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= Strontium =
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Introduction
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Strontium is a chemical element; it has symbol Sr and atomic number
38. An alkaline earth metal, it is a soft silver-white yellowish
metallic element that is highly chemically reactive. The metal forms a
dark oxide layer when it is exposed to air. Strontium has physical and
chemical properties similar to those of its two vertical neighbors in
the periodic table, calcium and barium. It occurs naturally mainly in
the minerals celestine and strontianite, and is mostly mined from
these.
Both strontium and strontianite are named after Strontian, a village
in Scotland near which the mineral was discovered in 1790 by Adair
Crawford and William Cruickshank; it was identified as a new element
the next year from its crimson-red flame test color. Strontium was
first isolated as a metal in 1808 by Humphry Davy using the then newly
discovered process of electrolysis. During the 19th century, strontium
was mostly used in the production of sugar from sugar beets (see
strontian process). At the peak of production of television
cathode-ray tubes, as much as 75% of strontium consumption in the
United States was used for the faceplate glass. With the replacement
of cathode-ray tubes with other display methods, consumption of
strontium has dramatically declined.
While natural strontium (which is mostly the isotope strontium-88) is
stable, the synthetic strontium-90 is radioactive and is one of the
most dangerous components of nuclear fallout, as strontium is absorbed
by the body in a similar manner to calcium. Natural stable strontium,
on the other hand, is not hazardous to health.
Characteristics
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Strontium is a divalent silvery metal with a pale yellow tint whose
properties are mostly intermediate between and similar to those of its
group neighbors calcium and barium. It is softer than calcium and
harder than barium. Its melting (777 °C) and boiling (1377 °C) points
are lower than those of calcium (842 °C and 1484 °C respectively);
barium continues this downward trend in the melting point (727 °C),
but not in the boiling point (1900 °C). The density of strontium (2.64
g/cm3) is similarly intermediate between those of calcium (1.54 g/cm3)
and barium (3.594 g/cm3). Three allotropes of metallic strontium
exist, with transition points at 235 and 540 °C.
The standard electrode potential for the Sr2+/Sr couple is −2.89 V,
approximately midway between those of the Ca2+/Ca (−2.84 V) and
Ba2+/Ba (−2.92 V) couples, and close to those of the neighboring
alkali metals. Strontium is intermediate between calcium and barium in
its reactivity toward water, with which it reacts on contact to
produce strontium hydroxide and hydrogen gas. Strontium metal burns in
air to produce both strontium oxide and strontium nitride, but since
it does not react with nitrogen below 380 °C, at room temperature it
forms only the oxide spontaneously. Besides the simple oxide SrO, the
peroxide can be made by direct oxidation of strontium metal under a
high pressure of oxygen, and there is some evidence for a yellow
superoxide . Strontium hydroxide, , is a strong base, though it is not
as strong as the hydroxides of barium or the alkali metals. All four
dihalides of strontium are known.
Due to the large size of the heavy s-block elements, including
strontium, a vast range of coordination numbers is known, from 2, 3,
or 4 all the way to 22 or 24 in and . The Sr2+ ion is quite large, so
that high coordination numbers are the rule. The large size of
strontium and barium plays a significant part in stabilising strontium
complexes with polydentate macrocyclic ligands such as crown ethers:
for example, while 18-crown-6 forms relatively weak complexes with
calcium and the alkali metals, its strontium and barium complexes are
much stronger.
Organostrontium compounds contain one or more strontium-carbon bonds.
They have been reported as intermediates in Barbier-type reactions.
Although strontium is in the same group as magnesium, and
organomagnesium compounds are very commonly used throughout chemistry,
organostrontium compounds are not similarly widespread because they
are more difficult to make and more reactive. Organostrontium
compounds tend to be more similar to organoeuropium or organosamarium
compounds due to the similar ionic radii of these elements (Sr2+ 118
pm; Eu2+ 117 pm; Sm2+ 122 pm). Most of these compounds can only be
prepared at low temperatures; bulky ligands tend to favor stability.
For example, strontium dicyclopentadienyl, , must be made by directly
reacting strontium metal with mercurocene or cyclopentadiene itself;
replacing the ligand with the bulkier ligand on the other hand
increases the compound's solubility, volatility, and kinetic
stability.
Because of its extreme reactivity with oxygen and water, strontium
occurs naturally only in compounds with other elements, such as in the
minerals strontianite and celestine. It is kept under a liquid
hydrocarbon such as mineral oil or kerosene to prevent oxidation;
freshly exposed strontium metal rapidly turns a yellowish color with
the formation of the oxide. Finely powdered strontium metal is
pyrophoric, meaning that it will ignite spontaneously in air at room
temperature. Volatile strontium salts impart a bright red color to
flames, and these salts are used in pyrotechnics and in the production
of flares. Like calcium and barium, as well as the alkali metals and
the divalent lanthanides europium and ytterbium, strontium metal
dissolves directly in liquid ammonia to give a dark blue solution of
solvated electrons.
