Caesium

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Caesium ( IUPAC ), standard language cesium or cesium ( Cesium in American English ), a chemical element with the chemical symbol Cs and atomic number 55 is in the periodic table it is in the 1st main group and is one of the alkali metals. Caesium is the heaviest stable alkali metal.

Cesium was discovered in 1861 by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff in the mineral water Dürkheimer Maxquelle. Due to the two blue lines, with which the element was detected, they named it after the Latin caesius for sky blue. The pure element was presented in 1881 by Carl Setter Mountain for the first time.

Caesium is an extremely reactive, very soft, golden, in a highly pure state silvery metal. Since it immediately and reacts very violently with air, it is kept in sealed glass ampoules under inert gas. A biological significance of the element is not known, it does not normally occur in the body and is not toxic.

The radioactive isotope 137Cs, a product of nuclear fission, found in public special attention, as it came as a result of the Chernobyl disaster on 26 April 1986 in larger amounts in the environment.

History

Cesium was first described in 1861 by Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They examined and found Dürkheim mineral water, after separation of calcium, strontium, magnesium, lithium and two previously unknown lines in the blue spectral range. They concluded from their observations that in the investigated mineral water there must be another, as yet unknown element that for sky blue, named because of the blue spectral cesium, after the Latin caesius.

Bunsen also tried to separate cesium from other alkali metals, to explore further properties of the element. To this end, he put the solution with a platinum chloride solution to precipitate potassium and the new heavier alkali metals rubidium and cesium as insoluble Hexachloridoplatinat. The potassium was removed by repeated boiling in a little water. To obtain the pure platinum chlorides with hydrogen was reduced to the element, so that the now water-soluble cesium and Rubidiumchloride could be leached. The separation of cesium and rubidium carried out under utilization of the different solubility of the carbonate in absolute ethanol, wherein in contrast to the corresponding cesium rubidium compound is soluble. Cesium chloride Bunsen and Kirchhoff also served for a first determination of the molar mass of the new element, which they found the value of 123.35 g / mol.

The two researchers were able to win no elemental cesium, because during the electrolysis of molten cesium chloride instead of the metal was a blue compound, which they described as subchloride, which is but probably was a colloidal mixture of cesium and cesium chloride. In the electrolysis of an aqueous solution with a mercury anode, the easily decomposable Caesiumamalgam formed.

The representation of the elemental cesium finally succeeded in 1881 Carl Setter Mountain, which avoided the problems with the chloride by used for the fused-salt electrolysis Caesiumcyanid. Initially disturbed the melting of the Caesiumcyanids required relatively high temperature, but it could reduce by the eutectic with barium cyanide.

Occurrence

With a content of 3 ppm in the Earth's continental crust cesium on earth is a rare element. It is the rarest alkali metal according to the unstable francium. Due to its high reactivity, it is not elementary, but only in the form of compounds. Cesium usually is a rare element in accompanying potassium or other alkali metal salts, such as lepidolite, but also some known Caesiumminerale. The most common Caesiummineral is pollucite, (Cs, Na) 2Al2Si4O12 · H2O, the near Lac du Bonnet in the Canadian province of Manitoba found in greater numbers, especially on Bernic Lake in the Tanco Mine. Other major deposits are located in Bikita, Zimbabwe and Namibia. The occurrence in the Tanco mine at Lac du Bonnet are the only ones in which cesium is reduced. Rarer Caesiumminerale example, Cesstibtantit (Cs, Na) SbTa4O12 and Pautovit CsFe2S3.

Due to the water solubility of most cesium compounds the element dissolved in seawater; a liter, while an average of 0.3 to 4 micrograms of cesium. In comparable amounts, we find such frequent, but less soluble elements such as nickel, chromium, or copper.

Production and representation

Cesium is produced only in small scale. In 1978 was the world's produced amount of cesium and cesium compounds about 20 tons. Starting material for the production of elemental cesium and cesium compounds all is pollucite, which can be digested with acids or bases. Acids as hydrochloric, sulfuric or hydrobromic acid can be used. In each case a cesium and aluminum-containing solution, can be recovered from the by precipitation, ion exchange or extraction, the pure cesium is produced. Another possibility is to heat pollucite with calcium or sodium carbonate and the corresponding chlorides and subsequently leaching with water. This produces an impure Caesiumchloridlösung.

Cesium metal can be obtained chemically by reduction of Caesiumhalogeniden with calcium or barium. It has been distilled from volatile cesium metal in a vacuum.

More Ways to Caesiummetallherstellung are the reduction of cesium hydroxide with magnesium and the reduction of Caesiumdichromat with zirconium.

High-purity cesium can be generated by the decomposition of Caesiumazid, which can be obtained from cesium carbonate, and represent subsequent distillation. The reaction occurs at 380 ° C to an iron or copper catalyst.

Properties

Physical Properties

Caesium is in its purest state a silver-white light metal with a density of 1.873 g/cm3, which already appear golden yellow with slight impurities. In many properties, it is between those of rubidium and - if known - those of the unstable francium. It has 28.7 ° C except francium the lowest melting point of all the alkali metals and has at the same time for mercury and compared with one of the lowest melting point of gallium metals at all. Caesium is very soft (Mohs hardness: 0.2 ) and very stretchy.

