxide decomposes at 500°C.

Orthogonal crystalline powder; refractive index (for montroydite) 2.37; density 11.14 g/cm3; Moh’s hardness 2.5; insoluble in water and ethanol; soluble in dilute acids and aqueous solutions of alkali iodides and cyanides.

Thermochemical Properties (Red Modification)

Preparation

Yellow mercury(II) oxide is precipitated upon the addition of excess caustic soda or caustic potash to an aqueous solution of mercury(II) nitrate or chloride:

Hg2+ + 2OH¯ → HgO + H2O (yellow)

Most precipitation methods give yellow mercury(II) oxide. However, if barium hydroxide is added to a hot solution of mercury(II) nitrate, the product is red oxide.

Red mercury(II) oxide is obtained by heating mercury in air or oxygen (preferably under pressure) at 450°C:

2Hg + O2 → 2HgO

(red)

or by thermal decomposition of mercury(II) nitrate or mercury(I) nitrate: Hg(NO3)2 → HgO + 2NO2 + ½ O2

Hg2(NO3)2 → 2HgO + 2NO2

or by boiling the yellow oxide in a solution of sodium or potassium chloride. Mercury(II) oxide also is obtained as an intermediate when cinnabar (HgS)

MERCURY(II) OXIDE 577

is roasted in air in the extraction of mercury.

Reactions

The yellow form of the HgO is more reactive than the red form due to finer particle size.

When heated at 500°C, mercury(II) oxide decomposes to its elements:

HgO → Hg + ½ O2

Mercury(II) oxide is a basic oxide. It reacts with acids forming their mercury(II) salts:

HgO + 2HCl → HgCl2 + 2H2O

HgO + H2SO4 → HgSO4 + H2O

Mercury(II) oxide dissolves in neutral sodium or potassium sulfite solution forming sulfitomercurate(II) ion:

HgO + 2HSO3¯ → [Hg(SO3)2]2¯ + H2O

Upon evaporation of the solution, complex salt crystals such as Na2[Hg(SO3)2] or K2[Hg(SO3)2] form.

Mercury(II) oxide reacts with potassium iodide in solution, forming potassium tetraiodomercurate(II):

HgO + 4KI + H2O → K2HgI4 + 2KOH

Reaction with chlorine yields chlorine monoxide, ClO, and in aqueous solution or with chlorine water, the product is hypochlorous acid, HOCl:

HgO + Cl → Hg + ClO

The oxide is reduced to mercury metal when heated with mercury(II) sulfide:

2HgO + HgS → 3Hg + SO2

Similarly, reaction with zinc and other reducing agents converts the oxide to metallic mercury:

HgO + Zn → ZnO + Hg

Mercury(II) oxide dissolves in a saturated solution of mercury(II) cyanide, forming mercury oxycyanide, Hg(CN)2•HgO.

Mercury(II) oxide dissolves in an aqueous solution of acetamide forming mercury(II) acetamide. In this reaction, mercury replaces both the hydrogen atoms of the amide functional group, forming covalent N—Hg bond:

578 MERCURY(II) SULFATE

HgO + 2CH3CONH2 → (CH3CONH)2Hg + H2O

Similarly, reaction with succinimide yields mercury succinimide [584-43-0]. The yellow oxide reacts with aqueous ammonia to yield a bright yellow

crystalline powder, dihydroxymercury(II)–ammonium hydroxide, known as Millon’s base, [(HO–Hg)2NH2]OH [12529-66-7], a photosensitive unstable product.

Mercury(II) oxide yields mercury(II) salicylate [5970-32-1] when added to an aqueous soution of salicyclic acid:

The product was used medicinally for the treatment of syphilis.

Analysis

Elemental composition: Hg 92.61%, O 7.39%. The compound is digested in nitric acid and mercury in an appropriately diluted acid extract and then is measured by cold-vapor AA or by ICP. The compound may be characterized nondestructively by x-ray methods.

Toxicity

It is highly toxic by ingestion and inhalation as dust. Toxic symptoms are those of mercury (see Mercury). The oral LD50 in rodents is between 15 to 20 mg/kg.

MERCURY(II) SULFATE

[7783-35-9]

Formula: HgSO4; MW 296.65 Synonym: mercuric sulfate

Uses

Mercury(II) sulfate is an electrolyte for primary batteries. Other applications are in extraction of gold and silver from roasted pyrites; as a catalyst in organic reactions such as conversion of acetylene to acetaldehyde; and as a reagent for wine coloring.

Physical Properties

White monoclinic crystal; hygroscopic; density 6.47 g/cm3; decomposes in water; soluble in hot dilute sulfuric acid, hydrochloric acid and concentrated solution of sodium chloride.