Chlorine And Argon

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Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, with fluorine being the lightest. Chlorine is found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine. It has the highest electron affinity and the third highest electronegativity of all the elementsSony VAIO VPCF135Z1E/B battery; for this reason, chlorine is a strong oxidizing agent.

The most common compound of chlorine, sodium chloride, has been known since ancient times; however, around 1630, chlorine gas was obtained by the Belgian chemist and physician Jan Baptist van Helmont. The synthesis and characterization of elemental chlorine occurred in 1774 by Swedish chemist Carl Wilhelm ScheeleSony VAIO VPCF137HG/BI battery, who called it "dephlogisticated muriatic acid air," having thought he synthesized the oxide obtained from the hydrochloric acid. Because acids were thought at the time to necessarily contain oxygen, a number of chemists, including Claude Berthollet, suggested that Scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered elementSony VAIO VPCF136FG/BI battery, and Scheele named the supposed new element within this oxide as muriaticum. The suggestion that this newly discovered gas was a simple element was made in 1809 by Joseph Louis Gay-Lussac and Louis-Jacques. This was confirmed by Sir Humphry Davy in 1810, who named it chlorine, from the Greek word χλωρος (chlōros), meaning "green-yellow."

Chlorine is a component of various compoundsSony VAIO VPCF135FG/B battery, including table salt. It is the second most abundant halogen and 21st most abundant chemical element in Earth's crust. The great oxidizing potential of chlorine led it to its bleaching and disinfectant uses, as well as uses of an essential reagent in the chemical industry. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitarySony VAIO VPCF127HG/BI battery. In the upper atmosphere, chlorine-containing molecules such as chlorofluorocarbons have been implicated in ozone depletion. Elemental chlorine is extremely dangerous and poisonous for all lifeforms; however, chlorine is necessary to most forms of life, including humans, in form of chloride ions.

Characteristics

Physical characteristics

Chlorine, liquified under a pressure of 7.4 bar at room temperature, displayed in a quartz ampule embedded in acrylic glassSony VAIO VPCF11Z1E battery.

At standard temperature and pressure, two chlorine atoms form the diatomic molecule Cl2.[3] This is a yellow-green gas that has a distinctive strong odor, familiar to most from common Household bleach.[4] The bonding between the two atoms is relatively weak (only 242.580 ± 0.004 kJ/mol), which makes the Cl2 molecule highly reactive. The boiling point at regular atmosphere is around −34 ˚CSony VAIO VPCF11S1E battery, but it can be liquefied at room temperature with pressures above 740 kPa.[5]

Chemical characteristics

Along with fluorine, bromine, iodine, and astatine, chlorine is a member of the halogen series that forms the group 17 (formerly VII, VIIA, or VIIB) of the periodic table. Chlorine forms compounds with almost all of the elements to give compounds that are usually called chlorides. Chlorine gas reacts with most organic compounds, and will even sluggishly support the combustion of hydrocarbonsSony VAIO VPCF11M1E/H battery.[6]

Hydrolysis

At 25 °C and atmospheric pressure, one liter of water dissolves 3.26 g or 1.02 L of gaseous chlorine.[7] Solutions of chlorine in water contain chlorine (Cl2), hydrochloric acid, and hypochlorous acid:

Cl2 + H2O  HCl + HClO

This conversion to the right is called disproportionation, because the ingredient chlorine both increases and decreases in formal oxidation state. The solubility of chlorine in water is increased if the water contains dissolved alkali hydroxide, and in this way, chlorine bleach is producedSony VAIO VPCF11M1E battery.[8]

Cl2 + 2 OH– → ClO– + Cl– + H2O

Chlorine gas only exists in a neutral or acidic solution.

