Overblog Suivre ce blog
Editer l'article Administration Créer mon blog
17 novembre 2011 4 17 /11 /novembre /2011 03:56

Pharmaceuticals, agricultural chemicals, and poisons

Several important pharmaceuticals contain fluorine. Because of the considerable stability of the carbon-fluorine bond, many drugs are fluorinated to prevent their metabolism and prolong their half-lives, allowing for longer times between dosing and activation.Sony VGP-BPS9A/B Battery

For example, an aromatic ring may add to prevent the metabolism of a drug, but this presents a safety problem, because enzymes in the body metabolize some aromatic compounds into poisonous epoxides. Substituting a fluorine into a para position, however, protects the aromatic ring and prevents the epoxide from being produced.Sony VGP-BPS9A/S Battery

Adding fluorine to biologically active organics increases theirlipophilicity, because the carbon–fluorine bond is even more hydrophobic than the carbon–hydrogen bond. This effect often increases a drug's bioavailability due to increased cell membrane penetration.Sony VGP-BPS9B Battery

Since the carbon–fluorine bond is strong, organofluorides are generally very stable, although the potential of the fluorine to be released as a fluoride leaving group is heavily dependent on the its position in the molecule.

Of drugs that have been commercialized in the past 50 years, 5–15% contain fluorine, and the percentage of currently available fluorine-containing drugs is increasing.Sony VGP-BPX11 Battery

For example, fludrocortisone is one of the most common mineralocorticoids, a class of drugs that mimics the actions of aldosterone. The anti-inflammatories dexamethasone and triamcinolone, which are among the most potent of the synthetic corticosteroids class of drugs, contain fluorine.Sony VGP-BPL18 Battery

Several inhaled general anesthetic agents, including the most commonly used inhaled agents, also contain fluorine. Examples include sevoflurane, desflurane, and isoflurane, which arehydrofluorocarbon derivatives.

Many SSRI antidepressants are fluorinated organics,including citalopram, escitalopram, fluoxetine, fluvoxamine, and paroxetine.Sony VGP-BPS18 Battery

Fluoroquinolones are a commonly used family of broad-spectrum antibiotics. Because biological systems do not metabolize fluorinated molecules easily, fluorinated pharmaceuticals (often antibiotics and antidepressants) are among the major fluorinated organics found in treated city sewage and wastewater.Sony VGP-BPL20 Battery

In addition to pharmaceuticals, an estimated 30% of agrochemical compounds contain fluorine. Because of these, water from agricultural sites contaminates rivers with runoff organofluorines. Synthetic sodium fluoroacetate has been used as an insecticide, especially against cockroaches, and is effective as a bait-poison against mammalian pests.Sony VGP-BPS20/B Battery

Several other insecticides contain sodium fluoride, which is much less toxic than fluoroacetate.

Chronology

Because groundwater contains fluorine ions, organic items such as bone that are buried in soil will absorb those ions over time. Sony VGP-BPS20/S Battery

As such, it is possible to determine the relative age of an object by comparing the amount of fluoride with another object found in the same area. It is important as a separation technique in intra-site chronological analysis and inter-site comparisons.

However, if no actual age of any object is known, the ages can only be expressed in terms of one of the objects being older or younger than the other. Sony VGP-BPL21 Battery

The fluctuating amount of fluoride found in groundwater means the objects being compared must be in the same local area in order for the comparisons to be accurate. This technique is not always reliable, given that not all objects absorb fluorine at the same rates.Sony VGP-BPS21 Battery

Environmental concerns

Chlorofluorocarbons and bromofluorocarbons have come under strict environmental regulation due to their long residence times in the atmosphere, and their contribution to ozone depletion. Sony VGP-BPS21/S Battery

Since it is specifically chlorine and bromine radicals that harm the ozone layer, not fluorine, compounds that do not contain chlorine or bromine but contain only fluorine, carbon, and hydrogen (called hydrofluorocarbons) are not on the United States Environmental Protection Agency list of ozone-depleting substances, and have been widely used as replacements for halocarbons containing chlorine and bromine.Sony VGP-BPS21A Battery

Hydrofluorocarbons and perfluorocarbons are greenhouse gases about 4,000 to 10,000 times as potent as carbon dioxide. Sulfur hexafluoride exhibits an even stronger effect, having 20,000 times the global warming potential of carbon dioxide.

Because of the strength of the carbon–fluorine bond, many synthetic fluorocarbons and fluorocarbon-based compounds are persistent in the environment. Sony VGP-BPS21A/B Battery

The fluorosurfactants perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), used in waterproofing sprays, and other related chemicals, are persistent global contaminants. PFOS is apersistent organic pollutant and may be harming the health of wildlife.Sony VGP-BPS21B Battery

The potential health effects of PFOA to humans are unclear; its tissue distribution in humans is unknown, but studies in rats suggest it is likely to be present primarily in the liver, kidney, and blood, as it is absorbed easily via the gastrointestinal tract in rats. PFOA has been shown not to metabolize in the body, and, unlike chlorinated hydrocarbons, it is neither genotoxic nor lipophilic.Sony VGP-BPL22 Battery

It binds to serum albumin and is excreted primarily from the kidney.

