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= Barium =
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Introduction
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Barium is a chemical element; it has symbol Ba and atomic number 56.
It is the fifth element in group 2 and is a soft, silvery alkaline
earth metal. Because of its high chemical reactivity, barium is never
found in nature as a free element.
The most common minerals of barium are barite (barium sulfate, BaSO4)
and witherite (barium carbonate, BaCO3). The name 'barium' originates
from the alchemical derivative "baryta", from Greek (), 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 of electrolysis.
Barium has few industrial applications. Historically, it was used as a
getter for vacuum tubes and in oxide form as the emissive coating on
indirectly heated cathodes. It is a component of YBCO
(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 to oil
well drilling fluid. In a purer form it is used as X-ray radiocontrast
agents for imaging the human gastrointestinal tract. Water-soluble
barium compounds are poisonous and have been used as rodenticides.
Physical properties
=====================
Barium is a soft, silvery-white metal, with a slight golden shade when
ultrapure. The silvery-white color of barium metal rapidly vanishes
upon oxidation in air yielding a dark gray layer containing the oxide.
Barium has a medium specific weight and high electrical conductivity.
Because barium is difficult to purify, many of its properties have not
been accurately determined.
At room temperature and pressure, barium metal adopts a body-centered
cubic structure, with a barium-barium distance of 503 picometers,
expanding with heating at a rate of approximately 1.8/°C. It is a soft
metal with a Mohs hardness of 1.25. Its melting temperature of 1000 K
is intermediate between those of the lighter strontium (1050 K) and
heavier radium (973 K); however, its boiling point of 2170 K exceeds
that of strontium (1655 K). The density (3.62 g/cm3) is again
intermediate between those of strontium (2.36 g/cm3) and radium (≈5
g/cm3).
Chemical reactivity
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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 with chalcogens is highly exothermic (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. Reactions with other nonmetals, such as
carbon, nitrogen, phosphorus, silicon, and hydrogen, proceed upon
heating. Reactions with water and alcohols are also exothermic and
release hydrogen gas:
:↑ (R is an alkyl group or a hydrogen atom)
Barium reacts with ammonia to form the electride , which near room
temperature gives the amide .
The metal is readily attacked by acids. Sulfuric acid is a notable
exception because passivation stops the reaction by forming the
insoluble barium sulfate on the surface. Barium combines with several
other metals, including aluminium, zinc, lead, and tin, forming
intermetallic phases and alloys.
Compounds
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|Selected alkaline earth and zinc salts densities, g/cm3
!
|3.34 |2.59 |3.18 |2.15 |2.96 |2.83 |2.9 |1.7
|5.1 |3.7 |4.24 |3.05 |3.96 |3.5 |4.78 |3.26
**'**'
'5.72' '4.3' '4.89' '3.89' '4.49' '4.29' '4.96' '4.16'
|5.6 |4.09 |4.95 |2.09 |3.54 |4.4 |1.57 |--
Barium salts are typically white when solid and colorless when
dissolved. They are denser than the strontium or calcium analogs,
except for the halides (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 green flame, which is an
efficient test to detect a barium compound. The color results from
spectral lines at 455.4, 493.4, 553.6, and 611.1 nm.
Organobarium compounds are a growing field of knowledge: recently
discovered are dialkylbariums and alkylhalobariums.
Isotopes
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Barium found in the Earth's crust is a mixture of seven primordial
nuclides, barium-130, 132, and 134 through 138. Barium-130 undergoes
very slow radioactive decay to xenon-130 by double beta 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.
Theoretically, barium-132 can similarly undergo double beta decay to
xenon-132; this decay has not been detected. 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 decreasing mass number.
In total, barium has 40 known isotopes, ranging in mass between 114
and 153. The most stable artificial radioisotope is barium-133 with a
half-life of approximately 10.51 years. Five other isotopes have
half-lives longer than a day. Barium also has 10 meta states, of which
barium-133m1 is the most stable with a half-life of about 39 hours.
History
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Alchemists in the early Middle Ages knew about some barium minerals.