Compounds

See also Category: Chlorine compounds

Chlorine exists in all odd numbered oxidation states from −1 to +7, as well as the elemental state of zero and four in chlorine dioxide (see table below, and also structures in chlorite).[9] Progressing through the states, hydrochloric acid can be oxidized using manganese dioxide, or hydrogen chloride gas oxidized catalytically by air to form elemental chlorine gasSony VAIO VPCF11JFX/B battery.[10]

Chlorine oxides

Chlorine forms a variety of oxides, as seen above: chlorine dioxide (ClO2), dichlorine monoxide (Cl2O), dichlorine hexoxide (Cl2O6), dichlorine heptoxide (Cl2O7). The anionic derivatives of these same oxides are also well known including chlorate (ClO−

3), chlorite (ClO−

2), hypochlorite (ClO−), and perchlorate (ClO−

4). The acid derivatives of these anions are hypochlorous acid (HOCl), chloric acid (HClO3) and perchloric acid (HClO4) Sony VAIO VPCF119FJ battery. The chloroxy cation chloryl (ClO2+) is known and has the same structure as chlorite but with a positive charge and chlorine in the +5 oxidation state.[11] The compound "chlorine trioxide", rather than being the expected +6 oxidation state, is instead a mixture of +5 and +7 states, occurring as the ionic compound chloryl perchlorate, [ClO2]+[ClO4]− commonly called dichlorine hexoxide.[12]

In hot concentrated alkali solution hypochlorite disproportionatesSony VAIO VPCF119FC/BI battery:

2 ClO− → Cl− + ClO−2

ClO− + ClO−2 → Cl− + ClO−3

Sodium chlorate and potassium chlorate can be crystallized from solutions formed by the above reactions. If their crystals are heated, they undergo a further, final disproportionation:

4 ClO−3 → Cl− + 3 ClO−4

This same progression from chloride to perchlorate can be accomplished by electrolysis. The anode reaction progression is:[13]

Each step is accompanied at the cathode by

2 H2O + 2 e− → 2 OH− + H2 (−0.83 volts) Sony VAIO VPCF119FC battery

Interhalogen compounds

Chlorine oxidizes bromide and iodide salts to bromine and iodine, respectively. However, it cannot oxidize fluoride salts to fluorine. It makes a variety of interhalogen compounds such as the chlorine fluorides, chlorine monofluoride (ClF), chlorine trifluoride (ClF3), chlorine pentafluoride (ClF5). Chlorides of bromine and iodine are also knownSony VAIO VPCF117HG/BI battery.[14]

Organochlorine compounds

Main article: Organochloride

Chlorine is used extensively in organic chemistry in substitution and addition reactions. Chlorine often imparts many desired properties to an organic compound, in part owing to its electronegativity. Some organochlorine compounds are also serious pollutants, either as side products of industrial processes or as persistent pesticidesSony VAIO VPCF116FG/BI battery.

Many important industrial products are produced via organochlorine intermediates. Examples include polycarbonates, polyurethanes, silicones, polytetrafluoroethylene, carboxymethyl cellulose, and propylene oxide. Like the other halogens, chlorine participates in free-radical substitution reactions with hydrogen-containing organic compoundsSony VAIO VPCF115FG/B battery. When applied to organic substrates, reaction is often—but not invariably—non-regioselective, and, hence, may result in a mixture of isomeric products. It is often difficult to control the degree of substitution as well, so multiple substitutions are common. If the different reaction products are easily separated, e.g., by distillation, substitutive free-radical chlorination (in some cases accompanied by concurrent thermal dehydrochlorination) may be a useful synthetic routeSony VAIO VPCF21ZHJ battery. Industrial examples of this are the production of methyl chloride, methylene chloride, chloroform, and carbon tetrachloride from methane, allyl chloride from propylene, and trichloroethylene, and tetrachloroethylene from 1,2-dichloroethane.