Elementary state, fluoride ion, and fluoroacetate

Elemental fluorine is a highly toxic, corrosive oxidant, and is extremely reactive to organic material (except for perfluorinated substances) even at very low concentrations and can cause ignition at higher ones.Sony VGP-BPS22 Battery

Significant irritation to humans can be caused by concentration of fluorine of 25 ppm; at this and higher concentrations fluorine attacks the eyes, respiratory tract, lungs, liver and kidneys. At a concentration of 100 ppm, human eyes and noses are irritated and seriously damaged.Sony VGP-BPS22/A Battery

Soluble fluorides are moderately toxic. In the case of the simple salt sodium fluoride, the lethal dose for most adult humans is estimated at 5 to 10 g, which is equivalent to 32 to 64 mg/kg elemental fluoride/kg body weight. A toxic dose that may lead to adverse health effects is estimated at 3 to 5 mg/kg of fluoride.Sony VGP-BPS22A Battery

The fluoride ion is somewhat toxic, in part because of its ability to form, by equilibration, small amounts of hydrogen fluoride in water. This mobile uncharged species diffuses across cell membranes to attack intracellular calcium. The fluoride ion is readily absorbed by the stomach, intestines and excretedthrough urine. Sony VGP-BPL23 Battery

Urine tests have been used to ascertain rates of excretion in order to set upper limits in exposure to fluoride compounds and associated detrimental health effects. Ingested fluoride initially acts locally on the intestinal mucosa, where it forms hydrofluoric acid in the stomach.Sony VGP-BPS23 Battery

Thereafter it binds with calcium and interferes with various enzymes. Excess fluoride consumption can lead to skeletal fluorosis, which currently affects millions of people.

Historically, most cases of fluoride poisoning have been caused by accidental ingestion of insecticides containing inorganic fluoride, or (more rarely)rodenticides containing sodium fluoroacetate ("Compound 1080").Sony VGP-BPS23/B Battery

Currently, most fluoride poisonings are due to the ingestion of fluoride-containing toothpaste. Malfunction of water fluoridation equipment has occurred several times, including an Alaskan incident, which affected nearly 300 people and killed one person. Sony VGP-BPS23/D Battery

Hydrofluoric acid

Hydrofluoric acid is a contact poison, and must be handled with extreme care far beyond that accorded to other mineral acids, even the analogous hydrochloric acid, HCl. Owing to its lesser chemical dissociation in water (remaining a neutral molecule), hydrogen fluoride penetrates tissue more quickly than typical acids. Sony VGP-BPS23/G Battery

Poisoning can occur readily through exposure of skin or eyes, or when inhaled or swallowed. Symptoms of exposure to hydrofluoric acid may not be immediately evident. Hydrogen fluoride interferes with nerve function, meaning that burns may not initially be painful. Accidental exposures can go unnoticed, delaying treatment and increasing the extent and seriousness of the injury.Sony VGP-BPS23/P Battery

Once absorbed into blood through the skin, hydrogen fluoride reacts with blood calcium and may cause cardiac arrest. Formation of insoluble calcium fluoride possibly causes both a fall in calcium serum and the strong pain associated with tissue toxicity. In some cases, exposures can lead to hypocalcemia.Sony VGP-BPS23/W Battery

Burns with areas larger than 160 cm2 (25 in2) can cause serious systemic toxicity from interference with blood and tissue calcium levels.

Hydrofluoric acid exposure is often treated with calcium gluconate, a source of Ca2+ that binds with the fluoride ions. Sony Limited Edition 007 Battery

Hydrogen fluoride chemical burns to the skin can be treated with a water wash and 2.5% calcium gluconate gel or special rinsing solutions. However, because it is absorbed, medical treatment is necessary; in some cases, amputation may be required.

Caesium or cesium (  /?si?zi?m/ see-zee-?m) is the chemical element with the symbol Cs and atomic number 55.Sony VAIO VGC-LB15 Battery

It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at (or near) room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium. Sony VGN-AR Series Battery

The metal is extremely reactive and pyrophoric, reacting with water even at ?116 °C (?177 °F). It is the least electronegative element that has stable isotopes, of which it has only one, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, a fission product, are extracted from waste produced by nuclear reactors.Sony VGN-CR Series Battery

Two German chemists, Robert Bunsen and Gustav Kirchhoff, discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium have been as a "getter" in vacuum tubes and in photoelectric cells. Sony VIAO VGN-G Series Battery

In 1967, a specific frequency from the emission spectrum of caesium-133 was chosen to be used in the definition of thesecond by the International System of Units. Since then, caesium has been widely used in atomic clocks.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids.Sony VIAO VGN-G118 Battery

It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has ahalf-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks.Sony VIAO VGN-G118CN/B Battery

Caesium is a very soft (it has the lowest Mohs hardness of all elements (0.2)), veryductile, silvery-white metal, which develops a silvery-gold hue in the presence of trace amounts of oxygen. It has a melting point of 28.4 °C (83.1 °F), making it one of the few elemental metals which are liquid near room temperature. Sony VIAO VGN-G118CN/T Battery

Mercury is the only elemental metal with a known melting point lower than caesium. In addition, the metal has a rather low boiling point, 641 °C (1,186 °F), the lowest of all metals other than mercury.Its compounds burn with a blue color.

