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= Lanthanum =
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Introduction
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Lanthanum is a chemical element; it has symbol La and atomic number
57. It is a soft, ductile, silvery-white metal that tarnishes slowly
when exposed to air. It is the eponym of the lanthanide series, a
group of 15 similar elements between lanthanum and lutetium in the
periodic table, of which lanthanum is the first and the prototype.
Lanthanum is traditionally counted among the rare earth elements. Like
most other rare earth elements, its usual oxidation state is +3,
although some compounds are known with an oxidation state of +2.
Lanthanum has no biological role in humans but is used by some
bacteria. It is not particularly toxic to humans but does show some
antimicrobial activity.
Lanthanum usually occurs together with cerium and the other rare earth
elements. Lanthanum was first found by the Swedish chemist Carl Gustaf
Mosander in 1839 as an impurity in cerium nitrate - hence the name
'lanthanum', from the ancient Greek (), meaning 'to lie hidden'.
Although it is classified as a rare earth element, lanthanum is the
28th most abundant element in the Earth's crust, almost three times as
abundant as lead. In minerals such as monazite and bastnäsite,
lanthanum composes about a quarter of the lanthanide content. It is
extracted from those minerals by a process of such complexity that
pure lanthanum metal was not isolated until 1923.
Lanthanum compounds have numerous applications including catalysts,
additives in glass, carbon arc lamps for studio lights and projectors,
ignition elements in lighters and torches, electron cathodes,
scintillators, and gas tungsten arc welding electrodes. Lanthanum
carbonate is used as a phosphate binder to treat high levels of
phosphate in the blood accompanied by kidney failure.
Physical
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Lanthanum is the first element and prototype of the lanthanide series.
In the periodic table, it appears to the right of the alkaline earth
metal barium and to the left of the lanthanide cerium. Lanthanum is
generally considered the first of the f-block elements by authors
writing on the subject. The 57 electrons of a lanthanum atom are
arranged in the configuration [Xe]5d(1)6s(2), with three valence
electrons outside the noble gas core. In chemical reactions, lanthanum
almost always gives up these three valence electrons from the 5d and
6s subshells to form the +3 oxidation state, achieving the stable
configuration of the preceding noble gas xenon. Some lanthanum(II)
compounds are also known, but they are usually much less stable.
Lanthanum monoxide (LaO) produces strong absorption bands in some
stellar spectra.
Among the lanthanides, lanthanum is exceptional as it has no 4f
electrons as a single gas-phase atom. Thus it is only very weakly
paramagnetic, unlike the strongly paramagnetic later lanthanides (with
the exceptions of the last two, ytterbium and lutetium, where the 4f
shell is completely full). However, the 4f shell of lanthanum can
become partially occupied in chemical environments and participate in
chemical bonding. For example, the melting points of the trivalent
lanthanides (all but europium and ytterbium) are related to the extent
of hybridisation of the 6s, 5d, and 4f electrons (lowering with
increasing 4f involvement), and lanthanum has the second-lowest
melting point among them: 920 °C. (Europium and ytterbium have lower
melting points because they delocalise about two electrons per atom
rather than three.) This chemical availability of f orbitals justifies
lanthanum's placement in the f-block despite its anomalous
ground-state configuration (which is merely the result of strong
interelectronic repulsion making it less profitable to occupy the 4f
shell, as it is small and close to the core electrons).
The lanthanides become harder as the series is traversed: as expected,
lanthanum is a soft metal. Lanthanum has a relatively high resistivity
of 615 nΩm at room temperature; in comparison, the value for the good
conductor aluminium is only 26.50 nΩm. Lanthanum is the least volatile
of the lanthanides. Like most of the lanthanides, lanthanum has a
hexagonal crystal structure at room temperature (-La). At 310 °C,
lanthanum changes to a face-centered cubic structure (-La), and at 865
°C, it changes to a body-centered cubic structure (-La).
