The most common minerals of barium arebarite (barium sulfate, BaSO4) andwitherite (barium carbonate, BaCO3). The namebarium originates from the alchemical derivative "baryta", fromGreekβαρὺς (barys), meaning 'heavy'.Baric is the adjectival form of barium. Barium was identified as a new element in 1772, but not reduced to a metal until 1808 with the advent ofelectrolysis.
Barium has few industrial applications. Historically, it was used as agetter forvacuum tubes and in oxide form as the emissive coating onindirectly heated cathodes. It is a component ofYBCO (high-temperature superconductors) and electroceramics, and is added to steel and cast iron to reduce the size of carbon grains within the microstructure. Barium compounds are added to fireworks to impart a green color.Barium sulfate is used as an insoluble additive tooil welldrilling fluid. In a purer form it is used as X-rayradiocontrast agents for imaging the human gastrointestinal tract. Water-soluble barium compounds are poisonous and have been used asrodenticides.
Barium is a soft, silvery-white metal, with a slight golden shade when ultrapure.[9]: 2 The silvery-white color of barium metal rapidly vanishes uponoxidation in air yielding a dark gray layer containing theoxide. Barium has a mediumspecific weight and high electrical conductivity. Because barium is difficult to purify, many of its properties have not been accurately determined.[9]: 2
At room temperature and pressure, barium metal adopts abody-centered cubic structure, with a barium–barium distance of 503picometers, expanding with heating at a rate of approximately 1.8×10−5/°C.[9]: 2 It is a soft metal with aMohs hardness of 1.25.[9]: 2 Its melting temperature of 1,000 K (730 °C; 1,340 °F)[10]: 4–43 is intermediate between those of the lighter strontium (1,050 K or 780 °C or 1,430 °F)[10]: 4–86 and heavier radium (973 K or 700 °C or 1,292 °F);[10]: 4–78 however, its boiling point of 2,170 K (1,900 °C; 3,450 °F) exceeds that of strontium (1,655 K or 1,382 °C or 2,519 °F).[10]: 4–86 The density (3.62 g/cm3)[10]: 4–43 is again intermediate between those of strontium (2.36 g/cm3)[10]: 4–86 and radium (≈5 g/cm3).[10]: 4–78
Barium is chemically similar to magnesium, calcium, and strontium, but more reactive. Its compounds are almost invariably found in the +2 oxidation state. As expected for a highly electropositive metal, barium's reaction withchalcogens is highlyexothermic (release energy). Barium reacts with atmospheric oxygen in air at room temperature. For this reason, metallic barium is often stored under oil or in an inert atmosphere.[9]: 2 Reactions with othernonmetals, such as carbon, nitrogen, phosphorus, silicon, and hydrogen, proceed upon heating.[9]: 2–3 Reactions with water and alcohols are also exothermic and release hydrogen gas:[9]: 3
Ba + 2 ROH → Ba(OR)2 + H2↑ (R is an alkyl group or a hydrogen atom)
Barium reacts withammonia to form the electride [Ba(NH3)6](e−)2, which near room temperature gives the amide Ba(NH2)2.[11]
Barium salts are typically white when solid and colorless when dissolved.[14] They are denser than thestrontium orcalcium analogs, except for thehalides (see table;zinc is given for comparison).
Barium hydroxide ("baryta") was known to alchemists, who produced it by heating barium carbonate. Unlike calcium hydroxide, it absorbs very little CO2 in aqueous solutions and is therefore insensitive to atmospheric fluctuations. This property is used in calibrating pH equipment.
Barium compounds burn with a green to pale greenflame, which is an efficient test to detect a barium compound. The color results fromspectral lines at 455.4, 493.4, 553.6, and 611.1 nm.[9]: 3
Organobarium compounds are a growing field of knowledge: recently discovered are dialkylbariums and alkylhalobariums.[9]: 3
Barium found in the Earth's crust is a mixture of sevenprimordial nuclides, barium-130, 132, and 134 through 138.[15] Barium-130 undergoes very slowradioactive decay toxenon-130 by doublebeta plus decay, with a half-life of (0.5–2.7)×1021 years (about 1011 times the age of the universe). Its abundance is ≈0.1% that of natural barium.[15] Theoretically, barium-132 can similarly undergo double beta decay to xenon-132; this decay has not been detected.[16] The radioactivity of these isotopes is so weak that they pose no danger to life.