Caesium forms alloys with the other alkali metals as well as with gold, and amalgams with mercury. Sony VIAO VGN-G218 Battery

At temperatures below 650 °C (1,202 °F), it alloys with cobalt, iron,molybdenum, nickel, platinum, tantalum or tungsten. It forms well-defined intermetallic compounds with antimony, gallium, indium and thorium, which are photosensitive. It mixes with the other alkali metals (except with lithium), and the alloy with a molardistribution of 41% caesium, Sony VGN-TZ121 Battery

47% potassium, and 12% sodium has the lowest melting point of any known metal alloy, at ?78 °C(?108 °F). A few amalgams have been studied: CsHg2 is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.

Chemical properties

Caesium metal is highly reactive and very pyrophoric.Sony VGN-TZ13 Battery

In addition to igniting spontaneously in air, it reacts explosively with water even at low temperatures, more so than other members of the first group of the periodic table. The reaction with solid water occurs at temperatures as low as ?116 °C (?177 °F). Because of its high reactivity, the metal is classified as a hazardous material. Sony VGN-TZ131 Battery

It is stored and shipped in dry saturated hydrocarbons such as mineral oil. Similarly, it must be handled under inert atmosphere such as argon. It can be stored in vacuum-sealed borosilicate glass ampoules. In quantities of more than about 100 grams (3.5 oz), caesium is shipped in hermetically sealed stainless steel containers.Sony VGN-TZ132N Battery

The chemistry of caesium is similar to that of other alkali metals, but is more closely similar to that of rubidium, the element above caesium in the periodic table. Some small differences arise from the fact that it has a higher atomic mass and is moreelectropositive than other (non-radioactive) alkali metals.Sony VGN-TZ132/N Battery

Caesium is the most electropositive stable chemical element. The caesium ion is also larger and less "hard" than those of the lighter alkali metals.

Compounds

The vast majority of caesium compounds contain the element as the cation Cs+, which binds ionicallyto a wide variety of anions. One noteworthy exception is provided by the caeside anion (Cs?).Sony VGN-TZ17N Battery

Other exceptions include the several suboxides (see section on oxides below).

Returning to more normal compounds, salts of Cs+ are almost invariably colorless unless the anion itself is colored. Many of the simple salts are hygroscopic, but less so than the corresponding salts of the lighter alkali metals. The acetate, carbonate, halides, oxide, nitrate, and sulfatesalts are water-soluble. Sony VGN-TZ16N Battery

Double salts are often less soluble, and the low solubility of caesium aluminium sulfate is exploited in the purification of Cs from its ores. The double salt with antimony (such as CsSbCl4), bismuth, cadmium, copper, iron, and lead are also poorly soluble.

Caesium hydroxide (CsOH) is hygroscopic and a very strong base.Sony VGN-TZ18N Battery

It rapidly etches the surface ofsemiconductors such as silicon.CsOH has been previously regarded by chemists as the "strongest base", reflecting the relatively weak attraction between the large Cs+ ion and OH–.; it is indeed the strongest Arrhenius base, but a number of compounds that cannot exist in aqueous solution, such as n-butyllithiumand sodium amide,Sony VAIO VGN-AR47G/E1 Battery

are more basic.A stoichiometric mixture of caesium and gold will react to form caesium auride.

Complexes

Like all metal cations, Cs+ forms complexes with Lewis bases in solution. Because of its large size, Cs+ usually adopts coordination numbers greater than six-coordination, which is typical for the lighter alkali metal cations. Sony VAIO VGN-AR520E Battery

This trend is already apparent by the 8-coordination in CsCl, vs the halite motif adopted by the other alkali metal chlorides. Its high coordination number and softness (tendency to form covalent bonds) are the basis of the separation of Cs+ from other cations, as is practiced in the remediation of nuclear wastes, where 137Cs+ is separated from large amounts of non-radioactive K+.Sony VAIO VGN-AR53DB Battery

Halides

Caesium chloride (CsCl) crystallizes in the simple cubic crystal system. Also called the "caesium chloride structure", this structural motif is composed of aprimitive cubic lattice with a two-atom basis, each with an eightfold coordination; the chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the center of the cubes. Sony VAIO VGN-AR54DB Battery

This structure is shared with CsBr and CsI, and many other compounds that do not contain Cs. In contrast, most other alkaline halides adopt the sodium chloride (NaCl) structure. The CsCl structure is preferred because Cs+ has an ionic radius of 174 pm andCl?181 pm.Sony VAIO VGN-AR550E Battery

Oxides

More so than the other alkali metals, caesium forms numerous binary compounds with oxygen. When caesium burns in air, the superoxide CsO2 is the main product. The "normal" caesium oxide (Cs2O) forms yellow-orange hexagonal crystals,  and is the only oxide of the anti-CdCl2 type.Sony VAIO VGN-AR55DB Battery

It vaporizes at 250 °C (482 °F), and decomposes to caesium metal and the peroxide Cs2O2 at temperatures above 400 °C (752 °F). Aside from the superoxide and the ozonide CsO3, several brightly colored suboxides have also been studied. These include Cs7O, Cs4O, Cs11O3, Cs3O (dark-green ), CsO, Cs3O2, as well as Cs7O2.Sony VAIO VGN-AR630E Battery

The latter may be heated under vacuum to generate Cs2O. Binary compounds with sulfur,selenium, and tellurium also exist.