Smooth pebble-like stones of mineral baryte were found in volcanic
rock near Bologna, Italy, and so were called "Bologna stones".
Alchemists were attracted to them because after exposure to light they
would glow for years. The phosphorescent properties of baryte heated
with organics were described by V. Casciorolus in 1602.
Carl Scheele determined that baryte contained a new element in 1772,
but could not isolate barium, only barium oxide. Johan Gottlieb Gahn
also isolated barium oxide two years later in similar studies.
Oxidized barium was at first called "barote" by Guyton de Morveau, a
name that was changed by Antoine Lavoisier to 'baryte' (in French) or
'baryta' (in Latin). Also in the 18th century, English mineralogist
William Withering noted a heavy mineral in the lead mines of
Cumberland, now known to be witherite. Barium was first isolated by
electrolysis of molten barium salts in 1808 by Sir Humphry Davy in
England. Davy, by analogy with calcium, named "barium" after baryta,
with the "-ium" ending signifying a metallic element. Robert Bunsen
and Augustus Matthiessen obtained pure barium by electrolysis of a
molten mixture of barium chloride and ammonium chloride.
The production of pure oxygen in the Brin process was a large-scale
application of barium peroxide in the 1880s, before it was replaced by
electrolysis and fractional distillation of liquefied air in the early
1900s. In this process barium oxide reacts at 500 - with air to form
barium peroxide, which decomposes above 700 C by releasing oxygen:
:
Barium sulfate was first applied as a radiocontrast agent in X-ray
imaging of the digestive system in 1908.
Occurrence and production
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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 is baryte (also
called barytes or heavy spar), a barium sulfate mineral. with deposits
in many parts of the world. Another commercial source, far less
important than baryte, is witherite, barium carbonate. The main
deposits are located in Britain, Romania, and the former USSR.
The baryte reserves are estimated between 0.7 and 2 billion tonnes.
The highest production, 8.3 million tonnes, was achieved in 1981, but
only 7-8% was used for barium metal or compounds. 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%).
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, then froth 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 and silicon dioxide.
It is then reduced by carbon to barium sulfide:
:
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. The nitrate can be thermally decomposed to yield the oxide. Barium
metal is produced by reduction with aluminium at 1100 C. The
intermetallic compound BaAl4 is produced first:
:
is an intermediate reacted with barium oxide to produce the metal.
Note that not all barium is reduced.
:
The remaining barium oxide reacts with the formed aluminium oxide:
:
and the overall reaction is
:
Barium vapor is condensed and packed into molds in an atmosphere of
argon. This method is used commercially, yielding ultrapure barium.
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%.
A similar reaction with silicon at 1200 C yields barium and barium
metasilicate. Electrolysis is not used because barium readily
dissolves in molten halides and the product is rather impure.
Gemstone
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The barium mineral, benitoite (barium titanium silicate), occurs as a
very rare blue fluorescent gemstone, and is the official state gem of
California.
Barium in seawater
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Barium exists in seawater as the Ba2+ ion with an average oceanic
concentration of 109 nmol/kg. Barium also exists in the ocean as
BaSO4, or barite. Barium has a nutrient-like profile with a residence
time of 10,000 years.
Barium shows a relatively consistent concentration in upper ocean
seawater, excepting regions of high river inputs and regions with
strong upwelling. There is little depletion of barium concentrations
in the upper ocean for an ion with a nutrient-like profile, thus
lateral mixing is important. Barium isotopic values show basin-scale
balances instead of local or short-term processes.
Metal and alloys
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Barium, as a metal or when alloyed with aluminium, is used to remove
unwanted gases (gettering) from vacuum tubes, such as TV picture
tubes. Barium is suitable for this purpose because of its low vapor
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.
Other uses of elemental barium are minor and include an additive to
silumin (aluminium-silicon alloys) that refines their structure, as
well as
* bearing alloys;
* lead-tin soldering alloys - to increase the creep resistance;
* alloy with nickel for spark plugs;
* additive to steel and cast iron as an inoculant;
* alloys with calcium, manganese, silicon, and aluminium as high-grade
steel deoxidizers.