Like the other halides, chlorine undergoes electrophilic addition reactions, the most notable one being the chlorination of alkenes and aromatic compounds with a Lewis acid catalyst. Organic chlorine compounds tend to be less reactive in nucleophilic substitution reactions than the corresponding bromine or iodine derivatives, but they tend to be cheaperSony VAIO VPCF21Z1E/BI battery. They may be activated for reaction by substituting with a tosylate group, or by the use of a catalytic amount of sodium iodide.[citation needed]

Chlorides

Main article: Chloride

Chlorine combines with almost all elements to give chlorides. Compounds with oxygen, nitrogen, xenon, and krypton are known, but do not form by direct reaction of the elements.[15] Chloride is one of the most common anions in nature. Hydrogen chloride and its aqueous solution, hydrochloric acid, are produced on megaton scale annually both as valued intermediates but sometimes as undesirable pollutantsSony VAIO VPCF21AHJ battery.

Occurrence

See also Category: Halide minerals

In nature, chlorine is found primarily as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans — about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits. Most chloride salts are soluble in water, thusSony VAIO VPCF21AGJ battery, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. In the Earth's crust, chlorine is present at average concentrations of about 126 parts per million,[16] predominantly in such minerals as halite (sodium chloride), sylvite (potassium chloride), and carnallite (potassium magnesium chloride hexahydrate). Over 2000 naturally occurring organic chlorine compounds are knownSony VAIO VPCF21AFJ battery.[17]

In the interstellar medium, chlorine is produced in supernovae via the r-process.[18]

Isotopes

Main article: Isotopes of chlorine

Chlorine has a wide range of isotopes. The two stable isotopes are 35Cl (75.77%) and 37Cl (24.23%).[19] Together they give chlorine an atomic weight of 35.4527 g/mol. The half-integer value for chlorine's weight caused some confusion in the early days of chemistry, when it had been postulated that atoms were composed of even units of hydrogen (see Proust's law), and the existence of chemical isotopes was unsuspectedSony VAIO VPCF219FJ/BI battery.[20]

Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 7x10−13 to 1 with stable isotopes. 36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic ray protons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to 36S and to 36ArSony VAIO VPCF217HG/BI battery, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 weekSony VAIO VPCF217HG battery. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, including dating ice and sediments.[19]

History

The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 BC and brine as early as 6000 BCSony VAIO VPCF14ZHJ battery.[21] Around 1630, chlorine was recognized as a gas by the Belgian chemist and physician Jan Baptist van Helmont.[22]

Carl Wilhelm Scheele

Elemental chlorine was first prepared and studied in 1774 by Swedish chemist Carl Wilhelm Scheele, and, therefore, he is credited for its discovery.[23] He called it "dephlogisticated muriatic acid air" since it is a gas (then called "airs") and it came from hydrochloric acid (then known as "muriatic acid")Sony VAIO VPCF14AHJ battery.[23] However, he failed to establish chlorine as an element, mistakenly thinking that it was the oxide obtained from the hydrochloric acid (see phlogiston theory). [23] He named the new element within this oxide as muriaticum.[23] Regardless of what he thought, Scheele did isolate chlorine by reacting MnO2 (as the mineral pyrolusite) with HCl:[22]

4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2

Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow green color, and the smell similar to aqua regiaSony VAIO VPCF14AGJ battery.[24]

At the time, common chemical theory was: any acid is a compound that contains oxygen (still sounding in the German and Dutch names of oxygen: sauerstoff or zuurstof, both translating into English as acid stuff), so a number of chemists, including Claude Berthollet, suggested that Scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum. Sony VAIO VPCF14AFJ battery

In 1809, Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to decompose dephlogisticated muriatic acid air by reacting it with charcoal to release the free element muriaticum (and carbon dioxide).[23] They did not succeed and published a report in which they considered the possibility that dephlogisticated muriatic acid air is an element, but were not convinced.Sony VAIO VPCF149FJ/BI battery