Isotopes

Caesium has a total of 39 known isotopes that range in their mass number (i.e. number of nucleons in its nucleus) from 112 to 151.Sony VAIO VGN-AR64DB Battery

Several of these are synthesized from lighter elements by the slow neutron capture process (S-process) inside old stars, as well as inside supernova explosions (R-process). However, the onlystable isotope is 133Cs, which has 78 neutrons. Although it has a large nuclear spin (7/2+), nuclear magnetic resonance studies can be done with this isotope at a resonating frequency of 11.7 MHz.Sony VAIO VGN-AR65DB Battery

The radioactive 135Cs has a very long half-life of about 2.3 million years, while 137Cs and 134Cs have half-lives of 30 and 2 years, respectively. 137Cs decomposes to a short-lived 137mBa by beta decay, and then to non-radioactive barium, while 134Cs transforms into 134Ba directly. Sony VAIO VGN-AR660U Battery

The isotopes with mass numbers of 129, 131, 132 and 136, have half-times between a day and two weeks, while most of the other isotopes have half-lives from a few seconds to fractions of a second. There are at least 21 metastable nuclear isomers. Other than 134mCs (with a half-life of just under 3 hours), all are very unstable and decay with half-lives of a few minutes or less.Sony VAIO VGN-AR690U Battery

The isotope 135Cs is one of the long-lived fission products of uranium which form in nuclear reactors.However, itsfission product yield is reduced in most reactors because its predecessor, 135Xe, is an extremely potent neutron poison and transmutes frequently to stable 136Xe before it can decay to 135Cs.Sony VAIO VGN-AR71ZU Battery

Because of its beta decay (to 137mBa), 137Cs is a strong emitter of gamma radiation. Its half-life makes it the principalmedium-lived fission product along with 90Sr—both are responsible for radioactivity of spent nuclear fuel after several years of cooling up to several hundred years after use.Sony VAIO VGN-AR730E/B Battery

For example 137Cs together with 90Sr currently generate the largest source of radioactivity generated in the area around the Chernobyl disaster. It is not feasible to dispose of 137Cs through neutron capture (due to the low capture rate) and as a result it must be allowed to decay.Sony VAIO VGN-AR73DB Battery

Almost all caesium produced from nuclear fission comes from beta decay of originally more neutron-rich fission products, passing through various isotopes of iodineand of xenon. Because iodine and xenon are volatile and can diffuse through nuclear fuel or air, radioactive caesium is often created far from the original site of fission.Sony VAIO VGN-AR74DB Battery

With the commencement of nuclear weapons testing around 1945, 137Cs was released into the atmosphere and then returned to the surface of the earth as a component of radioactive fallout.

Occurrence

Caesium is a relatively rare element as it is estimated to average approximately 3 parts per million in the Earth's crust.Sony VAIO VGN-AR75UDB Battery

This makes it the 45th most abundant of all elements and the 36th of all the metals. Nevertheless, it is more abundant than such elements as antimony, cadmium, tin and tungsten, and two orders of magnitude more abundant than mercury or silver, but 30 times less abundant than rubidium—with which it is so closely chemically associated.Sony VAIO VGN-AR790U/B Battery

Due to its large ionic radius, caesium is one of the "incompatible elements." During magma crystallization, caesium is concentrated in the liquid phase and crystallizes last. Therefore the largest deposits of caesium are zone pegmatite ore bodies formed by this enrichment process. Sony VAIO VGN-AR820E Battery

Because caesium does not substitute for potassium as readily as does rubidium, the alkali evaporite minerals sylvite (KCl) and carnallite (KMgCl3·6H2O) may contain only 0.002% caesium. Consequently, Cs is found in few minerals. Sony VAIO VGN-AR830E Battery

Percent amounts of caesium may be found in beryl (Be3Al2(SiO3)6) and avogadrite ((K,Cs)BF4), up to 15 wt% Cs2O in the closely related mineral pezzottaite (Cs(Be2Li)Al2Si6O18), up to 8.4 wt% Cs2O in the rare mineral londonite ((Cs,K)Al4Be4(B,Be)12O28), and less in the more widespreadrhodizite.Sony VAIO VGN-AR83S Battery

The only economically important source mineral for caesium is pollucite Cs(AlSi2O6), which is found in a few places around the world in zoned pegmatites, and is associated with the more commercially important lithium minerals lepidolite and petalite. Within the pegmatites, the large grain size and the strong separation of the minerals create high-grade ore for mining.Sony VAIO VGN-AR83US Battery

