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= Argon =
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Introduction
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Argon is a chemical element; it has symbol Ar and atomic number 18. It
is in group 18 of the periodic table and is a noble gas. Argon is the
third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv).
It is more than twice as abundant as water vapor (which averages about
4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide
(400 ppmv), and more than 500 times as abundant as neon (18 ppmv).
Argon is the most abundant noble gas in Earth's crust, comprising
0.00015% of the crust.
Nearly all argon in Earth's atmosphere is radiogenic argon-40, derived
from the decay of potassium-40 in Earth's crust. In the universe,
argon-36 is by far the most common argon isotope, as it is the most
easily produced by stellar nucleosynthesis in supernovas.
The name "argon" is derived from the Greek word , neuter singular form
of meaning 'lazy' or 'inactive', as a reference 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 83.8058 K is a defining fixed point in the International
Temperature Scale of 1990.
Argon is extracted industrially by the fractional distillation of
liquid air. It is mostly used as an inert shielding gas in welding and
other high-temperature industrial processes where ordinarily
unreactive substances become reactive; for example, an argon
atmosphere is used in graphite electric furnaces to prevent the
graphite from burning. It is also used in incandescent and fluorescent
lighting, and other gas-discharge tubes. It makes a distinctive
blue-green gas laser. It is also used in fluorescent glow starters.
Characteristics
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Argon has approximately the same solubility in water as oxygen and is
2.5 times more soluble in water than nitrogen. Argon is colorless,
odorless, nonflammable and nontoxic as a solid, liquid or gas. Argon
is chemically inert under most conditions and forms no 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 compound of
argon with fluorine and hydrogen that is stable below , has been
demonstrated.
Although the neutral ground-state chemical compounds of argon are
presently limited to HArF, argon can form clathrates with water when
atoms of argon are trapped in a lattice of water molecules.
Ions, such as , and excited-state complexes, such as ArF, have been
demonstrated. Theoretical calculation predicts several more argon
compounds that should be stable
but have not yet been synthesized.
History
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A: test-tube, B: dilute alkali, C: U-shaped glass tube, D: platinum
electrode
'Argon' (Greek , neuter singular form of meaning "lazy" or
"inactive") is named in reference to its chemical inactivity. This
chemical property of this first noble gas to be discovered impressed
the namers.
An unreactive gas was suspected to be a component of air by Henry
Cavendish in 1785.
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 sample of clean air. They first
accomplished this by replicating an experiment of Henry Cavendish's.
They trapped a mixture of atmospheric air with additional oxygen in a
test-tube (A) upside-down over a large quantity of dilute alkali
solution (B), which in Cavendish's original experiment was potassium
hydroxide, and conveyed a current through wires insulated by U-shaped
glass tubes (CC) which sealed around the platinum wire electrodes,
leaving the ends of the wires (DD) exposed to the gas and insulated
from the alkali solution. The arc was powered by a battery of five
Grove cells and a Ruhmkorff coil of medium size. The alkali absorbed
the oxides of nitrogen produced by the arc and also carbon dioxide.
They operated the arc until no more reduction of volume of the gas
could be seen for at least an hour or two and the spectral lines of
nitrogen disappeared when the gas was examined. The remaining oxygen
was reacted with alkaline pyrogallate to leave behind an apparently
non-reactive gas which they called argon.
Before isolating the gas, they had determined that nitrogen produced
from chemical compounds was 0.5% lighter than nitrogen from the
atmosphere. The difference was slight, but it was important enough to
attract their attention for many months. They concluded that there was
another gas in the air mixed in with the nitrogen.
Argon was also encountered in 1882 through independent research of H.
F. Newall and W. N. Hartley. Each observed new lines in the emission
spectrum of air that did not match known elements.
Prior to 1957, the symbol for argon was "A". This was changed to Ar
after the International Union of Pure and Applied Chemistry published
the work 'Nomenclature of Inorganic Chemistry' in 1957.
Occurrence
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Argon constitutes 0.934% by volume and 1.288% by mass of Earth's
atmosphere.
Air is the primary industrial source of purified argon products.
Argon is isolated from air by fractionation, most commonly by
cryogenic fractional distillation, a process that also produces
purified nitrogen, oxygen, neon, krypton and xenon.
Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon,
respectively.
Isotopes
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The main isotopes of argon found on Earth are (99.6%), (0.34%), and
(0.06%). Naturally occurring , with a half-life of 1.25 years, decays
to stable (11.2%) by electron capture or positron emission, and also
to stable (88.8%) by beta decay. These properties and ratios are used
to determine the age of rocks by K-Ar dating.
In Earth's atmosphere, is made by cosmic ray activity, primarily by
neutron capture of followed by two-neutron emission. In the
subsurface environment, it is also produced through neutron capture by
, followed by proton emission. is created from the neutron capture by
followed by an alpha particle emission as a result of subsurface
nuclear explosions. It has a half-life of 35 days.
