Compounds of Mercury

Compound of mercury

Calomel or mercurous chloride (Hg₂Cl₂)

Mercury (I) chloride is the chemical compound with the formula Hg₂Cl₂. it is also termed as calomel (i.e. a mineral form, rarely found in nature) or mercurous chloride, this is dense white or yellowish-white in appearance and characterized by odorless solid and is the principal example of a mercury (I) compound. It is a component of reference electrodes in electrochemistry.

The synonyms of mercury chloride are as follows:

• dimercury dichloride
• mercury(I) chloride
• calomel
• dimercury dichloride
• mercurous chloride
• mercury chloride

The oxidation number of mercury in dimercury dichloride is 1.

• Formula: Hg₂Cl₂
• Hill system formula: Cl₂Hg₂
• Formula weight: 472.085
• Class: chloride

• Colour: white
• Appearance: Crystalline solid and an addition to sensitivity of light
• Melting point: 525°C (turns yellow on heating)
• Boiling point: 383°C
• Density: 7160 kg m^-3

Calomel contains 84.98% mercury and 15.02% chlorine within it.

Preparation

• From mercurous nitrate Hg₂(NO₃)₂: mercurous chloride can be prepared by treating mercurous nitrate with sodium chloride.

Hg₂(NO₃)₂ + 2NaCl → Hg₂Cl₂ + 2NaNO₃

• From mercuric chloride HgCl₂: when mercuric chloride and mercury react in the iron vessel the ratio 4:3, calomel is obtained.

HgCl₂ + Hg → Hg₂Cl₂

• From mercuric sulphate HgSO₄: When mercuric sulphate is treated with sodium chloride in the presence of mercury, calomel is obtained.

HgSO₄ + NaCl + Hg → Hg₂Cl₂ + Na₂SO₄

Physical properties of mercurous chloride

• It is amorphous powder.
• It is insoluble in water, acid but soluble in aquaregia.

Chemical properties

• Action with heat: when heated calomel gives mercuric chloride and mercury.

Hg₂Cl₂ → HgCl₂ + Hg

• Action with ammonia: it reacts with ammonia to form black ppt. of amine complex of mercuric chloride.

Hg₂Cl₂ + 2NH₃ → {Hg(NH₂)Cl + Hg} + NH₄Cl

Uses

• It is used as fungicide.
• It is used to make electrodes.
• It is used as the purgative in medicine.
• It is used for making other compounds of mercury.

Mercuric chloride or corrosive sublimate (HgCl₂)

Mercury(II) chloride or mercuric chloride (archaically, corrosive sublimate) is the chemical compound of mercury and chlorine with the chemical formula HgCl₂. . Once used as a treatment for an STD disease i.e. syphilis, it is no longer used for medicinal purposes because of toxicity of mercury and the availability of superior treatments.

The synonyms of mercuric chloride are as below:

• mercury dichloride
• mercury(II) chloride
• corrosive sublimate
• mercuric chloride
• mercury chloride

The oxidation number of mercury in mercury dichloride is 2.

• Formula: HgCl₂
• Hill system formula: Cl₂Hg₁
• Formula weight: 271.495
• Class: chloride

• Colour: white
• Appearance: crystalline solid
• Melting point: 280°C
• Boiling point: 303°C
• Density: 5600 kg m^-3

Element analysis

Mercuric chloride contains 26.12% chlorine and 73.88% mercury within it.

Preparation

• From Hg: when excess chlorine is passed over mercury, mercuric chloride is obtained.

Hg + Cl₂ → HgCl₂

• From mercuric sulphate: it is prepared by heating mercuric sulphate with sodium chloride in the presence on MnO₂.

HgSO₄ + 2NaCl → HgCl₂ + Na₂SO₄

• From aquaregia: aquaregia dissolves mercury and forms mercuric chloride.

HNO₃ + 3HCl + Hg → NOCl + HgCl₂ + 2H₂O

Physical properties of mercuric chloride

• It is the white crystalline solid.
• It is soluble in water.
• It is poisonous in nature.
• It is corrosive in nature.

Chemical properties

• Action with SnCl₂: at first mercuric chloride gets reduced to mercurous chloride and then to metallic chloride.

2HgCl₂ + SnCl₂ → Hg₂Cl₂ + SnCl₄

Hg₂Cl₂ + SnCl₂ → 2Hg + SnCl₄

• Action with KI: with HgCl₂, it forms mercuric iodide and with excess KI it forms complex potassium mercuric chloride.

HgCl₂ + 2KI → HgI₂ + 2KCl

HgI₂ + 2KI → K₂HgI₄

Nessler’s reagent: the alkaline solution of potassium mercuric iodide is known as Nessler's reagent. It is used to detect ammonia.

• Action with ammonia: it forms white ppt. of amine complex of mercuric chloride.

HgCl₂ + NH₃ → Hg(NH₂)Cl + NH₄Cl

• Action of heat; it decompose to Hg & Cl₂.

HgCl₂ → Hg + Cl₂

Uses

• It is used to prepare Kessler's reagent.
• It is used for the preparation of calomel.
• It is used as antiseptic for sterilizing surgical instruments.

Reference

poulse, tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. kathmandu: Vidhyarthi Pustak Bhandar, 2012.