In 1810, Sir Humphry Davy tried the same experiment again, and concluded that it is an element, and not a compound.[23] He named this new element as chlorine, from the Greek word χλωρος (chlōros), meaning green-yellow.[29] The name halogen, meaning salt producer, was originally defined for chlorine (in 1811 by Johann Salomo Christoph Schweigger) Sony VAIO VPCF148FJ/B battery, and later in 1842, at a suggestion by Jöns Jakob Berzelius, this term was applied to the rest of the elements in this family.[30][31] In 1823, Michael Faraday liquefied chlorine for the first time,[32][33] and demonstrated that what was then known as "solid chlorine" had a structure of chlorine hydrate (Cl2·H2O).[22]

Chlorine was first used by Claude Berthollet to bleach textiles in 1785. In 1826, silver chloride was used to produce photographic images for the first timeSony VAIO VPCF13ZHJ battery.[34] Chloroform was first used as an anesthetic in 1847.[34] An elemental chlorine solution in water (which was expensive), then the less expensive chlorine gas dissolved in lime-water (calcium hypochlorite) was first used as an antiseptic to prevent the spread of puerperal fever in the maternity wards of Vienna General Hospital in Austria in 1847. Sony VAIO VPCF13Z0E/B battery In 1850, chlorine in lime-water was used by John Snow to purify the water supply in London after an outbreak of cholera. (Both uses preceded the germ theory of disease, and were based on destruction of odors and "putrid matter").

The US Department of Treasury called for all water to be disinfected with chlorine by 1918.[34] Polyvinyl chloride (PVC) was invented in 1912Sony VAIO VPCF13M8E/B battery, initially without a purpose.[34] Chlorine gas was first introduced as a weapon on April 22, 1915, at Ypres by the German Army,[36][37] and the results of this weapon were disastrous because gas masks had not been mass distributed and were tricky to get on quickly.

Production

Liquid chlorine analysis

Main article: Chlorine production

In industry, elemental chlorine is usually produced by the electrolysis of sodium chloride dissolved in water.[38] Along with chlorine, this chloralkali process yields hydrogen gas and sodium hydroxide, according to the following chemical equationSony VAIO VPCF13AHJ battery:[10]

2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH

The electrolysis of chloride solutions all proceed according to the following equations:

Cathode: 2 H+ (aq) + 2 e− → H2 (g)

Anode: 2 Cl− (aq) → Cl2 (g) + 2 e−

Overall process: 2 NaCl (or KCl) + 2 H2O → Cl2 + H2 + 2 NaOH (or KOH) In diaphragm cell electrolysis, an asbestos (or polymer-fiber) diaphragm separates a cathode and an anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathodeSony VAIO VPCF13AGJ battery.[39] The salt solution (brine) is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the caustic alkali is produced and the brine is partially depleted. Diaphragm methods produce dilute and slightly impure alkali but they are not burdened with the problem of preventing mercury discharge into the environment and they are more energy efficientSony VAIO VPCF13AFJ battery. Membrane cell electrolysis employ permeable membrane as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration.[40] This method is more efficient than the diaphragm cell and produces very pure sodium (or potassium) hydroxide at about 32% concentration, but requires very pure brineSony VAIO VPCF138FJ/BI battery.

Laboratory methods

Small amounts of chlorine gas can be made in the laboratory by combining hydrochloric acid and manganese dioxide. Alternatively a strong acid such as sulfuric acid or hydrochloric acid reacts with sodium hypochlorite solution to release chlorine gas but reacts with sodium chlorate to produce chlorine gas and chlorine dioxide gas as wellSony VAIO VPCF138FJ/B battery. In the home, accidents occur when hypochlorite bleach solutions are combined with certain acidic drain-cleaners.