Isotopes
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Natural strontium is a mixture of four stable isotopes: 84Sr, 86Sr,
87Sr, and 88Sr. On these isotopes, 88Sr is the most abundant, makes up
about 82.6% of all natural strontium, though the abundance varies due
to the production of radiogenic 87Sr as the daughter of long-lived
beta-decaying 87Rb. This is the basis of rubidium-strontium dating. Of
the unstable isotopes, the primary decay mode of the isotopes lighter
than 85Sr is electron capture or positron emission to isotopes of
rubidium, and that of the isotopes heavier than 88Sr is electron
emission to isotopes of yttrium. Of special note are 89Sr and 90Sr.
The former has a half-life of 50.6 days and is used to treat bone
cancer due to strontium's chemical similarity and hence ability to
replace calcium. While 90Sr (half-life 28.90 years) has been used
similarly, it is also an isotope of concern in fallout from nuclear
weapons and nuclear accidents due to its production as a fission
product. Its presence in bones can cause bone cancer, cancer of nearby
tissues, and leukemia. The 1986 Chernobyl nuclear accident
contaminated about 30,000 km2 with greater than 10 kBq/m2 with 90Sr,
which accounts for about 5% of the 90Sr which was in the reactor core.
History
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Strontium is named after the Scottish village of Strontian (), where
it was discovered in the ores of the lead mines.
In 1790, Adair Crawford, a physician engaged in the preparation of
barium, and his colleague William Cruickshank, recognised that the
Strontian ores exhibited properties that differed from those in other
"heavy spars" sources. This allowed Crawford to conclude on page 355
"... it is probable indeed, that the scotch mineral is a new species
of earth which has not hitherto been sufficiently examined." The
physician and mineral collector Friedrich Gabriel Sulzer analysed
together with Johann Friedrich Blumenbach the mineral from Strontian
and named it strontianite. He also came to the conclusion that it was
distinct from the witherite and contained a new earth (neue
Grunderde). In 1793 Thomas Charles Hope, a professor of chemistry at
the University of Glasgow studied the mineral and proposed the name
'strontites'. He confirmed the earlier work of Crawford and recounted:
"... Considering it a peculiar earth I thought it necessary to give it
an name. I have called it Strontites, from the place it was found; a
mode of derivation in my opinion, fully as proper as any quality it
may possess, which is the present fashion." The element was eventually
isolated by Sir Humphry Davy in 1808 by the electrolysis of a mixture
containing strontium chloride and mercuric oxide, and announced by him
in a lecture to the Royal Society on 30 June 1808. In keeping with the
naming of the other alkaline earths, he changed the name to
'strontium'.
The first large-scale application of strontium was in the production
of sugar from sugar beet. Although a crystallisation process using
strontium hydroxide was patented by Augustin-Pierre Dubrunfaut in 1849
the large scale introduction came with the improvement of the process
in the early 1870s. The German sugar industry used the process well
into the 20th century. Before World War I the beet sugar industry used
100,000 to 150,000 tons of strontium hydroxide for this process per
year. The strontium hydroxide was recycled in the process, but the
demand to substitute losses during production was high enough to
create a significant demand initiating mining of strontianite in the
Münsterland. The mining of strontianite in Germany ended when mining
of the celestine deposits in Gloucestershire started. These mines
supplied most of the world strontium supply from 1884 to 1941.
Although the celestine deposits in the Granada basin were known for
some time the large scale mining did not start before the 1950s.
During atmospheric nuclear weapons testing, it was observed that
strontium-90 is one of the nuclear fission products with a relatively
high yield. The similarity to calcium and the chance that the
strontium-90 might become enriched in bones made research on the
metabolism of strontium an important topic.
Occurrence
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The mineral celestine ()
Strontium commonly occurs in nature, being the 15th most abundant
element on Earth (its heavier congener barium being the 14th),
estimated to average approximately 360 parts per million in the
Earth's crust and is found chiefly as the sulfate mineral celestine ()
and the carbonate strontianite (). Of the two, celestine occurs much
more frequently in deposits of sufficient size for mining. Because
strontium is used most often in the carbonate form, strontianite would
be the more useful of the two common minerals, but few deposits have
been discovered that are suitable for development. Because of the way
it reacts with air and water, strontium only exists in nature when
combined to form minerals. Naturally occurring strontium is stable,
but its synthetic isotope Sr-90 is only produced by nuclear fallout.