Like the other alkali metals, cesium crystallizes at standard conditions in cubic crystal system with a body-centered cubic unit cell in the space group Im3m with the lattice parameters a = 614 pm and two formula units per unit cell. Under a pressure of 41 kilobars, a phase transformation occurs in a face-centered cubic crystal structure with the lattice parameters of a = 598 microns.

With the exception of lithium can mix any cesium with other alkali metals. At a ratio of 41% cesium, 12 % sodium and 47 % potassium alloy with the lowest rate ever known melting point of -78 ° C. produced

The cesium atom and the ion Cs have a large radius, they are - again with the exception of francium - the largest single atoms or ions. This is due to the very low effective nuclear charge, whereby in particular the outermost s- electron is bound only to a small extent to the core. This has the effect in addition to the large atomic radius, the low ionization potential of the cesium atom, and thus the high reactivity of the element.

Gaseous cesium has an abnormal refractive index is smaller than one. This means that the phase velocity of the electromagnetic wave - is larger than in the vacuum, which is not in contradiction with the theory of relativity - in this case light.

Chemical Properties

After francium cesium is the element having the lowest ionization potential. For the removal of the outermost electron it has the lowest electronegativity. Caesium is this contact with other elements very easily and forms monovalent cesium salts. Since the noble gas configuration is achieved by the elimination of this one electron, it does not form two - or higher- ions.

Reactions with cesium run very violent in general, it ignites on contact with oxygen immediately and forms such as potassium and rubidium the corresponding superoxide.

Even with water it reacts violently with formation of cesium, this reaction takes place even at temperatures of -116 ° C with ice.

Isotopes

A total of 39 isotopes and 23 more Kernisomere of cesium are known. In nature, only the isotope 133Cs occurs. Caesium is therefore a pure element. Of the artificial isotopes have 134Cs with 2.0648 years, 135Cs and 137Cs at 2.3 million years with 30.17 years of medium to very long half-lives, while the other isotopes between 17 microseconds for 113Cs and 136Cs at 13.16 days are.

An important artificial isotope is 137Cs, a beta emitter with a half -life of 30.17 years. 137Cs decays with a probability of 94.6 % in the first metastable intermediate 137mBa which converts to a half-life of 2.552 minutes by gamma decay in the stable barium isotope 137Ba. In the remaining 5.4% there is a direct transition to stable barium isotope 137Ba. Together with other Caesiumisotopen it arises either directly by nuclear fission in nuclear reactors or by the decay of short-lived fission products such other 137i or 137Xe.

137Cs gamma-ray source is an important addition to the cobalt and isotope 60 Co is used in radiation therapy for the treatment of cancers, for measuring the flow rate in pipes and thickness of test example paper, film or metal. It is also used in quality control in nuclear medicine as durable nuclide in test emitters.

Larger quantities of the isotope 137Cs reached by above-ground nuclear weapons tests and reactor accidents at Chernobyl and Fukushima in the environment. The liberated in all above-ground nuclear weapons tests 137Cs activity at was 948 × 1015 Bq. The total amount of 137Cs, which was released by the Chernobyl disaster, had an activity of about 85 × 1015 Bq. Added to this was an activity of about 47 × 1015 Bq 134Cs and by 36 × 1015 Bq 136Cs through. Due to the fallout were many areas in Europe, including in Germany, loaded with radioactive cesium. Especially to 137Cs accumulates in fungi that can degrade lignin and thus easier access to potassium and therefore also to the chemically very similar cesium than plants. In particular, the chestnuts Boletus (Boletus badius ) and the Flockenstielige witches Boletus (Boletus erythropus ) accumulate cesium, for example, while the related Common cep (Boletus edulis ) shows only a small cesium enrichment. The reason for the high cesium concentration of the first two fungi is due to their Hutfarbstoffe Badion A and Norbadion A, can complex the cesium. In porcini these two derivatives of Pulvinsäure are not available. Are also affected wild animals that eat fungi. The exact Caesiumbelastung depends on the amount of niedergegangenem fallout and soil conditions as soil cesium different bind strongly and can thus make it available to plants.

An incident in which people died because of the radiation exposure due to 137Cs, was the Goiânia accident in 1987 in Brazil, at the abandoned consists of a radiation clinic two garbage collectors stole a metal container. The 137Cs contained therein was distributed to friends and acquaintances due to the eye-catching fluorescent color.

Use

Due to the complicated manufacturing and high reactivity elemental cesium is used only in a small extent. It has its areas mainly in research. Since cesium has a small work function, it can be used as a thermionic cathode such as for the recovery of free electrons. And magneto- hydrodynamic generators are examined with cesium as a possible plasma material. In space cesium is used in addition to mercury and xenon, due to its high molar mass, which results in a larger recoil than lighter elements as the driving means in ion drives.