Applications

Production of industrial and consumer products

Principal applications of chlorine are in the production of a wide range of industrial and consumer products.[41][42] For example, it is used in making plastics, solvents for dry cleaning and metal degreasing, textiles, agrochemicals and pharmaceuticals, insecticides, dyestuffs, household cleaning products, etcSony VAIO VPCF138FC/BI battery. Quantitatively, about 63% and 18% of all chlorine are used in the manufacture of organic and inorganic chlorine compounds, respectively,[38] and about 15,000 chlorine compounds are being used commercially.[24] The remaining 19% is used for bleaches and disinfection products.[38] The most significant of organic compounds in terms of production volume are 1,2-dichloroethane and vinyl chloride, intermediates in the production of PVCSony VAIO VPCF135FG battery. Other particularly important organochlorines are methyl chloride, methylene chloride, chloroform, vinylidene chloride, trichloroethylene, perchloroethylene, allyl chloride, epichlorohydrin, chlorobenzene, dichlorobenzenes, and trichlorobenzenes. The major inorganic compounds include HCl, Cl2O, HOCl, NaClO3, chlorinated isocyanurates, AlCl3, SiCl4, SnCl4, PCl3, PCl5, POCl3, AsCl3, SbCl3, SbCl5, BiCl3, S2Cl2, SCl2, SOCI2, CIF3, ICl, ICl3, TiCl3, TiCl4, MoCl5, FeCl3, ZnCl2, etc. Sony VAIO VPCF12AHJ battery

Purification and disinfection

Main article: Chlorination

Chlorine is an important chemical for water purification (such as water treatment plants), in disinfectants, and in bleach. Chlorine in water is more than three times as effective as a disinfectant against Escherichia coli than an equivalent concentration of bromine, and is more than six times more effective than an equivalent concentration of iodineSony VAIO VPCF12AGJ battery.[44]

Chlorine is usually used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and public swimming pools. In most private swimming pools, chlorine itself is not used, but rather sodium hypochlorite, formed from chlorine and sodium hydroxide, or solid tablets of chlorinated isocyanurates. The drawback of using chlorine in swimming pools is that the chlorine reacts with a human's hair and skin because hair and skin are made from proteinSony VAIO VPCF12AFJ battery.(Reaction with protein amino groups) Even small water supplies are now routinely chlorinated.[6]

It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative methods of adding chlorine are used. These include hypochlorite solutions, which gradually release chlorine into the water, and compounds like sodium dichloro-s-triazinetrione (dihydrate or anhydrous), sometimes referred to as "dichlor", and trichloro-s-triazinetrioneSony VAIO VPCF129FJ/BI battery, sometimes referred to as "trichlor". These compounds are stable while solid and may be used in powdered, granular, or tablet form. When added in small amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule forming hypochlorous acid (HOCl), which acts as a general biocide, killing germs, micro-organisms, algae, and so on. Sony VAIO VPCF128FJ/B battery

Use as a weapon

World War I

Main article: Poison gas in World War I

Chlorine gas, also known as bertholite, was first used as a weapon in World War I by Germany on April 22, 1915 in the Second Battle of Ypres [47]. As described by the soldiers it had a distinctive smell of a mixture between pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. Chlorine can react with water in the mucosa of the lungs to form hydrochloric acid, an irritant that can be lethalSony VAIO VPCF127FJ/W battery. The damage done by chlorine gas can be prevented by a gas mask, or other filtration method, which makes the overall chance of death by chlorine gas much lower than those of other chemical weapons. It was pioneered by a German scientist later to be a Nobel laureate, Fritz Haber of the Kaiser Wilhelm Institute in Berlin, in collaboration with the German chemical conglomerate IG FarbenSony VAIO VPCF11ZHJ battery, who developed methods for discharging chlorine gas against an entrenched enemy. It is alleged that Haber's role in the use of chlorine as a deadly weapon drove his wife, Clara Immerwahr, to suicide.[48] After its first use, chlorine was utilized by both sides as a chemical weapon, but it was soon replaced by the more deadly phosgene and mustard gasSony VAIO VPCF11AHJ battery.[49]

Iraq War

Main article: 2007 chlorine bombings in Iraq

Chlorine "attack" on an acetal resin plumbing joint.

