|Name , symbol , atomic number||Strontium, Sr, 38|
|Element category||Alkaline earth metals|
|Group , period , block||2 , 5 , p|
|Appearance||silvery white metallic|
|Mass fraction of the earth's envelope||0.014%|
|Atomic mass||87.62 (1) u|
|Atomic radius (calculated)||200 (219) pm|
|Covalent radius||195 pm|
|Van der Waals radius||249 pm|
|Electron configuration||[ Kr ] 5 s 2|
|1. Ionization energy||5.694 867 40 (13) eV ≈ 549.47 kJ / mol|
|2. Ionization energy||11.030 276 4 (25) eV ≈ 1 064.26 kJ / mol|
|3. Ionization energy||42.88353 (19) eV ≈ 4 137.63 kJ / mol|
|4. Ionization energy||56.280 (3) eV ≈ 5 430.2 kJ / mol|
|5. Ionization energy||70.7 (6) eV ≈ 6 822 kJ / mol|
|Crystal structure||Cubic area-centered|
|density||2.63 g / cm 3 (20 ° C )|
|magnetism||paramagnetic ( Χ m = 3.5 10 −5 )|
|Melting point||1050 K (777 ° C)|
|boiling point||1653 K (1380 ° C)|
|Molar volume||33.94 10 −6 m 3 mol −1|
|Heat of evaporation||141 kJ / mol|
|Heat of fusion||8 kJ mol −1|
|Work function||2.59 eV|
|Electric conductivity||7.41 · 10 6 A · V −1 · m −1|
|Thermal conductivity||35 W m −1 K −1|
|Normal potential||−2.89 V (Sr 2+ + 2 e - → Sr)|
|Electronegativity||0.95 ( Pauling scale )|
|For other isotopes see list of isotopes|
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .
Strontium is a chemical element with the element symbol Sr and the atomic number 38. In the periodic table it is in the 5th period as well as the 2nd main group , or the 2nd IUPAC group and thus belongs to the alkaline earth metals . It is a soft ( Mohs hardness : 1.5) and very reactive metal.
The element was discovered by Adair Crawford in 1790 and named Strontian in Scotland after its first location . Elementary, but still contaminated by foreign admixtures, it was able to be represented in 1808 by means of electrolysis by Humphry Davy . Robert Bunsen also succeeded in depicting pure strontium in 1855. The element is only used in small quantities, especially for cathode ray tubes , pyrotechnics (red flame color), permanent magnets and in aluminum smelting .
The first indications of the existence of the element found Adair Crawford and William Cruickshank in 1790, when she was a from Strontian in Scotland derived mineral , the first for "air-containing barite" ( barium carbonate , Witherit was held), examined more closely. They produced the chloride and compared several properties of the later strontium chloride with those of barium chloride. Among other things, they determined different solubilities in water and other crystal forms. 1791 named Friedrich Gabriel Sulzer (1749-1830), the mineral after its place Strontian Strontianit . He and Johann Friedrich Blumenbach examined the mineral more closely and found further differences to witherite, such as the different toxicity and flame color . In the following years, chemists such as Martin Heinrich Klaproth , Richard Kirwan , Thomas Charles Hope or Johann Tobias Lowitz investigated strontianite further and obtained other strontium compounds from it.
In 1808, Humphry Davy succeeded in producing strontium amalgam by means of electrolytic reduction in the presence of red mercury oxide , which he then purified by distillation and thus obtained the - albeit still contaminated - metal. He named it after the strontianite analogous to the other alkaline earth metals strontium . Robert Bunsen obtained pure strontium in 1855 by electrolysis of a strontium chloride melt. He also determined properties of the metal such as the specific gravity of the element.