One of the world's most significant and richest sources of the metal is the Tanco mine at Bernic Lake in Manitoba, Canada. The deposits there are estimated to contain 350,000 metric tons of pollucite ore, which represents more than two-thirds of the world's reserve base.Sony VAIO VGN-AR84S Battery

Although the stoichiometric content of caesium in pollucite is 42.6%, pure pollucite samples from this deposit contain only about 34% caesium, while the average content is 24 wt%. Commercial pollucite contains over 19% caesium. The Bikita pegmatite deposit in Zimbabwe is mined for its petalite but it also contains a significant amount of pollucite. Sony VAIO VGN-AR84US Battery

Notable amounts of pollucite are also mined in the Karibib Desert, Namibia.At the present rate of world mine production of 5 to 10 metric tons per year, reserves will last for thousands of years.

Production

The mining of pollucite ore is a selective process and is conducted on a small scale in comparison with most metal mining operations.Sony VAIO VGN-AR85S Battery

The ore is crushed, hand-sorted, but not usually concentrated, and then ground. Caesium is then extracted from pollucite mainly by three methods: acid digestion, alkaline decomposition, and direct reduction.

In the acid digestion, the silicate pollucite rock is dissolved with strong acids such as hydrochloric (HCl), sulfuric (H2SO4), hydrobromic (HBr), or hydrofluoric(HF). Sony VAIO VGN-AR85US Battery

With hydrochloric acid, a mixture of soluble chlorides is produced, and the insoluble chloride double salts of caesium are precipitated as caesium antimony chloride (Cs4SbCl7), caesium iodine chloride (Cs2ICl), or caesium hexachlorocerate (Cs2(CeCl6)).Sony VAIO VGN-AR870EA Battery

After separation, the pure precipitated double salt is decomposed, and pure CsCl is obtained after evaporating the water. The method using sulfuric acid yields the insoluble double salt directly as caesium alum (CsAl(SO4)2·12H2O). The aluminium sulfate in it is converted to the insoluble aluminium oxide by roasting the alum with carbon, and the resulting product is leached with water to yield a Cs2SO4 solution. Sony VAIO VGN-AR890U Battery

The roasting of pollucite with calcium carbonate and calcium chloride yields insoluble calcium silicates and soluble caesium chloride. Leaching with water or diluteammonia (NH4OH) yields then a dilute chloride (CsCl) solution. This solution can be evaporated to produce caesium chloride or transformed into caesium alum or caesium carbonate.Sony VAIO VGN-AR93S Battery

Albeit not commercially feasible, direct reduction of the ore with potassium, sodium or calcium in vacuum can produce caesium metal directly.

Most of the mined caesium (as salts) is directly converted into caesium formate (HCOO?Cs+) for applications such as oil drilling.Sony VAIO VGN-AR93US Battery

To supply the developing market,Cabot Corporation built a production plant in 1997 at the Tanco Mine near Bernic Lake in Manitoba, Canada, with a capacity of 12,000 barrels (1,900 m3) per year of caesium formate solution. The primary smaller-scale commercial compounds of caesium are caesium chloride and its nitrate.Sony VAIO VGN-AR94S Battery

Alternatively, caesium metal may be obtained from the purified compounds derived from the ore. Caesium chloride, and the other caesium halides as well, can be reduced at 700 to 800 °C (1,292 to 1,472 °F) with calcium or barium, followed by distillation of the caesium metal. In the same way, the aluminate, carbonate, or hydroxide may be reduced by magnesium.Sony VAIO VGN-AR94US Battery

The metal can also be isolated by electrolysis of fused caesium cyanide (CsCN). Exceptionally pure and gas-free caesium can be made by the thermal decomposition at 390 °C (734 °F) of caesium azide CsN3, which is produced from aqueous caesium sulfate and barium azide.Sony VAIO VGN-AR95S Battery

In vacuum applications, caesium dichromate can be reacted with zirconium forming pure caesium metal without other gaseous products.

Cs2Cr2O7 + 2 Zr ? 2 Cs + 2 ZrO2+ Cr2O3

The price of 99.8% pure caesium (metal basis) in 2009 was about US$10 per gram ($280 per ounce), but its compounds are significantly cheaper.Sony VAIO VGN-AR95US Battery

History

In 1860, Robert Bunsen and Gustav Kirchhoff discovered caesium in the mineral water from Dürkheim, Germany. Due to the bright blue lines in its emission spectrum, they chose a name derived from the Latin word caesius, meaning sky-blue.Sony VAIO VGN-AW110J/H Battery

Caesium was the first element to be discovered spectroscopically, only one year after the invention of the spectroscope by Bunsen and Kirchhoff.