Between locations in the Solar System, the isotopic composition of
argon varies greatly. Where the major source of argon is the decay of
in rocks, will be the dominant isotope, as it is on Earth. Argon
produced directly by stellar nucleosynthesis is dominated by the
alpha-process nuclide . Correspondingly, solar argon contains 84.6%
(according to solar wind measurements),
and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the
atmospheres of the outer planets is 8400 : 1600 : 1. This contrasts
with the low abundance of primordial in Earth's atmosphere, which is
only 31.5 ppmv (= 9340 ppmv × 0.337%), comparable with that of neon
(18.18 ppmv) on Earth and with interplanetary gasses, measured by
probes.
The atmospheres of Mars, Mercury and Titan (the largest moon of
Saturn) contain argon, predominantly as .
The predominance of radiogenic is the reason the standard atomic
weight of terrestrial argon is greater than that of the next element,
potassium, a fact that was puzzling when argon was discovered.
Mendeleev positioned the elements on his periodic table in order of
atomic weight, but the inertness of argon suggested a placement
'before' the reactive alkali metal. Henry Moseley later solved this
problem by showing that the periodic table is actually arranged in
order of atomic number (see History of the periodic table).
Compounds
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Argon's complete octet of electrons indicates full s and p subshells.
This full valence shell makes argon very stable and extremely
resistant to bonding with other elements. Before 1962, argon and the
other noble gases were considered to be chemically inert and unable to
form compounds; however, compounds of the heavier noble gases have
since been synthesized. The first argon compound with tungsten
pentacarbonyl, W(CO)5Ar, was isolated in 1975. However, it was not
widely recognised at that time. In August 2000, another argon
compound, argon fluorohydride (HArF), was formed by researchers at the
University of Helsinki, by shining ultraviolet light onto frozen argon
containing a small amount of hydrogen fluoride with caesium iodide.
This discovery caused the recognition that argon could form weakly
bound compounds, even though it was not the first. It is stable up to
17 kelvins (−256 °C). The metastable dication, which is
valence-isoelectronic with carbonyl fluoride and phosgene, was
observed in 2010.
Argon-36, in the form of argon hydride (argonium) ions, has been
detected in interstellar medium associated with the Crab Nebula
supernova; this was the first noble-gas molecule detected in outer
space.
Solid argon hydride (Ar(H2)2) has the same crystal structure as the
MgZn2 Laves phase. It forms at pressures between 4.3 and 220 GPa,
though Raman measurements suggest that the H2 molecules in Ar(H2)2
dissociate above 175 GPa.
Production
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Argon is extracted industrially by the fractional 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 tonnes of argon are produced worldwide every year.
Applications
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Cylinders containing argon gas for use in extinguishing fire without
damaging server equipment
Argon has several desirable properties:
* Argon is a chemically inert gas.
* Argon is the cheapest alternative when nitrogen is not sufficiently
inert.
* Argon has low thermal conductivity.
* Argon has electronic properties (ionization and/or the emission
spectrum) desirable for some applications.
Other noble gases would be equally suitable for most of these
applications, but argon is by far the cheapest. It is inexpensive,
since it occurs naturally in air and is readily obtained as a
byproduct of cryogenic air separation in the production of liquid
oxygen and liquid nitrogen: the primary constituents of air are used
on a large industrial scale. The other noble gases (except helium) are
produced this way as well, but argon is the most plentiful by far. The
bulk of its applications arise simply because it is inert and
relatively cheap.
Industrial processes
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Argon is used in some high-temperature industrial processes where
ordinarily non-reactive substances become reactive. For example, an
argon atmosphere is used in graphite electric furnaces to prevent the
graphite from burning.
For some of these processes, the presence of nitrogen or oxygen gases
might cause defects within the material. Argon is used in some types
of 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. An argon atmosphere is also used for growing crystals of
silicon and germanium.
Argon is used in the poultry industry to asphyxiate birds, either for
mass culling following disease outbreaks, or as a means of slaughter
more humane than electric stunning. Argon is denser than air and
displaces oxygen close to the ground during inert gas asphyxiation.
Its non-reactive nature makes it suitable in a food product, and since
it replaces oxygen within the dead bird, argon also enhances shelf
life.
Argon is sometimes used for extinguishing fires where valuable
equipment may be damaged by water or foam.
Scientific research
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Liquid argon is used as the target for neutrino experiments and direct
dark matter searches. The interaction between the hypothetical WIMPs
and an argon nucleus produces scintillation light that is detected by
photomultiplier tubes. Two-phase detectors containing argon gas are
used to detect the ionized electrons produced during the WIMP-nucleus
scattering. As with most other liquefied noble gases, argon has a high
scintillation light yield (about 51 photons/keV
), is transparent to its own scintillation light, and is relatively
easy to purify. Compared to xenon, argon is cheaper and has a distinct
scintillation time profile, which allows the separation of electronic
recoils from nuclear recoils. On the other hand, its intrinsic
beta-ray background is larger due to contamination, unless one uses
argon from underground sources, which has much less contamination.
Most of the argon in Earth's atmosphere was produced by electron
capture of long-lived ( + e− → + ν) present in natural potassium
within Earth. The activity in the atmosphere is maintained by
cosmogenic production through the knockout reaction (n,2n) and similar
reactions. The half-life of is only 269 years. As a result, the
underground Ar, shielded by rock and water, has much less
contamination.