The second as a unit of time is defined since 1967 as the 9,192,631,770 times the period of a particular atomic transition in cesium (see International System of Units ). Matching is the cesium frequency determining element in atomic clocks, which form the basis for Coordinated Universal Time. The choice fell on cesium, because this is a pure element and in the 1960s, the transition between the two ground states with approximately 9 GHz were still detectable by electronic means. The width of this transition and thus the uncertainty of the measurement is not determined by properties of the atom. Due to the low evaporation temperature can be generated at low speed uncertainty with little effort an atomic beam.

A cloud of cesium atoms can be held in magneto- optical traps in limbo and cooled by means of lasers to a few micro- Kelvin at absolute zero. With this technique it was possible to improve the frequency stability and the accuracy of the cesium atomic clock clearly.

In addition, cesium is used in vacuum tubes, because it reacts with low residual traces of gases and so for a better vacuum ( getter ) provides. In this case, cesium is generated in situ by the reaction of Caesiumdichromat with zirconium. Cesium - alloyed with antimony and other alkali metals - a material for photocathodes, such as those used in photomultipliers.

Proof

For the detection of cesium, the spectral lines can be used at 455 and 459 nm in the blue. Quantitatively, this can be use in flame photometry for the determination of Caesiumspuren.

In polarography cesium shows a reversible cathodic step at -2.09 V (vs. a calomel electrode ). It must as supporting electrolyte quaternary ammonium compounds ( for example, tetramethylammonium hydroxide ) are used, as other alkali metal or alkaline earth metal ions have very similar half-wave potentials.

Gravimetric can cesium as potassium through various sparingly soluble salts prove. Examples of this are the perchlorate CsClO4 and Hexachloridoplatinat Cs2 [ PtCl 6 ].

Biological Significance

Ingested cesium is absorbed due to the similarity to potassium in the gastrointestinal tract and stored analogous to potassium mainly in muscle tissue. The biological half-life, is excreted with the cesium from the human body again, is dependent on age and gender, and is on average 110 days.

Caesium is chemically toxic only to a very limited degree. Typical LD50 values ​​for cesium are at 1000 mg / kg (rat, oral). However, of importance is the effect of the ionizing radiation recorded radioactive Caesiumisotope which may cause radiation sickness, depending on the dose. Because of the good water solubility of most of these cesium salts in the digestive tract are completely absorbed and is distributed mainly in the muscle tissue. Through the absorption of radioactive 137Cs after the Chernobyl disaster in 1986 resulted in the first three months of an average effective dose of 0.6 mSv for an adult of the Federal Republic of Germany.

Safety

On air, cesium ignited spontaneously, so it must be stored in ampoules under pure argon or in vacuum. Because of its high reactivity, it reacts with water explosively. The explosive can be enhanced by the inflammation of the resulting hydrogen. Burning cesium must be cleared with metal fire extinguishers or dry sand. The management is carried out as with other alkali metals by careful dropwise addition of alcohols such as 2- pentanol, tert- butanol, or octanol, and subsequent neutralization.

Compounds

A typical alkali metal is cesium exclusively ionic compounds present in the oxidation state 1. The most cesium compounds are readily soluble in water.

Halides

Caesium forms with all the halogens readily water-soluble halides of the form CsX (X = halide). Cesium chloride has a characteristic crystal structure, which forms an important structural type ( Caesiumchloridstruktur ). To crystallize with the exception of cesium fluoride and the other cesium halides. Caesium chloride is the starting material for the extraction of elemental cesium. Since a density gradient is automatically formed at a sufficiently langdauerndem centrifugation, it is used for the separation and purification of DNA in the ultracentrifuge. High-purity cesium iodide and cesium bromide are used as transparent scintillation in scintillation counters.

Oxygen compounds

Caesium is an unusually large number of oxygen compounds. This is mainly due to the low reactivity of the cesium ion, so that the formation of oxygen -oxygen bonds is possible. Are known several suboxides as Cs11O3 Cs3O and in which an excess of cesium is present, and accordingly exhibit the electrical conductivity. In addition, the peroxide Cs2O2, the superoxide and the ozonide CSO2 CSO3 are with increasing oxygen content, the oxide Cs2O known. All of these compounds are in contrast to most other cesium compounds are colored, which suboxides purple or blue-green, the other yellow, orange or red.

Cesium hydroxide is a strongly hygroscopic, white solid which is readily soluble in water. The resulting base is one of the strongest aqueous bases at all.

More cesium compounds

Cesium carbonate is a white solid and is soluble in many organic solvents. It is used in various organic syntheses as a base, for example, esterification or for the elimination of specific protecting groups.

Cesium is widely used in military pyrotechnics, in NIR flares and Infrarottarnnebeln. While the use in NIR flares due to the intense emission lines of the element at 852, 1359 and 1469 nm, the use is based in Tarnnebeln on the light of ionization of the element. The formed in the flame during combustion of the pyrotechnic active mass Cs ions act as condensation nuclei and thus reinforce the important for the absorption of radiation aerosol yield.

Caesiumchromat can be used together with zirconium as an easy source for obtaining elemental cesium in removing water and oxygen traces in vacuum tubes.

An overview of the cesium compounds are Category: cesium.