7.3 Mercury
🎯 Learning Outcomes
The students should be able to:
- Describe the occurrence of mercury
- Describe the extraction of mercury
- State properties of mercury
- Explain the chemistry (preparation, properties, and uses) of calomel and corrosive sublimate
1 Mercury — Occurrence
Generally, mercury occurs in a combined state. The main ore is the sulphide ore, cinnabar (HgS). It has also been found as an amalgam of gold and silver.
- Hg is mainly extracted in Russia, Spain, Mexico, and Algeria.
- In Nepal, Hg is reported from Tirche Pani/Taruka and the Khimti river.
2 Extraction — Concentration & Roasting
Mercury is extracted from cinnabar. Extraction involves the following processes.
The ore is crushed in big jaw crushers, finely powdered in ball mills and sieved.
- This is the process of removing gangue particles from the ore.
- Gangue — unwanted material or impurities in the form of sand, rock, etc. that surrounds the mineral in an ore deposit.
- The sulphide ore is concentrated by the Froth Flotation Process:
- The ore is powdered and a suspension is created in water, and pine or eucalyptus oil is added.
- The suspension is stirred by a constant blast of compressed air.
- The oil (froth) wets the metal, and the water wets the gangue.
- This frothy metal sulphide ore is skimmed off from the top.
- The gangue particles remain at the bottom and are removed.
- In the modern process, the concentrated ore is mixed with 2% coke and charged into a shaft furnace.
- A blast of air is introduced from the bottom to burn the fuel for heating.
- The sulphide ore gets converted into oxide, which then decomposes to give metallic mercury in the vapour state.
- The vapour of mercury, along with gases, comes out from the furnace and is passed through Y-shaped water-cooled iron pipes, which act as a condenser — mercury vapour condenses here, while waste gases escape from the outlets.
- Mercury thus obtained is about 99.7% pure.
3 Extraction — Purification
Mercury thus obtained is not pure and contains impurities like Zn, Cd, Fe, Pb, Ag, Au, etc. It is refined by the following methods:
Certain reactive metals form their oxides due to partial oxidation by air. These are removed by filtration through chamois leather or canvas.
The impurities like Zn, Fe, Cu, etc. are removed by introducing the crude mercury into a tube that contains 5% HNO₃.
The metals form their soluble nitrates and get separated from mercury:
If any mercurous nitrate is formed, it is displaced by these metals:
The mercury thus purified may still contain Au, Ag, Pt, etc. as impurities. These are removed by vacuum distillation, where the more volatile mercury is distilled off, leaving the impurities behind.
4 Properties & Poisoning
- Mercury is a silvery-white liquid.
- It solidifies at −38.9°C and boils at 357°C.
- It is the heaviest liquid known at ordinary temperature; its density is 13.6.
Action with Air
At ordinary temperature, mercury does not react with air. When heated up to 350°C, it slowly oxidises to mercuric oxide.
On strong heating, it decomposes back to Hg and O₂.
Action with Acids
(a) With HCl: Mercury is not affected by dilute or concentrated HCl.
(b) With H₂SO₄: It does not react with dilute H₂SO₄, but reacts with concentrated H₂SO₄ to give SO₂ gas.
(c) With HNO₃: Mercury reacts with different concentrations of HNO₃ to give different oxides of nitrogen.
(d) With aqua regia: Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid in a molar ratio of 1:3. Mercury dissolves in aqua regia to form mercuric chloride.
Action with Halogen
When mercury is heated with a halogen, it gives a different product under different conditions.
Mercury vapours are poisonous and can cause giddiness, tremors, lung and brain damage if inhaled. Many organic and inorganic mercuric compounds are also poisonous — they may cause deafness, brain damage, and even death.
5 Uses of Mercury
- In thermometers, barometers, etc.
- In mercury vapour lamps.
- In the extraction of gold & silver by the amalgamation process.
- To make amalgams — some amalgams are used in dentistry.
- In the manufacture of ayurvedic medicine, e.g., “Paro Vasma”.
6 Calomel (Hg₂Cl₂)
Mercurous chloride
Preparation
1. From Mercuric Chloride
When HgCl₂ and Hg are mixed in a 4:3 ratio and heated in an iron vessel, Hg₂Cl₂ sublimes off.
It may also be prepared by reducing mercuric chloride with stannous chloride:
2. From Mercuric Sulphate
When mercuric sulphate is heated with sodium chloride in the presence of mercury, Hg₂Cl₂ is formed.
3. From Mercuric Nitrate
When mercuric nitrate solution is treated with any soluble chloride, Hg₂Cl₂ is formed.
Properties
- White powder, insoluble in cold water, soluble in hot water.
- It sublimes when heated.
Action of Heat
It dissociates into Hg and mercuric chloride when strongly heated.
Action with Ammonia
When Hg₂Cl₂ is treated with ammonia, a black precipitate of metallic mercury and basic mercuric aminochloride is formed.
Action with Stannous Chloride
When Hg₂Cl₂ is treated with SnCl₂, it reduces Hg₂Cl₂ to Hg.
Action with Aqua Regia
Mercurous chloride dissolves in aqua regia due to the formation of mercuric chloride.
Uses of Calomel
- In medicine as a purgative (to clean the intestines).
- In the manufacture of calomel electrodes.
7 Corrosive Sublimate (HgCl₂)
Mercuric chloride
Preparation
1. From Mercury
When mercury is heated with chlorine gas, HgCl₂ is formed.
It is also prepared by treating mercury, or mercurous chloride, with aqua regia:
2. From HgSO₄
When mercuric sulphate is heated with sodium chloride in the presence of MnO₂, HgCl₂ sublimes off.
3. From HgO
When mercuric oxide is treated with dilute HCl, HgCl₂ is formed.
Properties
- White crystalline solid.
- Slightly soluble in cold water, but strongly soluble in hot water.
- Poisonous and corrosive in nature.
Action of Heat
It is stable towards heat, but on strong heating it decomposes to free mercury and chlorine.
Action with Stannous Chloride
When HgCl₂ is treated with SnCl₂, mercuric chloride is reduced to mercurous chloride, and then further to Hg.
Action with Sulphur Dioxide
When SO₂ gas is passed through a solution of mercuric chloride, a white precipitate of mercurous chloride is formed.
Action with Potassium Iodide
When mercuric chloride solution is treated with KI solution, a red scarlet precipitate of HgI₂ is formed. On further addition of KI solution, the scarlet precipitate dissolves due to the formation of the complex potassium mercuric iodide.
Action with Ammonia
When mercuric chloride is treated with ammonia, a white precipitate of ammonobasic mercuric chloride is formed.
Uses
- As an antiseptic to sterilize surgical instruments.
- To prepare Nessler’s reagent.
- In preserving skin, wood, fur, etc.
- Used as an antidote (medicine used against poison).
📘 Chapter Summary
- Mercury occurs combined as cinnabar (HgS); it does not occur free in nature.
- Extracted by roasting and distillation in a shaft furnace, then purified with dilute HNO₃ and vacuum distillation.
- The only metal that is liquid at room temperature — silvery-white, dense, and volatile.
- Forms two important chloride compounds: calomel (Hg₂Cl₂) and corrosive sublimate (HgCl₂).
- Mercury and its compounds are toxic — handle with care.