With a proportion of 370 ppm in the continental crust of the earth, strontium is relatively abundant on earth; the abundance of elements in the earth's crust is comparable to that of barium , sulfur or carbon . A large amount of strontium is also present in seawater . The element does not appear solid , but always in various combinations. Due to the low solubility, the most important strontium minerals are strontium sulfate or celestine with a strontium content of up to 47.7%. as well as strontium carbonate or strontianite with a strontium content of up to 59.4%. A total of around 200 minerals containing strontium are known to date (as of 2011).
The deposits of the most important strontium mineral, Celestine, were created by precipitating the poorly soluble strontium sulfate from seawater. A hydrothermal formation of the mineral is also possible. Strontianite is also formed hydrothermally or as a secondary mineral from Celestine. The most important strontium deposits and mining locations are in Spain , Mexico , Turkey , China and Iran . Great Britain was also an important producer for a long time, but production ended in 1992. The production of strontium minerals in 2008 was 496,000 tons worldwide.
Extraction and presentation
The starting material for the production of strontium and strontium compounds is usually Celestine (strontium sulfate). As a rule, strontium carbonate is first obtained from this. This is the industrially most important strontium compound and basic material for the extraction of the metal and other compounds.
To produce strontium carbonate, strontium sulphate is first reacted with carbon at 1100–1200 ° C. The sulfate is reduced to sulfide and strontium sulfide and carbon dioxide are formed . The strontium sulfide is purified by extraction with hot water.
Then either carbon dioxide is passed through the strontium sulfide solution or the strontium sulfide is reacted with sodium carbonate . In addition to strontium carbonate, hydrogen sulfide and sodium sulfide are formed . Which of the two variants is used depends on the availability of the raw materials and the possibility of selling the by-products.
Finely ground strontium sulfate can also be reacted directly with sodium or ammonium carbonate to form strontium carbonate. However, complex cleaning steps are necessary here.
To obtain strontium metal , strontium oxide is reduced with aluminum ( aluminothermie ). In addition to elemental strontium, a mixture of aluminum and strontium oxide is formed. The reaction takes place in a vacuum , since under these conditions the strontium is in gaseous form, can be easily separated off and collected in a cooler.
Strontium is an extremely pure, bright gold-yellow, shiny, otherwise silvery-white alkaline earth metal . With a melting point of 777 ° C and a boiling point of 1380 ° C, the boiling point is between the lighter calcium and the heavier barium , with calcium having a higher melting point and barium a lower melting point. After magnesium and radium, strontium has the lowest boiling point of all alkaline earth metals. With a density of 2.6 g / cm 3, it is one of the light metals . Strontium is very soft with a Mohs hardness of 1.5 and can be easily bent or rolled.
Like calcium, strontium crystallizes at room temperature in a face-centered cubic crystal structure in the space group Fm 3 m (space group no. 225) ( copper type) with the lattice parameter a = 608.5 pm and four formula units per unit cell . In addition, two other high-temperature modifications are known. At temperatures above 215 ° C, the structure changes into a hexagonal close packing of spheres ( magnesium type) with the lattice parameters a = 432 pm and c = 706 pm. Finally, above 605 ° C, a body-centered cubic structure ( tungsten type) is most stable.
After barium and radium, strontium is the most reactive alkaline earth metal. It reacts directly with halogens , oxygen , nitrogen and sulfur . It always forms compounds in which it is present as a divalent cation. When heated in air, the metal burns with the typical carmine-red flame color to form strontium oxide and strontium nitride .
As a very base metal, strontium reacts with water to form hydrogen and hydroxide. Strontium hydroxide is already formed when the metal comes into contact with moist air. Strontium is also soluble in ammonia , and bluish-black ammonia is formed .
In the groundwater , strontium mostly behaves in a similar way to calcium . Strontium compounds are insoluble under weakly acidic to basic conditions. Strontium only appears in dissolved form at lower pH values . If the decomposition of carbon dioxide (CO 2 ) occurs as a result of weathering processes or the like , the precipitation of strontium together with calcium (as strontium or calcium carbonate ) is increased. In addition, a high cation exchange capacity of the soil can promote the binding of strontium.