Chlorine gas has also been used by insurgents against the local population and coalition forces in the Iraq War in the form of chlorine bombs. On March 17, 2007, for example, three chlorine filled trucks were detonated in the Anbar province killing two and sickening over 350.[50] Other chlorine bomb attacks resulted in higher death tollsSony VAIO VPCF11AGJ battery, with more than 30 deaths on two separate occasions.[51] Most of the deaths were caused by the force of the explosions rather than the effects of chlorine, since the toxic gas is readily dispersed and diluted in the atmosphere by the blast. The Iraqi authorities have tightened up security for chlorine, which is essential for providing safe drinking water for the populationSony VAIO VPCF11AFJ battery.

Chlorine cracking

The element is widely used for purifying water owing to its powerful oxidizing properties, especially potable water supplies and water used in swimming pools. Several catastrophic collapses of swimming pool ceilings have occurred owing to stress corrosion cracking of stainless steel rods used to suspend them.[52] Some polymers are also sensitive to attackSony VAIO VPCF118FJ battery, including acetal resin and polybutene. Both materials were used in hot and cold water domestic supplies, and stress corrosion cracking caused widespread failures in the USA in the 1980s and 1990s. One example shows an acetal joint in a water supply system, which, when it fractured, caused substantial physical damage to computers in the labs below the supply. The cracks started at injection molding defects in the joint and grew slowly until finally triggeredSony VAIO VPCF117FJ battery. The fracture surface shows iron and calcium salts that were deposited in the leaking joint from the water supply before failure.[53]

Health effects

NFPA 704

030OX

Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials.[54]

Chlorine is detectable with measuring devices in concentrations of as low as 0.2 parts per million (ppm), and by smell at 3 ppm. Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gasSony VAIO VGN-CS33H battery.[24] Breathing lower concentrations can aggravate the respiratory system, and exposure to the gas can irritate the eyes.[55] The toxicity of chlorine comes from its oxidizing power. When chlorine is inhaled at concentrations above 30 ppm, it begins to react with water and cells, which change it into hydrochloric acid (HCl) and hypochlorous acid (HClO) Sony VAIO VGN-CS33H/Z battery.

When used at specified levels for water disinfection, the reaction of chlorine with water is not a major concern for human health. However, other materials present in the water may generate disinfection by-products that can damage human health.

Argon ( /ˈɑrɡɒn/ ar-gon) is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases) Sony VAIO VGN-CS33H/B battery. Argon is the third most common gas in the Earth's atmosphere, at 0.93% (9,300 ppm), making it approximately 23.8 times more abundant than carbon dioxide (390 ppm). Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, being the preferred argon isotope produced by stellar nucleosynthesis in supernovasSony VAIO VGN-CS31Z/Q battery.

The name "argon" is derived from the Greek word αργον meaning "lazy" or "the inactive one", 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 1990Sony VAIO VGN-CS31S/W battery.

Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other 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. Argon gas also has uses in incandescent and fluorescent lighting, and other types of gas discharge tubesSony VAIO VGN-CS31S/V battery. Argon makes a distinctive blue-green gas laser.

Characteristics

A small piece of rapidly melting solid argon.

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, and nontoxic as a solid, liquid, and gas. Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperatureSony VAIO VGN-CS31S/T battery.

Although argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of argon fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000.[2] Although the neutral ground-state chemical compounds of argon are presently limited to HArFSony VAIO VGN-CS31S/R battery, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules.[3] Argon-containing ions and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have predicted several argon compounds that should be stable,[4] but for which no synthesis routes are currently knownSony VAIO VGN-CS31S/P battery.

[edit]History

Lord Rayleigh's method for the isolation of argon, based on an experiment of Henry Cavendish's. The gases are contained in a test-tube (A) standing over a large quantity of weak alkali (B), and the current is conveyed in wires insulated by U-shaped glass tubes (CC) passing through the liquid and round the mouth of the test-tube. The inner platinum ends (DD) of the wire receive a current from a battery of five Grove cells and a Ruhmkorff coil of medium sizeSony VAIO VGN-CS28 battery.