To obtain a pure sample of caesium, 44,000 litres (9,700 imp gal; 12,000 US gal) of mineral water had to be evaporated to yield 240 kilograms (530 lb) of concentrated salt solution. Sony VAIO VGN-AW11M/H Battery

The alkaline earth metals were precipitated either as sulfates or oxalates, leaving the alkali metal in the solution. After conversion to the nitrates and extraction with ethanol, a sodium-free mixture was obtained. From this mixture, the lithium was precipitated by ammonium carbonate. Sony VAIO VGN-AW11S/B Battery

Potassium, rubidium and caesium form insoluble salts with chloroplatinic acid, but these salts show a slight difference in solubility in hot water. Therefore, the less-soluble caesium and rubidium hexachloroplatinate ((Cs,Rb)2PtCl6) could be obtained by fractional crystallization.Sony VAIO VGN-AW11XU/Q Battery

After reduction of the hexachloroplatinate with hydrogen, caesium and rubidium could be separated by the difference in solubility of their carbonates in alcohol. The process yielded 9.2 grams (0.32 oz) of rubidium chloride and 7.3 grams (0.26 oz) of caesium chloride from the initial 44,000 liters of mineral water.Sony VAIO VGN-AW11Z/B Battery

The two scientists used the caesium chloride thus obtained to estimate the atomic weight of the new element at 123.35 (compared to the currently accepted one of 132.9). They tried to generate elemental caesium by electrolysis of molten caesium chloride, but instead of a metal, they obtained a blue homogenous substance which "neither under the naked eye nor under the microscope" showed the slightest trace of metallic substance;" as a result, they assigned it as a subchloride (Cs2Cl). Sony VAIO VGN-AW120J/H Battery

In reality, the product was probably a colloidal mixture of the metal and caesium chloride.[58] The electrolysis of the aqueous solution of chloride with a mercury anode produced a caesium amalgam which readily decomposed under the aqueous conditions.Sony VAIO VGN-AW150Y/H Battery

The pure metal was eventually isolated by the German chemist Carl Setterberg while working on his doctorate with Kekulé and Bunsen. In 1882 he produced caesium metal by electrolyzing caesium cyanide, and thus avoiding the problems with the chloride.Sony VAIO VGN-AW160J/Q Battery

Historically, the most important use for caesium has been in research and development, primarily in chemical and electrical fields. Very few applications existed for caesium until the 1920s when it became used in radio vacuum tubes. It had two functions: as a getter it removed excess oxygen after manufacture, and as a coating on the heated cathode, it increased its electrical conductivity.Sony VAIO VGN-AW170Y/Q Battery

Caesium did not become recognized as a high-performance industrial metal until the 1950s. Applications of non-radioactive caesium included photoelectric cells, photomultiplier tubes, optical components of infrared spectrophotometers, catalysts for several organic reactions, crystals for scintillation counters, and in magnetohydrodynamic power generators.Sony VAIO VGN-AW180Y/Q Battery

Since 1967, the International System of Measurements has based its unit of time, the second, on the properties of caesium. The International System of Units (SI) defines the second as 9,192,631,770 cycles of the radiation, which corresponds to the transition between two hyperfine energy levels of the ground state of the caesium-133 atom.Sony VAIO VGN-AW190JAH Battery

The 13th General Conference on Weights and Measures of 1967 defined a second as: "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of caesium-133 atoms in their ground state undisturbed by external fields".Sony VAIO VGN-AW190NAB Battery

Petroleum exploration

The largest current end-use of caesium is in caesium formate-based drilling fluids for the extractive oil industry. Aqueous solutions of caesium formate (HCOO–Cs+)—made by reacting caesium hydroxide with formic acid—were developed in the mid-1990s for use as oil well drilling and completion fluids. Sony VAIO VGN-AW190NBB Battery

The function of caesium formate as a drilling fluid is to lubricate drill bits, to bring rock cuttings to the surface, and to maintain pressure on the formation during drilling of the well. As completion fluid, which assists the emplacement of control hardware after drilling but prior to production, the function of caesium formate is to maintain the pressure.Sony VAIO VGN-AW190NCB Battery

The high density of the caesium formate brine (up to 2.3 g·cm?3, or 19.2 pounds per gallon), coupled with the relatively benign nature of most caesium compounds, reduces the requirement for toxic high-density suspended solids in the drilling fluid—a significant technological, engineering and environmental advantage. Sony VAIO VGN-AW190YAB Battery

Unlike the components of many other heavy liquids, caesium formate is relatively environment-friendly. The caesium formate brine can be blended with potassium and sodium formates to decrease the density of the fluids down to that of water (1.0 g·cm?3, or 8.3 pounds per gallon). Sony VAIO VGN-AW190YBB Battery

Furthermore, it is biodegradable and reclaimable, and may be recycled, which is important in view of its high cost (about $4,000 per barrel in 2001). Alkali formates are safe to handle and do not damage the producing formation or downhole metals as their corrosive alternative, high-density brines (such as zinc bromide ZnBr2 solutions),sometimes do; they also require less cleanup and disposal costs. Sony VAIO VGN-AW190YCB Battery

Atomic clocks

Caesium-based atomic clocks observe electromagnetic transitions in the hyperfine structure of caesium-133 atoms and use it as a reference point. The first accurate caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK.Sony VAIO VGN-AW190YDB Battery

Since then, they have been improved repeatedly over the past half-century, and form the basis for standards-compliant time and frequency measurements. These clocks measure frequency with an error of 2 to 3 parts in 1014, which would correspond to a time measurement accuracy of 2 nanoseconds per day, or one second in 1.4 million years. Sony VAIO VGN-AW220J/B Battery

The latest versions are accurate to better than 1 part in 1015, which means they would be off by about 2 seconds since the extinction of the dinosaurs 65 million years ago, and has been regarded as "the most accurate realization of a unit that mankind has yet achieved." Sony VAIO VGN-AW230J/H Battery

Caesium clocks are also used in networks that oversee the timing of cell phone transmissions and the information flow on the Internet.