A total of 34 isotopes and a further nine core isomers are known. Of these, four, 84 Sr, 86 Sr, 87 Sr, and 88 Sr, occur naturally. In the natural isotopic composition, the isotope 88 Sr predominates with a share of 82.58%. 86 Sr with 9.86% and 87 Sr with 7.0%, and 84 Sr with a share of 0.56% are less common.
90 Sr is a beta emitter with a decay energy of 0.546 MeV and decays to 90 Y with a half-life of 28.78 years , which in turn rapidly ( t 1/2 = 64.1 h ) emitting high-energy beta radiation ( ZE = 2.282 MeV) and decays from gamma radiation to the stable 90 Zr. It mostly occurs as a secondary cleavage product . It is formed within a few minutes through multiple beta decays from primary fission products of mass number 90, which occur in 5.7% of all nuclear fission of 235 U in nuclear power plants and atomic bomb explosions . This makes 90 Sr one of the most common fission products.
Larger amounts of 90 Sr get into the environment in all nuclear disasters. Accidents in which 90 Sr was released into the environment were the Windscale fire , in which 0.07 TBq 90 Sr was released, and the Chernobyl disaster , in which the activity at 90 Sr was 800 TBq. After the aboveground nuclear weapons tests, especially in the years 1955–58 and 1961–63, the pollution of the atmosphere with 90 Sr increased sharply. This, along with the charge at 137 Cs in 1963, led to the adoption of the Treaty on the Prohibition of Nuclear Weapons Tests in the Atmosphere, Space and Underwater , which banned such tests in the signatory states. As a result, the pollution of the atmosphere fell significantly again in the following years. The total activity of 90 Sr released by nuclear weapons was approximately 6 · 10 17 Bq (600 PBq).
The intake of 90 Sr, which can get into the body through contaminated milk, is dangerous. The high-energy beta radiation of the isotope can change cells in bones or bone marrow and thus trigger bone tumors or leukemia . A decorporation of the captured in the bone strontium with chelating agents is impossible, since this preferred calcium complex and the strontium in the bone remains. A decoration with barium sulfate is only possible if it is done quickly after the incorporation, before it can be incorporated into the bone. The degradation through biological processes is also very slow, the biological half-life in bones is 49 years, the effective half-life of 90 Sr is 18.1 years. Possibly 90 Sr binds to cells of the parathyroid glands . This would reduce the accumulation of cases of hyperparathyroidism in liquidators of the reactor in Chernobyl explain.
The beta radiation of 90 Sr and 90 Y in radionuclide , such as for remote beacons and beacon in the former Soviet Union , the long-lived isotope labeling , for measuring the thickness of materials or to calibrate from Geiger counters are used.
87 Sr is the decay product of the rubidium isotope 87 Rb , which has a half-life of 48 billion years . The age of rubidium- and strontium-containing rocks such as granite can therefore be determined from the ratio of the different strontium isotopes in the context of a strontium isotope analysis .
Strontium is stored in different amounts in bones and teeth under different conditions . At the same time, the isotope ratio of 86 Sr and 87 Sr depends on the rocks in the area. Therefore, one can sometimes draw conclusions about the migration of prehistoric humans from the isotope ratios of strontium.
According to the operator, the small German pebble bed reactor called AVR next to the site of the research center in Jülich is considered to be the nuclear plant most heavily contaminated with 90 Sr worldwide. There is also strontium in the floor under the reactor. This should be laboriously removed when the reactor is dismantled by 2025 .
Metallic strontium is mainly used in the aluminum industry (primary and secondary aluminum smelters and foundries) as is sodium as a structure- influencing agent in aluminum-silicon alloys with 7-12% silicon . Small admixtures of strontium change the eutectic in silicon-aluminum alloys and thus improve the mechanical properties of the alloy. This is due to the fact that in aluminum-silicon alloys without strontium, coarse, needle-shaped, mechanically less resilient grains precipitate on the eutectic, which is prevented by the strontium. Its “refining” effect lasts longer in castable melts (casting and holding furnaces) than that of sodium, since it is less easily oxidizable. In the field of slowly solidifying melts ( sand casting ), strontium has in part already replaced sodium, which has been used alone for decades. In the case of rapid solidification in permanent metallic form, especially with die casting , the use of strontium is not mandatory in every case, the formation of the desired fine, "refined" structure is already promoted by the rapid solidification.