Argon (αργος, Greek meaning "inactive", in reference to its chemical inactivity)[5][6] was suspected to be present in air by Henry Cavendish in 1785 but was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay in Scotland in an experiment in which they removed all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air. Sony VAIO VGN-CS28/Q battery They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, 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.[10] Argon was also encountered in 1882 through independent research of H. F. Newall and W.N. HartleySony VAIO VGN-CS27 battery. Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered. The symbol for argon is now Ar, but up until 1957 it was A.[11]

[edit]Occurrence

Argon constitutes 0.934% by volume and 1.28% by mass of the Earth's atmosphere, and air is the primary raw material used by industry to produce purified argon productsSony VAIO VGN-CS27/W battery. 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.[12]

Isotopes

Main article: Isotopes of argon

The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar (0.34%), and 38Ar (0.06%). Naturally occurring 40K with a half-life of 1.25×109 years, decays to stable 40Ar (11.2%) by electron capture or positron emission, and also to stable 40Ca (88.8%) via beta decaySony VAIO VGN-CS27/R battery. These properties and ratios are used to determine the age of rocks by the method of K-Ar dating.[13]

In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 40Ar. In the subsurface environment, it is also produced through neutron capture by 39K or alpha emission by calcium. 37Ar is created from the neutron spallation of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 daysSony VAIO VGN-CS27/P battery.[13]

Argon is notable in that its isotopic composition varies greatly between different locations in the solar system. Where the major source of argon is the decay of 40K in rocks, 40Ar will be the dominant isotope, as it is on earth. Argon produced directly by stellar nucleosynthesis, in contrast, is dominated by the alpha process nuclide, 36Ar. Correspondingly, solar argon contains 84.6% 36Ar based on solar wind measurements. Sony VAIO VGN-CS27/C battery

The predominance of radiogenic 40Ar is responsible for the fact that the standard atomic weight of terrestrial argon is greater than that of the next element, potassium. This was puzzling at the time when argon was discovered, since Mendeleev had placed the elements in his periodic table in order of atomic weight, although the inertness of argon implies that it must be placed before the reactive alkali metal potassiumSony VAIO VGN-CS26T/W battery. 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).

The much greater atmospheric abundance of argon relative to the other noble gases is also due to the presence of radiogenic 40Ar. Primordial 36Ar has an abundance of only 31.5 ppmv (= 9340 ppmv x 0.337%), comparable to that of neon (18.18 ppmv)Sony VAIO VGN-CS26T/V battery.

The Martian atmosphere contains 1.6% of 40Ar and 5 ppm of 36Ar. The Mariner space probe fly-by of the planet Mercury in 1973 found that Mercury has a very thin atmosphere with 70% argon, believed to result from releases of the gas as a decay product from radioactive materials on the planet. In 2005, the Huygens probe also discovered the presence of 40Ar on Titan, the largest moon of Saturn. Sony VAIO VGN-CS26T/T battery

[edit]Compounds

See also: Van der Waals molecule

Argon’s complete octet of electrons indicates full s and p subshells. This full outer energy level 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 synthesizedSony VAIO VGN-CS26T/R battery. In August 2000, the first argon compound was formed by researchers at the University of Helsinki. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, argon fluorohydride (HArF) was formed.[2][16] It is stable up to 40 kelvin (−233 °C). The ArCF2+

2 metastable dication, which is valence isoelectronic with carbonyl fluoride, was observed in 2010Sony VAIO VGN-CS26T/Q battery.[17]

Production

Industrial

Argon is produced 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 yearSony VAIO VGN-CS26T/P battery.[18]

[edit]In radioactive decays

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

[edit]Applications

Cylinders containing argon gas for use in extinguishing fire without damaging server equipment

There are several different reasons argon is used in particular applications:

An inert gas is needed. In particular, argon is the cheapest alternative when nitrogen is not sufficiently inertSony VAIO VGN-CS26T/C battery.

Low thermal conductivity is required.

The electronic properties (ionization and/or the emission spectrum) are necessary.