Dark-matter detectors currently operating with liquid argon include
DarkSide, WArP, ArDM, microCLEAN and DEAP. Neutrino experiments
include ICARUS and MicroBooNE, both of which use high-purity liquid
argon in a time projection chamber for fine grained three-dimensional
imaging of neutrino interactions.
At Linköping University, Sweden, the inert gas is being utilized in a
vacuum chamber in which plasma is introduced to ionize metallic films.
This process results in a film usable for manufacturing computer
processors. The new process would eliminate the need for chemical
baths and use of expensive, dangerous and rare materials.
Preservative
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Argon is used to displace oxygen- and moisture-containing air in
packaging material to extend the shelf-lives of the contents (argon
has the European food additive code E938). Aerial oxidation,
hydrolysis, and other chemical reactions that degrade the products are
retarded or prevented entirely. High-purity chemicals and
pharmaceuticals are sometimes packed and sealed in argon.
In winemaking, argon is used in a variety of activities to provide a
barrier against oxygen at the liquid surface, which can spoil wine by
fueling both microbial metabolism (as with acetic acid bacteria) and
standard redox chemistry.
Argon is sometimes used as the propellant in aerosol cans.
Argon is also used as a preservative for such products as varnish,
polyurethane, and paint, by displacing air to prepare a container for
storage.
Since 2002, the American National Archives stores important national
documents such as the Declaration of Independence and the Constitution
within argon-filled cases to inhibit their degradation. Argon is
preferable to the helium that had been used in the preceding five
decades, because helium gas escapes through the intermolecular pores
in most containers and must be regularly replaced.
Laboratory equipment
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Argon may be used as the inert gas within Schlenk lines and
gloveboxes. Argon is preferred to less expensive nitrogen in cases
where nitrogen may react with the reagents or apparatus.
Argon may be used as the carrier gas in gas chromatography and in
electrospray ionization mass spectrometry; it is the gas of choice for
the plasma used in ICP spectroscopy. Argon is preferred for the
sputter coating of specimens for scanning electron microscopy. Argon
gas is also commonly used for sputter deposition of thin films as in
microelectronics and for wafer cleaning in microfabrication.
Medical use
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Cryosurgery procedures such as cryoablation use liquid argon to
destroy tissue such as cancer cells. It is used in a procedure called
"argon-enhanced coagulation", a form of argon plasma beam
electrosurgery. The procedure carries a risk of producing gas embolism
and has resulted in the death of at least one patient.
Blue argon lasers are used in surgery to weld arteries, destroy
tumors, and correct eye defects.
Argon has also been used experimentally to replace nitrogen in the
breathing or decompression mix known as Argox, to speed the
elimination of dissolved nitrogen from the blood.
Lighting
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Incandescent lights are filled with argon, to preserve the filaments
at high temperature from oxidation. It is used for the specific way it
ionizes and emits light, such as in plasma globes and calorimetry in
experimental particle physics. Gas-discharge lamps filled with pure
argon provide lilac/violet light; with argon and some mercury, blue
light. Argon is also used for blue and green argon-ion lasers.
Miscellaneous uses
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Argon is used for thermal insulation in energy-efficient windows.
Argon is also used in technical scuba diving to inflate a dry suit
because it is inert and has low thermal conductivity.
Argon is used as a propellant in the development of the Variable
Specific Impulse Magnetoplasma Rocket (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 cooled thermal
seeker heads. The gas is stored at high pressure.
Argon-39, with a half-life of 269 years, has been used for a number of
applications, primarily ice core and ground water dating. Also,
potassium-argon dating and related argon-argon dating are used to date
sedimentary, metamorphic, and igneous rocks.
Argon has been used by athletes as a doping agent to simulate hypoxic
conditions. In 2014, the World Anti-Doping Agency (WADA) added argon
and xenon to the list of prohibited substances and methods, although
at this time there is no reliable test for abuse.
Safety
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Although argon is non-toxic, it is 38% more dense than air and
therefore considered a dangerous asphyxiant in closed areas. It is
difficult to detect because it is colorless, odorless, and tasteless.
A 1994 incident, in which a man was asphyxiated after entering an
argon-filled section of oil pipe under construction in Alaska,
highlights the dangers of argon tank leakage in confined spaces and
emphasizes the need for proper use, storage and handling.
See also
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* Industrial gas
* Oxygen-argon ratio, a ratio of two physically similar gases, which
has importance in various sectors.
Further reading
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*
* On triple point pressure at 69 kPa.
* On triple point pressure at 83.8058 K.
External links
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* [
http://www.periodicvideos.com/videos/018.htm Argon] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://wwwrcamnl.wr.usgs.gov/isoig/period/ar_iig.html USGS Periodic
Table - Argon]
* Diving applications:
[
http://www.decompression.org/maiken/Why_Argon.htm Why Argon?]
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=========
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Original Article:
http://en.wikipedia.org/wiki/Argon