Of the stable isotopes, barium-138 composes 71.7% of all barium; other isotopes have decreasing abundance with decreasingmass number.[15]
In total, barium has 40 known isotopes, ranging in mass between 114 and 153. The most stableartificial radioisotope is barium-133 with a half-life of approximately 10.51 years. Five other isotopes have half-lives longer than a day.[16] Barium also has 10meta states, of which barium-133m1 is the most stable with a half-life of about 39 hours.[16]
Alchemists in the early Middle Ages knew about some barium minerals. Smooth pebble-like stones of mineral baryte were found in volcanic rock nearBologna,Italy, and so were called "Bologna stones". Alchemists were attracted to them because after exposure to light they would glow for years.[17] The phosphorescent properties of baryte heated with organics were described by V. Casciorolus in 1602.[9]: 5
The production of pure oxygen in theBrin process was a large-scale application of barium peroxide in the 1880s, before it was replaced by electrolysis andfractional distillation of liquefied air in the early 1900s. In this process barium oxide reacts at 500–600 °C (932–1,112 °F) with air to form barium peroxide, which decomposes above 700 °C (1,292 °F) by releasing oxygen:[21][22]
The abundance of barium is 0.0425% in the Earth's crust and 13 μg/L in sea water. The primary commercial source of barium isbaryte (also called barytes or heavy spar), a barium sulfate mineral.[9]: 5 with deposits in many parts of the world. Another commercial source, far less important than baryte, iswitherite, barium carbonate. The main deposits are located in Britain, Romania, and the former USSR.[9]: 5
Barite, left to right: appearance, graph showing trends in production over time, and the map showing shares of the most important producer countries in 2010.
The baryte reserves are estimated between 0.7 and 2 billiontonnes. The highest production, 8.3 million tonnes, was achieved in 1981, but only 7–8% was used for barium metal or compounds.[9]: 5 Baryte production has risen since the second half of the 1990s from 5.6 million tonnes in 1996 to 7.6 in 2005 and 7.8 in 2011. China accounts for more than 50% of this output, followed by India (14% in 2011), Morocco (8.3%), US (8.2%), Iran and Kazakhstan (2.6% each) and Turkey (2.5%).[24]
The mined ore is washed, crushed, classified, and separated from quartz. If the quartz penetrates too deeply into the ore, or the iron, zinc, or lead content is abnormally high, thenfroth flotation is used. The product is a 98% pure baryte (by mass); the purity should be no less than 95%, with a minimal content of iron andsilicon dioxide.[9]: 7 It is then reduced by carbon tobarium sulfide:[9]: 6
BaSO4 + 2 C → BaS + 2 CO2
The water-soluble barium sulfide is the starting point for other compounds: treating BaS with oxygen produces the sulfate, with nitric acid the nitrate, with aqueous carbon dioxide the carbonate, and so on.[9]: 6 The nitrate can be thermally decomposed to yield the oxide.[9]: 6 Barium metal is produced by reduction withaluminium at 1,100 °C (2,010 °F). Theintermetallic compound BaAl4 is produced first:[9]: 3
3 BaO + 14 Al → 3 BaAl4 + Al2O3
BaAl4 is an intermediate reacted with barium oxide to produce the metal. Note that not all barium is reduced.[9]: 3
8 BaO + BaAl4 → Ba↓ + 7 BaAl2O4
The remaining barium oxide reacts with the formed aluminium oxide:[9]: 3
Barium vapor is condensed and packed into molds in an atmosphere of argon.[9]: 3 This method is used commercially, yielding ultrapure barium.[9]: 3 Commonly sold barium is about 99% pure, with main impurities being strontium and calcium (up to 0.8% and 0.25%) and other contaminants contributing less than 0.1%.[9]: 4
A similar reaction with silicon at 1,200 °C (2,190 °F) yields barium andbarium metasilicate.[9]: 3 Electrolysis is not used because barium readily dissolves in molten halides and the product is rather impure.[9]: 3
Barium exists in seawater as the Ba2+ ion with an average oceanic concentration of 109 nmol/kg.[25] Barium also exists in the ocean as BaSO4, or barite.[26] Barium has a nutrient-like profile[27] with a residence time of 10,000 years.[25]
Barium shows a relatively consistent concentration in upper ocean seawater, excepting regions of high river inputs and regions with strong upwelling.[28] There is little depletion of barium concentrations in the upper ocean for an ion with a nutrient-like profile, thus lateral mixing is important.[28] Barium isotopic values show basin-scale balances instead of local or short-term processes.[28]