In groundwater strontium behaves chemically much like calcium. At
intermediate to acidic pH Sr2+ is the dominant strontium species. In
the presence of calcium ions, strontium commonly forms coprecipitates
with calcium minerals such as calcite and anhydrite at an increased
pH. At intermediate to acidic pH, dissolved strontium is bound to soil
particles by cation exchange.
The mean strontium content of ocean water is 8 mg/L. At a
concentration between 82 and 90 μmol/L of strontium, the concentration
is considerably lower than the calcium concentration, which is
normally between 9.6 and 11.6 mmol/L. It is nevertheless much higher
than that of barium, 13 μg/L.
Production
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The major producers of strontium as celestine as of January 2024 are
Spain (200,000 t), Iran (200,000 t), China (80,000 t), Mexico (35,000
t); and Argentina (700 t). Although strontium deposits occur widely in
the United States, they have not been mined since 1959.
A large proportion of mined celestine is converted to the carbonate by
two processes. Either the celestine is directly leached with sodium
carbonate solution or the celestine is roasted with coal to form the
sulfide. The second stage produces a dark-coloured material containing
mostly strontium sulfide. This so-called "black ash" is dissolved in
water and filtered. Strontium carbonate is precipitated from the
strontium sulfide solution by introduction of carbon dioxide. The
sulfate is reduced to the sulfide by the carbothermic reduction:
:
About 300,000 tons are processed in this way annually.
The metal is produced commercially by reducing strontium oxide with
aluminium. The strontium is distilled from the mixture. Strontium
metal can also be prepared on a small scale by electrolysis of a
solution of strontium chloride in molten potassium chloride:
:
:
Applications
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Consuming 75% of production, the primary use for strontium was in
glass for colour television cathode-ray tubes, where it prevented
X-ray emission. This application for strontium has been declining
because CRTs are being replaced by other display methods. This decline
has a significant influence on the mining and refining of strontium.
All parts of the CRT must absorb X-rays. In the neck and the funnel of
the tube, lead glass is used for this purpose, but this type of glass
shows a browning effect due to the interaction of the X-rays with the
glass. Therefore, the front panel is made from a different glass
mixture with strontium and barium to absorb the X-rays. The average
values for the glass mixture determined for a recycling study in 2005
is 8.5% strontium oxide and 10% barium oxide.
Because strontium is so similar to calcium, it is incorporated in the
bone. All four stable isotopes are incorporated, in roughly the same
proportions they are found in nature. However, the actual distribution
of the isotopes tends to vary greatly from one geographical location
to another. Thus, analyzing the bone of an individual can help
determine the region it came from. This approach helps to identify the
ancient migration patterns and the origin of commingled human remains
in battlefield burial sites.
87Sr/86Sr ratios are commonly used to determine the likely provenance
areas of sediment in natural systems, especially in marine and fluvial
environments. Dasch (1969) showed that surface sediments of Atlantic
displayed 87Sr/86Sr ratios that could be regarded as bulk averages of
the 87Sr/86Sr ratios of geological terrains from adjacent landmasses.
A good example of a fluvial-marine system to which Sr isotope
provenance studies have been successfully employed is the River
Nile-Mediterranean system. Due to the differing ages of the rocks that
constitute the majority of the Blue and White Nile, catchment areas of
the changing provenance of sediment reaching the River Nile Delta and
East Mediterranean Sea can be discerned through strontium isotopic
studies. Such changes are climatically controlled in the Late
Quaternary.
More recently, 87Sr/86Sr ratios have also been used to determine the
source of ancient archaeological materials such as timbers and corn in
Chaco Canyon, New Mexico. 87Sr/86Sr ratios in teeth may also be used
to track animal migrations.
Strontium aluminate is frequently used in glow in the dark toys, as it
is chemically and biologically inert.
Strontium carbonate and other strontium salts are added to fireworks
to give a deep red colour. This same effect identifies strontium
cations in the flame test. Fireworks consume about 5% of the world's
production. Strontium carbonate is used in the manufacturing of hard
ferrite magnets.
Strontium chloride is sometimes used in toothpastes for sensitive
teeth. One popular brand includes 10% total strontium chloride
hexahydrate by weight. Small amounts are used in the refining of zinc
to remove small amounts of lead impurities. The metal itself has a
limited use as a getter, to remove unwanted gases in vacuums by
reacting with them, although barium may also be used for this purpose.
The ultra-narrow optical transition between the [Kr]5s2 1S0 electronic
ground state and the metastable [Kr]5s5p 3P0 excited state of 87Sr is
one of the leading candidates for the future re-definition of the
second in terms of an optical transition as opposed to the current
definition derived from a microwave transition between different
hyperfine ground states of 133Cs. Current optical atomic clocks
operating on this transition already surpass the precision and
accuracy of the current definition of the second.