Strontium is added to ferrosilicon, it regulates the structure of the carbon and prevents uneven solidification during casting.
Strontium can also be used as a getter material in electron tubes, to remove sulfur and phosphorus from steel and to harden battery plates made of lead.
Few living things use strontium in biological processes. These include Acantharia , single-celled eukaryotic organisms that belong to the group of radiant animals and are a frequent component of the zooplankton in the sea. These are the only protists to use strontium sulfate as a building material for the skeleton. As a result, they also cause changes in the strontium content in individual layers of the sea by initially absorbing strontium and then sinking into deeper layers after they die, where they dissolve.
Physiological and therapeutic importance
Strontium is very similar in its properties to calcium . However, in contrast to calcium, it is only absorbed in small quantities via the intestine . The reason for this is possibly the larger ionic radius of the element. The average strontium content of a man weighing 70 kilograms is only 0.32 g, compared to about 1000 g calcium in the body. The ingested strontium, like calcium, is mainly stored in the bones , which is a treatment option for osteoporosis . A correspondingly high bioavailability is achieved through salt formation with organic acids such as ranelic acid or malonic acid .
Like other alkaline earth metals, strontium is flammable. It reacts with water or carbon dioxide , so that these cannot be used as an extinguishing agent . Metal fire extinguishers (class D) should be used for extinguishing , and dry sand, salt and extinguishing powder can also be used. Hydrogen , which is explosive, is also formed on contact with water . To remove small amounts, strontium can be reacted with isopropanol , tert- butanol or octanol .
Like all alkaline earth metals, strontium occurs in stable compounds only in the +2 oxidation state . As a rule, they are colorless, often readily water-soluble salts.
With the halogens fluorine , chlorine , bromine and iodine , strontium forms a halide with the general formula SrX 2 . They are typical, colorless salts which, apart from strontium fluoride, are readily soluble in water. They can be prepared by reacting strontium carbonate with hydrohalic acids such as hydrofluoric acid or hydrochloric acid . Among other things, strontium chloride is used as an intermediate product for the production of other strontium compounds and in toothpaste , where it is supposed to act against pain-sensitive teeth.
Salts of oxo acids
The strontium salts of oxygen acids such as strontium carbonate , strontium nitrate , strontium sulfate or strontium chromate are of particular industrial importance . Strontium carbonate is the most important commercial form of strontium compounds, the majority of the extracted coelestin is converted to strontium carbonate. It is used primarily for the production of X-ray absorbing glass for cathode ray tubes , but also for the production of strontium ferrite for permanent magnets or electroceramics . Strontium nitrate is mainly used in pyrotechnics for the typical strontium red coloration of the flames, the yellow strontium chromate serves as a primer against corrosion of aluminum in aircraft or shipbuilding .
Other strontium compounds
Strontium (I) compounds
Strontium (I) compounds were found to be unstable intermediates in hot flames. Strontium (I) hydroxide, SrOH, like strontium (I) chloride, SrCl, is a strong emitter in the red spectral range and acts as the sole color source in bright and deeply saturated red pyrotechnic flares.
Organic strontium compounds
Organic strontium compounds are only known and investigated to a limited extent because they are very reactive and can also react with many solvents such as ethers. In contrast, they are insoluble in non-polar solvents. Among other things, a metallocene with pentamethylcyclopentadienyl anions (Cp *) was shown, which in contrast to other metallocenes such as ferrocene is angled in the gas phase .
An overview of strontium compounds is given in the category: strontium compounds .
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