Mercury: a liquid metal
Chemical properties and areas of application
Mercury is a heavy, silvery and very toxic metal. One of the most well-known features of mercury is that in normal conditions this metal is liquid and only freezes at –38.8°C! In nature there are only two elements that are liquid in normal conditions – mercury and bromine.
In the earth’s crust, mercury is a very rare element, but rocks in which mercury is found usually contain it in rather high concentrations. This is because mercury mixes poorly with elements that make up most of the earth’s crust, and so “accumulations” of minerals containing mercury are formed. The most abundant of them are cinnabar (mercury sulfide HgS) and corderoit (Hg₃S₂Cl₂).
The history of metal
Humanity has known about this metal for a very long time. It was ascribed both mystical and healing properties. Cinnabar was used by humans in the Paleolithic era, 30,000 years ago. They painted caves with it. The Ancient Greeks used mercury sulfide in ointments, and Ancient Egyptians and Romans used it to make cosmetics. Metallic mercury was found in the tombs of Ancient Egypt, dated 1,600 BCE and the pyramids of Teotihuacan. The Ancient Egyptians and inhabitants of Mesoamerica may have used mercury to make a prototype of the rivers of the afterworld.
Alchemists believed that mercury was the “primary material”, the “substrate” – the foundation of everything in existence. They thought that mercury could turn ordinary metals into gold.
The metal takes its name from the Roman God, the patron of traders and travelers, who is also responsible for communication and luck. The chemical symbol Hg comes from the Greek word “ὑδράργυρος” (“hydrargyros”), which literally means “water-silver”, as mercury is liquid like water, and shiny like silver.
Chemical properties of mercury
Mercury does not react with the majority of acids, but dissolves in concentrated sulfuric acid, in nitric acid and in aqua regia. If mercury is placed in nitric acid, mercury nitrate Hg(NO₃)₂ forms:
Hg + 4HNO₃ → Hg(NO₃)₂ + 2NO₂↑ + H₂O
Mercury is less reactive than copper, so copper forces mercury out of its salts. For example, if a copper coin is immersed in a solution of mercury nitrate, metallic mercury settles on the surface of the coin:
Cu + Hg(NO₃)₂ → Cu(NO₃)₂ + Hg↓
One famous experiment with mercury is the mercury heart. Metallic mercury has a solution of sulfuric acid poured over it, and potassium dichromate is added. In this oxidizing medium, on the surface of the drop of mercury the film of its sulfate forms, reducing the surface tension that gives the drop its form. As a result the drop of mercury spreads out a little.
2Hg + SO₄²⁻ → Hg₂SO₄(film) + 2e⁻
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
If the surface of a drop of mercury is touched with an iron wire or nail, mercury sulfate is reduced to metallic mercury, the film is broken, the surface tension increases, and the drop “contracts”.
Hg₂SO₄(film) + 2e⁻ → 2Hg↓ + SO₄²⁻
Fe → Fe²⁺ + 2e⁻
When the drop “contracts”, contact with iron is broken, and the oxidation process of the mercury starts again. The drop once more spreads out, touches iron, and then contracts. This process repeats again and again, and the drop throbs like a beating heart.
Where mercury is used
Owing to its toxicity, the use of mercury today has been severely restricted. It can still be found in mercury thermometers, which are becoming a thing of the past. Mercury expands evenly on heating in a rather wide range of temperatures, and as it expands and rises up the glass capillary, it is easy to judge a change in temperature. The fact that it is opaque and has a characteristic shine makes it convenient to see where the “column” of mercury in the thermometer is located.
Additionally, mercury fumes are used in some luminescent lamps. Mercury fumes radiate ultraviolet light if an electric current is passed through them. A special coating on the surface of the lamp converts ultraviolet light into visible light. One of these lamps contains around 10 mg of mercury, and in lamps with a low content of this metal, around 4 mg. Additionally, mercury is encountered in some electric switches and current rectifiers.
Electric batteries are made on the basis of mercury, but nowadays they have been replaced by other types of batteries and accumulators which do not contain mercury. There was even an attempt to use mercury as “fuel” for ionic engines of SERT (Space Electric Rocket Test) spacecraft.
