Let’s Learn about mercury compounds, their properties, industrial uses, toxicity, preparation, and environmental impact in this detailed guide. Mercury (Hg) is a unique metal that exists in both liquid and solid states under specific conditions. Its compounds are widely studied due to their significant roles in industries, environmental impact, and health concerns. Mercury is found in two oxidation states: mercurous (Hg⁺¹) and mercuric (Hg⁺²). These oxidation states give rise to various compounds with distinct chemical properties and uses.
Introduction to Mercury Compounds
Calomel or mercurous chloride (Hg2Cl2)
Mercury (I) chloride is a 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 an odorless solid and is the principal example of a mercury (I) compound. It is a component of reference electrodes in electrochemistry.
Basic Information:
| Chemical formula | Hg2Cl2 |
| Molar mass | 472.09 g/mol |
| Appearance | White solid |
| Density | 7.150 g/cm3 |
| Melting point | 525°C (977°F; 798K) (triple point) |
| Boiling point | 383°C (721°F; 656K) (sublimes) |
| Solubility in water | 0.2 mg/100 mL |
| Solubility | insoluble in ethanol, ether |
| Refractive index | 1.973mercury compounds, mercuric chloride, mercury oxide, methylmercury toxicity, mercury pollution |
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
- Color: 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
| Element percentages in dimercury dichloride | |
| Element | % |
| Cl | 15.02 |
| Hg | 84.98 |
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 an amorphous powder.
- It is insoluble in water, and 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 a fungicide.
- It is used to make electrodes.
- It is used as a 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 the toxicity of mercury and the availability of superior treatments.
Basic information
| Chemical formula | HgCl2 |
| Molar mass | 271.52 g/mol |
| Appearance | colorless or white solid |
|
Odor |
odorless |
| Density | 5.43 g/cm3 |
| Melting point | 276°C (529°F; 549K) |
| Boiling point | 304°C (579°F; 577K) |
| Solubility in water | 3.6 g/100 mL (0 °C) 7.4 g/100 mL (20 °C) 48 g/100 mL (100 °C) |
| Solubility | 4 g/100 mL (ether) soluble in alcohol, acetone, etc. slightly soluble in benzene, pyrimidine |
| Acidity | 3.2 (0.2M solution) |
| Refractive index | 1.859 |
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
- Color: white
- Appearance: crystalline solid
- Melting point: 280°C
- Boiling point: 303°C
- Density: 5600 kg m-3
Element analysis
| Element percentages in mercury dichloride | |
| Element | % |
| Cl | 26.12 |
| Hg | 73.88 |
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
- 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 a 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 an antiseptic for sterilizing surgical instruments.
