Argon

 

Argon is a chemical element with the symbol Ar and atomic number 18, which is in-group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in the Earth's atmosphere with 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times more abundant than carbon dioxide (400 ppmv), and more than 500 times more abundant than neon (18 ppmv). Argon is the most abundant noble gas in the Earth's crust, making up 0.00015% of the crust. Almost all the argon in the Earth's atmosphere is radioactive argon-40, which is obtained from the decay of potassium-40 in the earth's crust. In the universe, argon-36 is by far the most common isotope of argon because it is the easiest argon isotope to produce by stellar nuclear fusion in supernovae. The name "argon" is derived from the Greek word ἀργόν, the neuter singular form of ἀργός, meaning "lazy" or "inactive", as it refers to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its triple point temperature of 8058.83 K is a defining fixed point in the 1990 International Temperature Scale.

Specifications

The solubility of argon in water is almost equal to that of oxygen, and it dissolves 2.5 times more than nitrogen in water. Argon is colorless, odorless, non-flammable and non-toxic as a solid, liquid or gas. Argon is chemically inert under most conditions and does not form any confirmed stable compounds at room temperature. Although argon is a noble gas, it can form some compounds under various extreme conditions. Argon fluorohydride (HArF), a combination of argon with fluorine and hydrogen, which is stable below 17 K (−256.1 °C; −429.1 °F) is shown. Although the chemical compounds of the neutral ground state of argon are currently limited to HArF, argon can form clathrates with water where the argon atoms are trapped in a network of water molecules. Ions such as ArH+ and excited state complexes such as ArF are shown. Theoretical calculations predict several other argon compounds that should be stable but have not yet been synthesized.

History

Argon (Greek ἀργόν, neuter singular form ἀργός meaning "lazy" or inactive) is named for its chemical inactivity. This chemical property influenced the names of the first noble gases to be discovered. In 1785, Henry Cavendish suspected a non-reactive gas that is a component of air. Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water and nitrogen from a clean air sample. They first did this by replicating an experiment by Henry Cavendish. They trapped a mixture of atmospheric air with excess oxygen in an inverted test tube (A) over a large amount of dilute alkaline solution (B), which in Cavendish's original experiment was potassium hydroxide, and passed a current through it. U-shaped glass tubes (CC) that wrap around the platinum wire electrodes, expose the ends of the wires (DD) to the gas, and are insulated from the alkaline solution insulate the wires. A battery of five Grove cells and a medium-sized Ruhmkorff coil powered the arc.

Absorbs alkaline oxides of nitrogen produced by the arc as well as carbon dioxide. They worked the arc to such an extent that no reduction in gas volume was observed for at least another hour or two, and the nitrogen spectral lines disappeared when the gas was examined. The remaining oxygen was reacted with alkaline pyrogallate to leave an apparently unreactive gas called argon. Entitled "Argon," Lord Riley's cartoon in Vanity Fair, 1899 before the gas was isolated; they had determined that nitrogen produced from chemical compounds was 0.5% lighter than atmospheric nitrogen. The difference was small, but significant enough to hold their attention for several months. They concluded that another gas was mixed with the nitrogen in the air. Argon was also encountered in 1882 through the independent research of H. F. Newall and W. N. Hartley. Each observed new lines in the air emission spectrum that did not correspond to known elements.

The event

Argon constitutes 0.934% by volume and 1.288% by mass of the Earth's atmosphere. Air is the primary industrial source of pure argon products. Argon is separated from air by dissociation, usually by partial cryogenic distillation, a process that also produces pure nitrogen, oxygen, neon, krypton, and xenon. The earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.

Applications

Cylinders containing argon gas have several desirable properties for use in extinguishing fires without damaging server equipment:

Argon is a chemically inert gas. When nitrogen is not sufficiently inert, argon is the cheapest alternative. Argon has a low thermal conductivity. Argon has electronic properties (ionization and/or emission spectrum) that are desirable for some applications.

 Other noble gases are equally suitable for many of these applications, but argon is by far the least expensive. Argon is cheap because it occurs naturally in air and is easily obtained as a byproduct of cryogenic air separation in the production of liquid oxygen and liquid nitrogen: the primary compounds of air used on a large industrial scale. Other noble gases (except helium) are also produced this way, but argon is the most abundant. Argon's major uses are simply because it is inert and relatively cheap.

Industrial processes

Argon is used in some high-temperature industrial processes where normally unreactive materials become reactive. For example, to prevent graphite from burning, argon atmosphere is used in electric graphite furnaces.

For some of these processes, the presence of nitrogen or oxygen gases may cause material defects. Argon is used in some types of electric arc welding, such as gas metal arc welding and gas tungsten arc welding, as well as in the processing of titanium and other reactive elements. Argon atmosphere is also used to grow silicon and germanium crystals.

Applications

Medical

In cryosurgery methods such as cryoablation, liquid argon is used to destroy tissues such as cancer cells. This method is used in a procedure called "argon-enhanced coagulation", a type of electrosurgery with an argon plasma beam. This method carries the risk of gas embolism and has led to the death of at least one patient. Argon blue lasers are used in surgery to weld blood vessels, remove tumors and correct eye defects. Argon has also been used experimentally to replace nitrogen in the breathing or decompression mixture known as Argox, to speed the removal of dissolved nitrogen from the blood.

Miscellaneous

Argon is used for thermal insulation in energy-efficient windows. Argon is also used in technical diving to inflate drysuits because it is inert and has a low thermal conductivity. Argon is used as a propellant in the development of the Variable Impulse Magnetic Missile (VASIMR). Compressed argon gas is allowed to expand to cool the seeker heads of some versions of the AIM-9 Sidewinder missile and other missiles that use a thermally cooled seeker head. Gas is stored under high pressure. Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily ice core and groundwater dating. In addition, potassium-argon dating and corresponding argon-argon dating are used to date sedimentary, metamorphic and igneous rocks. Athletes have used argon as a doping agent to simulate hypoxic conditions. In 2014, the World Anti-Doping Agency (WADA) added argon and xenon to its list of banned substances and methods, although there are currently no reliable tests for abuse.

Safety

Although argon is non-toxic, it is 38% denser than air and is therefore considered a dangerous asphyxiating gas in enclosed areas. It is difficult to detect because it is colorless, odorless and tasteless. A 1994 incident, in which a man suffocated after entering an argon-filled section of an oil pipeline under construction in Alaska, highlights the dangers of leaking argon tanks in confined spaces and emphasizes the need for proper use, storage, and handling. .

Argon production methods

The gas mixture containing argon is first processed in a cryogenic separation unit to produce a crude argon stream with an argon concentration between 80 and 98%. The raw argon stream is transferred to the membrane separation unit where it is separated to produce an oxygen-free argon stream and an oxygen-rich stream. The oxygen-rich stream is recycled to the cryogenic separation unit, and the oxygen-free argon stream is recovered as product or further purification.

Industrial

Argon is extracted industrially by partial distillation of liquid air in a cryogenic air separation unit. A process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K, and liquid oxygen, which boils at 90.2 K. About 700,000 tons of argon are produced worldwide each year.

In radioactive decays

  Argon-40, the most abundant isotope of argon, is produced by the decay of 40K with a half-life of 1.25 x 109 years by electron absorption or positron emission. For this reason, it is used in potassium-argon dating to determine the age of rocks.

 

Refrence

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