Radioactive strontium
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89Sr is the active ingredient in Metastron, a radiopharmaceutical used
for bone pain secondary to metastatic bone cancer. The strontium is
processed like calcium by the body, preferentially incorporating it
into bone at sites of increased osteogenesis. This localization
focuses the radiation exposure on the cancerous lesion.
90Sr has been used as a power source for radioisotope thermoelectric
generators (RTGs). 90Sr produces approximately 0.93 watts of heat per
gram (it is lower for the form of 90Sr used in RTGs, which is
strontium fluoride). However, 90Sr has one third the lifetime and a
lower density than 238Pu, another RTG fuel. The main advantage of 90Sr
is that it is significantly cheaper than 238Pu and is found in nuclear
waste. The latter must be prepared by irradiating 237Np with neutrons
then separating the modest amounts of 238Pu. The principal
disadvantage of 90Sr is the high energy beta particles produce
Bremsstrahlung as they encounter nuclei of other nearby heavy atoms
such as adjacent strontium. This is mostly in the range of X-rays.
Thus strong beta emitters also emit significant secondary X-rays in
most cases. This requires significant shielding measures which
complicates the design of RTGs using 90Sr. The Soviet Union deployed
nearly 1000 of these RTGs on its northern coast as a power source for
lighthouses and meteorology stations.
Biological role
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Acantharea, a relatively large group of marine radiolarian protozoa,
produce intricate mineral skeletons composed of strontium sulfate. In
biological systems, calcium is substituted to a small extent by
strontium. In the human body, most of the absorbed strontium is
deposited in the bones. The ratio of strontium to calcium in human
bones is between 1:1000 and 1:2000, roughly in the same range as in
the blood serum.
Effect on the human body
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The human body absorbs strontium as if it were its lighter congener
calcium. Because the elements are chemically very similar, stable
strontium isotopes do not pose a significant health threat. The
average human has an intake of about two milligrams of strontium a
day. In adults, strontium consumed tends to attach only to the surface
of bones, but in children, strontium can replace calcium in the
mineral of the growing bones and thus lead to bone growth problems.
The biological half-life of strontium in humans has variously been
reported as from 14 to 600 days, 1,000 days, 18 years, 30 years and,
at an upper limit, 49 years. The wide-ranging published biological
half-life figures are explained by strontium's complex metabolism
within the body. However, by averaging all excretion paths, the
overall biological half-life is estimated to be about 18 years. The
elimination rate of strontium is strongly affected by age and sex, due
to differences in bone metabolism.
The drug strontium ranelate aids bone growth, increases bone density,
and lessens the incidence of vertebral, peripheral, and hip fractures.
However, strontium ranelate also increases the risk of venous
thromboembolism, pulmonary embolism, and serious cardiovascular
disorders, including myocardial infarction. Its use is therefore now
restricted. Its beneficial effects are also questionable, since the
increased bone density is partially caused by the increased density of
strontium over the calcium which it replaces. Strontium also
bioaccumulates in the body. Despite restrictions on strontium
ranelate, strontium is still contained in some supplements. There is
not much scientific evidence on risks of strontium chloride when taken
by mouth. Those with a personal or family history of blood clotting
disorders are advised to avoid strontium.
Strontium has been shown to inhibit sensory irritation when applied
topically to the skin. Topically applied, strontium has been shown to
accelerate the recovery rate of the epidermal permeability barrier
(skin barrier).
Nuclear waste
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Strontium-90 is a radioactive fission product produced by nuclear
reactors used in nuclear power. It is a major component of high-level
radioactivity of nuclear waste and spent nuclear fuel. Its 29-year
half-life is short enough that its decay heat has been used to power
arctic lighthouses, but long enough that it can take hundreds of years
to decay to safe levels. Exposure from contaminated water and food may
increase the risk of leukemia, bone cancer and primary
hyperparathyroidism.
Remediation
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Algae has shown selectivity for strontium in studies, where most
plants used in bioremediation have not shown selectivity between
calcium and strontium, often becoming saturated with calcium, which is
greater in quantity and also present in nuclear waste.
Researchers have looked at the bioaccumulation of strontium by
'Scenedesmus spinosus' (algae) in simulated wastewater. The study
claims a highly selective biosorption capacity for strontium of 'S.
spinosus', suggesting that it may be appropriate for use in treating
nuclear wastewater.
A study of the pond alga 'Closterium moniliferum' using
non-radioactive strontium found that varying the ratio of barium to
strontium in water improved strontium selectivity.
External links
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* [
http://www.webelements.com/strontium/ WebElements.com - Strontium]
* [
http://www.periodicvideos.com/videos/038.htm Strontium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
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Original Article:
http://en.wikipedia.org/wiki/Strontium