In industry, mercury and its compounds as used as catalysts, but one of their widespread applications is the electrolytical production of active metals, chlorine and alkalis (the Castner-Keller process). In this process, a very important property of mercury is used – the ability to form amalgams, and alloys with other metals. Only with iron, platinum, wolfram and tantalum mercury does not form amalgams.
Amalgams of mercury played a major role in the development of chemistry and the discovery of new elements. The renowned English chemist Sir Humphrey Davy used mercury extensively to extract potassium, magnesium, calcium, strontium and barium by electrochemical methods – amalgams of these metals were formed in the electrolysis of salts and oxides. Davy then removed the mercury and extracted pure (or almost pure) metals. Additionally, mercury was used to extract gold from ore, as gold also formed an amalgam with mercury.
Going back in history, we should discuss other spheres in which mercury was widely used. Compounds of this metal were used in medicine as diuretics, laxatives, and also medicine for syphilis. Medicine containing mercury was taken by the English king Henry VIII, Ivan the Terrible and Isaac Newton. In China and Tibet, it was believed that mercury extended life.
In the 20th century, the compound Thiomersal that contained mercury was used to extend the shelf life of vaccines. Mercury and its nitrate were used by manufacturers of felt hats in the 18th-19th centuries. Felt was soaked in a solution of mercury nitrate. Workers had to handle mercury and breath in its fumes, so poisoning from this metal was a professional illness for them. As mercury poisoning affected mental health, the expression “mad as a hatter” arose. There is a legend that Lewis Carrol based the Mad Hatter from “Alice in Wonderland” on this type of worker. However, this is probably not the case. The prototype for the Mad Hatter was an acquaintance of Carrol, the furniture dealer Theophilus Carter.
Mercury is highly toxic. The maximum acceptable concentration of mercury in a room during a working day must not exceed 0.05 mg per cubic meter of air. If a person inhales mercury fumes over a long period of time, serious poisoning will occur, damaging the internal organs and the nervous system. The person develops symptoms of “madness”: excessive shyness, indecisiveness, phobias, loss of self-identification etc. Additionally, the gums and the respiratory tract become inflamed, bronchitis develops and the digestive organs suffer.
What do you do if mercury is spilt at home or at work – if a mercury thermometer or lamp containing mercury breaks? If a mercury lamp breaks, everyone not involved in cleaning up the mess should leave the room, and take pets with them. Open the windows and turn on the air conditioning. Put the lamp shards in a container that can be closed or a plastic bag. Small shards can be gathered with sticky tape. Then air the room for several hours. Don’t use a vacuum cleaner! This may cause the mercury fumes to spread even more!
If a mercury thermometer breaks, then you should also open the windows, and place the shards of the thermometer in a container or plastic bag. Balls of mercury should be gathered very carefully with a pipette, as they have the property of moving swiftly in different directions. To make sure that no unnoticed drops of mercury have been left behind, turn off the light and shine a torch across the floor – you may notice shiny drops which were not visible in normal light.
The remains of the mercury which cannot be collected with a pipette should be sprinkled with sulfur powder, which is sold in special sets for removing mercury, or at an ordinary pharmacy. You can also use iron(III) chloride, which is sold at shops for radio enthusiasts. But if the mercury has fallen onto porous or wooly surfaces (for example a rug), then it is better to throw this item away, as it will be almost impossible to remove mercury from it.
Remember that the gathered shards and remains of mercury should be dispensed with according to the rules that apply in your area. You may find collection stations for this type of waste.
- Paul Parsons, Gail Dixon — The Periodic Table A visual guide to the elements;
- Kristi Lew – Mercury. The Rosen Publishing Group, 2008;
- Barry R. Leopold – Use and Release of Mercury in the United States, Contract Nos. 68-C-0027 and 68-C7-0011, 2002;
- Waldron, H. A. (1983). Did the Mad Hatter have mercury poisoning? British Medical Journal (Clinical Research Ed.), 287(6409), 1961;
- John Burke Sullivan, Gary R. Krieger – Clinical Environmental Health and Toxic Exposures. Lippincott Williams & Wilkins, 2001;
- Sir Humphry Davy – The Collected Works of Sir Humphry Davy …: Bakerian lectures and miscellaneous papers from 1806 to 1815. Smith, Elder and Company, 1840;
- Royal Society of Chemistry/Mercury;
- Wikipedia/Castner Kellner;