Electric power and electronics

Caesium vapor thermionic generators are low-power devices that convert heat energy to electrical energy. Sony VAIO VGN-AW235J/B Battery

In the two-electrodevacuum tube converter, it neutralizes the space charge that builds up near the cathode, and in doing so, it enhances the current flow.

Caesium is also important for its photoemissive properties by which light energy is converted to electron flow. Sony VAIO VGN-AW290JFQ Battery

It is used inphotoelectric cells because caesium-based cathodes such as the intermetallic compound K2CsSb have low threshold voltage for emission of electrons.  The range of photoemissive devices using caesium include optical character recognition devices,photomultiplier tubes, and video camera tubes.Sony VAIO VGN-AW41JF Battery

Nevertheless, germanium, rubidium, selenium, silicon, tellurium, and several other elements can substitute caesium in photosensitive materials. Sony VAIO VGN-AW41JF/H Battery

Caesium iodide (CsI), bromide (CsBr) and caesium fluoride (CsF) crystals are employed for

 scintillators in scintillation counterswidely used in mineral exploration and particle physics research as they are well suited for the detection of gamma and x-rayradiation.Sony VAIO VGN-AW41MF Battery

Caesium, being a heavy element, provides good stopping power contributing to better detectivity. Caesium compounds may also provide a faster response (CsF) and be less hygroscopic (CsI).

Caesium vapor is used in many common magnetometers. The element is also used as an internal standard in spectrophotometry.Sony VAIO VGN-AW41MF/H Battery

Like other alkali metals, caesium has a great affinity for oxygen and is used as a "getter" in vacuum tubes.  Other uses of the metal include high-energy lasers,vapor glow lamps, and vapor rectifiers. Sony VAIO VGN-AW41XH Battery

Centrifugation fluids

Because of their high density, solutions of caesium chloride (CsCl), sulfate (Cs2SO4), and trifluoroacetate (Cs(O2CCF3)) are commonly used in molecular biology for density gradient ultracentrifugation. Sony VAIO VGN-AW41XH/Q Battery

This technology is primarily applied to the isolation of viral particles, sub-cellular organelles and fractions, andnucleic acids from biological samples.

Chemical and medical use

Relatively few chemical applications exist for caesium.Sony VAIO VGN-AW41ZF Battery

Doping with caesium compounds is used to enhance the effectiveness of several metal-ion catalysts used in the production of chemicals, such as acrylic acid, anthraquinone, ethylene oxide, methanol,phthalic anhydride, styrene, methyl methacrylate monomers, and various olefins.Sony VAIO VGN-AW41ZF/B Battery

It is also used in the catalytic conversion ofsulfur dioxide into sulfur trioxide in the production of sulfuric acid.

Caesium fluoride enjoys niche use in organic chemistry as a base, or as an anhydrous source of fluoride ion. Caesium salts sometimes replace potassium or sodium salts in organic synthesis, such as cyclization, esterification, and polymerization.Sony VAIO VGN-AW50DB/H Battery

Nuclear and isotope applications

Caesium-137 is a very common radioisotope used as a gamma-emitter in industrial applications. Its advantages include a half-life of roughly 30 years, its availability from the nuclear fuel cycle, and having 137Ba as stable end product. Sony VAIO VGN-AW51JGB Battery

The high water solubility is a disadvantage which makes it incompatible with irradiation of food and medical supplies. It has been used in agriculture, cancer treatment, and the sterilization of food, sewage sludge, and surgical equipment. Radioactive isotopes of caesium in radiation devices were used in the medical field to treat certain types of cancer,Sony VAIO VGN-AW52JGB Battery

but emergence of better alternatives and the use of water-soluble caesium chloride in the sources, which could create wide-ranging contamination, gradually put some of these caesium sources out of use. Caesium-137 has been employed in a variety of industrial measurement gauges, including moisture, density, leveling, and thickness gauges.Sony VAIO VGN-AW53FB Battery

It has also been used in well logging devices for measuring the electron density of the rock formations, which is analogous to the bulk density of the formations.