Other noble gases would probably work as well in most of these applications, but argon is by far the cheapest. Argon is inexpensive since it is a byproduct of the production of liquid oxygen and liquid nitrogen from a cryogenic air separation unit, both of which are used on a large industrial scale. The other noble gases (except helium) are produced this way as wellSony VAIO VGN-CS25H battery, but argon is the most plentiful since it has the highest concentration in the atmosphere. The bulk of argon applications arise simply because it is inert and relatively cheap.

[edit]Industrial processes

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 burningSony VAIO VGN-CS25H/W battery.

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in various 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 germaniumSony VAIO VGN-CS25H/R battery.

See also: shielding gas

Argon is an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon's relatively high density causes it to remain close to the ground during gassing. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf lifeSony VAIO VGN-CS25H/Q battery.[19]

Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided.

[edit]Scientific research

Argon is used, primarily in liquid form, as the target for direct dark matter searches. The interaction of a hypothetical WIMP particle with the argon nucleus produces scintillation light that is then detected by photomultiplier tubes. Two-phase detectors also use argon gas to detect the ionized electrons produced during the WIMP-nucleus scatteringSony VAIO VGN-CS25H/P battery. As with most other liquefied noble gases, argon has a high scintillation lightyield (~ 51 photons / keV[20]), 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 gamma-ray background is larger due to 39Ar contaminationSony VAIO VGN-CS25H/C battery, unless one uses underground argon sources with a low level of radioactivity. Dark matter detectors currently operating with liquid argon include WArP, ArDM, microCLEAN and DEAP-I.

Preservative

A sample of caesium is packed under argon to avoid reactions with air

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 of E938). Aerial oxidation, hydrolysisSony VAIO VGN-CS23T/W battery, and other chemical reactions which degrade the products are retarded or prevented entirely. Bottles of high-purity chemicals and certain pharmaceutical products are available in sealed bottles or ampoules packed in argon. In wine making, argon is used to top-off barrels to avoid the aerial oxidation of ethanol to acetic acid during the aging process.

Argon is also available in aerosol-type cans, which may be used to preserve compounds such as varnish, polyurethane, paint, etc. for storage after openingSony VAIO VGN-CS23T/Q battery.[21]

Since 2001, the American National Archives stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to retard their degradation. Using argon reduces gas leakage, compared with the helium used in the preceding five decadesSony VAIO VGN-CS23H battery.[22]

Laboratory equipment

Gloveboxes are often filled with argon, which recirculates over scrubbers to maintain an oxygen-, nitrogen-, and moisture-free atmosphere

See also: Air-free technique

Argon may be used as the inert gas within Schlenk lines and gloveboxes. The use of argon over comparatively less expensive nitrogen is preferred where nitrogen may react with the experimental reagents or apparatusSony VAIO VGN-CS23H/S battery.

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 ions are also used for sputtering in microelectronicsSony VAIO VGN-CS23H/B battery.

[edit]Medical use

Cryosurgery procedures such as cryoablation use liquefied argon to destroy cancer cells. In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism in the patient and has resulted in the death of one person via this type of accidentSony VAIO VGN-CS23G battery.[23] Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.[24] It has also been used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood.[25] See Argox.

Lighting

Argon gas-discharge lamp forming the symbol for argon "Ar". Small amounts of mercury are sometimes added to argon to produce an electric blue color, as in this pictureSony VAIO VGN-CS23G/W battery.

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 argon provide blue light. Argon is also used for the creation of blue and green laser lightSony VAIO VGN-CS23G/Q battery.

[edit]Miscellaneous uses

It is used for thermal insulation in energy efficient windows.[26] Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.[27]

Compressed argon is allowed to expand, to cool the seeker heads of the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressureSony VAIO VGN-CS23G/P battery.[28]

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 is used in dating igneous rocks.

Safety

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. Argon is 25% more dense than air and is considered highly dangerous in closed areasSony VAIO VGN-CS21Z/Q battery. It is also 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 handlingSony VAIO VGN-CS21S/W battery.

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