Barium, as a metal or when alloyed with aluminium, is used to remove unwanted gases (gettering) from vacuum tubes, such as TV picture tubes.[9]: 4 Barium is suitable for this purpose because of its lowvapor pressure and reactivity towards oxygen, nitrogen, carbon dioxide, and water; it can even partly remove noble gases by dissolving them in the crystal lattice. This application is gradually disappearing due to the rising popularity of the tubeless LCD, LED, and plasma sets.[9]: 4
Other uses of elemental barium are minor and include an additive tosilumin (aluminium–silicon alloys) that refines their structure, as well as[9]: 4
Amoebiasis as seen in a radiograph of a barium-filled colon
Barium sulfate (the mineral baryte, BaSO4) is important to the petroleum industry as adrilling fluid inoil and gas wells.[10]: 4–5 The precipitate of the compound (called "blanc fixe", from the French for "permanent white") is used in paints and varnishes; as a filler inringing ink, plastics, and rubbers; as a paper coating pigment; and innanoparticles, to improve physical properties of some polymers, such as epoxies.[9]: 9
Barium sulfate has a low toxicity and relatively high density of ca. 4.5 g/cm3 (and thus opacity to X-rays). For this reason it is used as aradiocontrast agent inX-ray imaging of the digestive system ("barium meals" and "barium enemas").[10]: 4–5 Lithopone, apigment that contains barium sulfate andzinc sulfide, is a permanent white with good covering power that does not darken when exposed to sulfides.[29]
Other compounds of barium find only niche applications, limited by the toxicity of Ba2+ ions (barium carbonate is arat poison), which is not a problem for the insoluble BaSO4.
Barium fluoride is used for optics in infrared applications because of its wide transparency range of 0.15–12 micrometers.[33]
YBCO was the firsthigh-temperature superconductor cooled by liquid nitrogen, with a transition temperature of 93 K (−180.2 °C; −292.3 °F) greater than the boiling point of nitrogen (77 K or −196.2 °C or −321.1 °F).[34]
The lateral mixing of barium is caused by water mass mixing and ocean circulation.[35] Global ocean circulation reveals a strong correlation between dissolved barium and silicic acid.[35] The large-scale ocean circulation combined with remineralization of barium show a similar correlation between dissolved barium and ocean alkalinity.[35]
Dissolved barium's correlation with silicic acid can be seen both vertically and spatially.[36] Particulate barium shows a strong correlation withparticulate organic carbon or POC.[36] Barium is becoming more popular as a base for palaeoceanographic proxies.[36] With both dissolved and particulate barium's links with silicic acid and POC, it can be used to determine historical variations in the biological pump, carbon cycle, and global climate.[36]
The barium particulate barite (BaSO4), as one of many proxies, can be used to provide a host of historical information on processes in different oceanic settings (water column, sediments, and hydrothermal sites).[26] In each setting there are differences in isotopic and elemental composition of the barite particulate.[26] Barite in the water column, known as marine or pelagic barite, reveals information on seawater chemistry variation over time.[26] Barite in sediments, known as diagenetic or cold seeps barite, gives information about sedimentary redox processes.[26] Barite formed via hydrothermal activity at hydrothermal vents, known as hydrothermal barite, reveals alterations in the condition of the earth's crust around those vents.[26]
Soluble barium compounds haveLD50 near 10 mg/kg (oral rats). Symptoms include "convulsions... paralysis of the peripheral nerve system ... severe inflammation of the gastrointestinal tract".[9]: 18 The insoluble sulfate is nontoxic and is not classified as a dangerous goods in transport regulations.[9]: 9
Little is known about the long term effects of barium exposure.[38] The USEPA considers it unlikely that barium is carcinogenic when consumed orally. Inhaled dust containing insoluble barium compounds can accumulate in the lungs, causing abenign condition calledbaritosis.[39]
^abcArblaster, John W. (2018).Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International.ISBN978-1-62708-155-9.
^Xu, Wei; Lerner, Michael M. (2018). "A New and Facile Route Using Electride Solutions to Intercalate Alkaline Earth Ions into Graphite".Chemistry of Materials.30 (19):6930–6935.doi:10.1021/acs.chemmater.8b03421.S2CID105295721.
^Ferro, Riccardo & Saccone, Adriana (2008).Intermetallic Chemistry. Elsevier. p. 355.ISBN978-0-08-044099-6.
^Slowinski, Emil J.; Masterton, William L. (1990).Qualitative analysis and the properties of ions in aqueous solution (2nd ed.). Saunders. p. 87.ISBN978-0-03-031234-2.
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