Isotope 137 has also been used in hydrologic studies analogous to those using tritium. It is a daughter product of nuclear fission reactions. Sony VAIO VGN-AW70B/Q Battery

With the commencement of nuclear testing around 1945, and continuing through the mid-1980s, caesium-137 was released into the atmosphere where it is absorbed readily into solution. Known year-to-year variation within that period allows correlation with soil and sediment layers. Sony VAIO VGN-AW71JB Battery

Caesium-134, and to a lesser extent caesium-135, have also been used in hydrology as a measure of caesium output by the nuclear power industry. While they are less prevalent than either caesium-133 or caesium-137, these isotopes have the advantage of being produced solely from anthropogenic sources. Sony VAIO VGN-AW72JB Battery

Other uses

Caesium and mercury were used as a propellant in early ion engines designed for spacecraft propulsion on very long interplanetary or extraplanetary missions. The ionization method was to strip the outer electron from the propellant upon contact with a tungsten electrode that had voltage applied.Sony VAIO VGN-AW73FB Battery

Concerns about the corrosive action of caesium on spacecraft components have pushed development in the direction of use of inert gas propellants such as xenon; this is easier to handle in ground-based tests and has less potential to interfere with the spacecraft.Sony VAIO VGN-AW80NS Battery

Eventually, xenon was used in the experimental spacecraft Deep Space 1 launched in 1998.  Nevertheless, field emission electric propulsion thrusters which use a simple system of accelerating liquid metal ions such as of caesium to create thrust have been built.Sony VAIO VGN-AW80S Battery

Caesium nitrate is used as an oxidizer and pyrotechnic colorant to burn silicon in infrared flares such as the LUU-19 flare,because it emits much of its light in the near infrared spectrum.Caesium has been used to reduce theradar signature of exhaust plumes in the SR-71 Blackbird military aircraft.Sony VAIO VGN-AW80US Battery

Caesium, along with rubidium, has been added as a carbonate to glass because it reduces electrical conductivity and improves stability and durability offiber optics and night vision devices. Caesium fluoride or caesium aluminium fluoride are used in fluxes formulated for the brazing of aluminium alloys that contain magnesium.Sony VAIO VGN-AW81DS Battery

Prognostications

Magnetohydrodynamic (MHD) power-generating systems were researched but failed to gain widespread acceptance.Caesium metal has also been considered as the working fluid in high-temperature Rankine cycle turboelectric generators.Sony VAIO VGN-AW81JS Battery

Caesium salts have been evaluated as antishock reagents to be used following the administration of arsenical drugs. Because of their effect on heart rhythms, however, they are less likely to be used than potassium or rubidium salts. They have also been used to treat epilepsy.Sony VAIO VGN-AW81YS Battery

Health and safety hazards

Caesium compounds are rarely encountered by most people, but most caesium compounds are mildly toxic because of chemical similarity of caesium to potassium. Exposure to large amounts of caesium compounds can causehyperirritability and spasms, Sony VAIO VGN-AW82DS Battery

but as such amounts would not ordinarily be encountered in natural sources, caesium is not a major chemical environmental pollutant. The median lethal dose (LD50) value for caesium chloride in mice is 2.3 g per kilogram, which is comparable to the LD50values of potassium chloride and sodium chloride.Sony VAIO VGN-AW82JS Battery

Caesium metal is one of the most reactive elements and is highly explosive when it comes in contact with water. The hydrogen gas produced by the reaction is heated by the thermal energy released at the same time, causing ignition and a violent explosion. Sony VAIO VGN-AW82YS Battery

This can occur with other alkali metals, but caesium is so potent that this explosive reaction can even be triggered by cold water. The metal is highly pyrophoric, and ignites spontaneously in air to form caesium hydroxide and various oxides. Caesium hydroxide is a very strong base, and can rapidly corrode glass.Sony VAIO VGN-AW83FS Battery

The isotopes 134 and 137 (present in the biosphere in small amounts from radiation leaks) represent a radioactivity burden which varies depending on location. Radiocaesium does not accumulate in the body as effectively as many other fission products (such as radioiodine and radiostrontium).Sony VAIO VGN-AW83GS Battery

As with other alkali metals, radiocaesium washes out of the body relatively quickly in sweat and urine. However, radiocaesium follows potassium and tends to accumulate in plant tissues, including fruits and vegetables. It is also well-documented that mushrooms from contaminated forests accumulate radiocaesium (caesium-137) in their fungalsporocarps.Sony VAIO VGN-AW83HS Battery

Accumulation of caesium-137 in lakes has been a high concern after the Chernobyl disaster. Experiments with dogs showed that a single dose of 3.8 millicuries (140 MBq, 4.1 ?g of caesium-137) per kilogram is lethal within three weeks; smaller amounts may cause infertility and cancer.Sony VAIO VGN-AW90NS Battery

TheInternational Atomic Energy Agency and other sources have warned that radioactive materials, such as caesium-137, could be used in radiological dispersion devices, or "dirty bombs".Sony VAIO VGN-AW90S Battery,Sony VAIO VGN-AW90US Battery,Sony VAIO VGN-AW91CDS Battery,Sony VAIO VGN-AW91CJS Battery,Sony VAIO VGN-AW91CYS Battery,Sony VAIO VGN-AW91DS Battery

Partager cet article

Repost 0
Published by batterys - dans Laptop Battery
commenter cet article

commentaires