Barium sulfate and baryte
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Barium sulfate (the mineral baryte, BaSO4) is important to the
petroleum industry as a drilling fluid in oil and gas wells. The
precipitate of the compound (called "blanc fixe", from the French for
"permanent white") is used in paints and varnishes; as a filler in
ringing ink, plastics, and rubbers; as a paper coating pigment; and in
nanoparticles, to improve physical properties of some polymers, such
as epoxies.
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
a radiocontrast agent in X-ray imaging of the digestive system
("barium meals" and "barium enemas"). Lithopone, a pigment that
contains barium sulfate and zinc sulfide, is a permanent white with
good covering power that does not darken when exposed to sulfides.
Other barium compounds
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Other compounds of barium find only niche applications, limited by the
toxicity of ions (see ), which is not a problem for the insoluble .
* Barium oxide coating on the electrodes of fluorescent lamps
facilitates the release of electrons.
* By its great atomic density, barium carbonate increases the
refractive index and luster of glass and reduces leaks of X-rays from
CRT screens.
* Barium, typically as barium nitrate imparts a yellow or "apple"
green color to fireworks when no chlorine donors are present.; emerald
greens are generated using chlorine donors (barium chlorate acting as
the donor and oxidizer in many formulas) to produce barium chloride
in-situ.
* Barium peroxide is a catalyst in the aluminothermic reaction
(thermite) for welding rail tracks. It is also a green flare in tracer
ammunition and a bleaching agent.
* Barium titanate is a promising electroceramic.
* Barium fluoride is used for optics in infrared applications because
of its wide transparency range of 0.15-12 micrometers.
* YBCO was the first high-temperature superconductor cooled by liquid
nitrogen, with a transition temperature of 93 K greater than the
boiling point of nitrogen (77 K).
* Ferrite, a type of sintered ceramic composed of iron oxide () and
barium oxide (BaO), is both electrically nonconductive and
ferrimagnetic, and can be temporarily or permanently magnetized.
Palaeoceanography
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The lateral mixing of barium is caused by water mass mixing and ocean
circulation. Global ocean circulation reveals a strong correlation
between dissolved barium and silicic acid. The large-scale ocean
circulation combined with remineralization of barium show a similar
correlation between dissolved barium and ocean alkalinity.
Dissolved barium's correlation with silicic acid can be seen both
vertically and spatially. Particulate barium shows a strong
correlation with particulate organic carbon or POC. Barium is becoming
more popular as a base for palaeoceanographic proxies. 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.
The barium particulate barite (), 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). In
each setting there are differences in isotopic and elemental
composition of the barite particulate. Barite in the water column,
known as marine or pelagic barite, reveals information on seawater
chemistry variation over time. Barite in sediments, known as
diagenetic or cold seeps barite, gives information about sedimentary
redox processes. 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.
Toxicity
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Soluble barium compounds have LD50 near 10 mg/kg (oral rats).
Symptoms include "convulsions... paralysis of the peripheral nerve
system ... severe inflammation of the gastrointestinal tract". The
insoluble sulfate is nontoxic and is not classified as a dangerous
goods in transport regulations.
Little is known about the long term effects of barium exposure. The US
EPA considers it unlikely that barium is carcinogenic when consumed
orally. Inhaled dust containing insoluble barium compounds can
accumulate in the lungs, causing a benign condition called baritosis.
Barium carbonate has been used as a rodenticide.
Though considered obsolete, it may still be in use in some countries.
See also
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* Han purple and Han blue - synthetic barium copper silicate pigments
developed and used in ancient and imperial China
External links
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* [
http://www.periodicvideos.com/videos/056.htm Barium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://elements.vanderkrogt.net/element.php?sym=Ba Elementymology
& Elements Multidict]
*
[
https://web.archive.org/web/20120227061852/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA338490
3-D Holographic Display Using Strontium Barium Niobate]
License
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Original Article:
http://en.wikipedia.org/wiki/Barium