Chemical
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As expected from periodic trends, lanthanum has the largest atomic
radius of the lanthanides. Hence, it is the most reactive among them,
tarnishing quite rapidly in air, turning completely dark after several
hours and can readily burn to form lanthanum(III) oxide, , which is
almost as basic as calcium oxide. A centimeter-sized sample of
lanthanum will corrode completely in a year as its oxide spalls off
like iron rust, instead of forming a protective oxide coating like
aluminium, scandium, yttrium, and lutetium. Lanthanum reacts with the
halogens at room temperature to form the trihalides, and upon warming
will form binary compounds with the nonmetals nitrogen, carbon,
sulfur, phosphorus, boron, selenium, silicon and arsenic. Lanthanum
reacts slowly with water to form lanthanum(III) hydroxide, . In dilute
sulfuric acid, lanthanum readily forms the aquated tripositive ion :
This is colorless in aqueous solution since has no d or f electrons.
Lanthanum is the strongest and hardest base among the rare earth
elements, which is again expected from its being the largest of them.
Some lanthanum(II) compounds are also known, but they are much less
stable. Therefore, in officially naming compounds of lanthanum its
oxidation number always is to be mentioned.
Isotopes
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Naturally occurring lanthanum is made up of two isotopes, the stable
and the primordial long-lived radioisotope . is by far the most
abundant, making up 99.910% of natural lanthanum: it is produced in
the s-process (slow neutron capture, which occurs in low- to
medium-mass stars) and the r-process (rapid neutron capture, which
occurs in core-collapse supernovae). It is the only stable isotope of
lanthanum. The very rare isotope is one of the few primordial odd-odd
nuclei, with a long half-life of It is one of the proton-rich
p-nuclei which cannot be produced in the s- or r-processes. , along
with the even rarer tantalum-180m, is produced in the ν-process, where
neutrinos interact with stable nuclei. All other lanthanum isotopes
are synthetic: With the exception of with a half-life of about 60,000
years, all of them have half-lives less than two days, and most have
half-lives less than a minute. The isotopes and occur as fission
products of uranium.
Compounds
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Lanthanum oxide is a white solid that can be prepared by direct
reaction of its constituent elements. Due to the large size of the
ion, adopts a hexagonal 7-coordinate structure that changes to the
6-coordinate structure of scandium oxide () and yttrium oxide () at
high temperature. When it reacts with water, lanthanum hydroxide is
formed: a lot of heat is evolved in the reaction and a hissing sound
is heard. Lanthanum hydroxide will react with atmospheric carbon
dioxide to form the basic carbonate.
Lanthanum fluoride is insoluble in water and can be used as a
qualitative test for the presence of . The heavier halides are all
very soluble deliquescent compounds. The anhydrous halides are
produced by direct reaction of their elements, as heating the hydrates
causes hydrolysis: for example, heating hydrated produces .
Lanthanum reacts exothermically with hydrogen to produce the dihydride
, a black, pyrophoric, brittle, conducting compound with the calcium
fluoride structure. This is a non-stoichiometric compound, and further
absorption of hydrogen is possible, with a concomitant loss of
electrical conductivity, until the more salt-like is reached. Like
and , is probably an electride compound.
Due to the large ionic radius and great electropositivity of , there
is not much covalent contribution to its bonding and hence it has a
limited coordination chemistry, like yttrium and the other
lanthanides. Lanthanum oxalate does not dissolve very much in
alkali-metal oxalate solutions, and decomposes around 500 °C. Oxygen
is the most common donor atom in lanthanum complexes, which are mostly
ionic and often have high coordination numbers over is the most
characteristic, forming square antiprismatic and dodecadeltahedral
structures. These high-coordinate species, reaching up to coordination
number 12 with the use of chelating ligands such as in , often have a
low degree of symmetry because of stereo-chemical factors.
Lanthanum chemistry tends not to involve due to the electron
configuration of the element: thus its organometallic chemistry is
quite limited. The best characterized organolanthanum compounds are
the cyclopentadienyl complex , which is produced by reacting anhydrous
with in tetrahydrofuran, and its methyl-substituted derivatives.
History
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In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a
heavy mineral from the mine at Bastnäs, later named cerite. Thirty
years later, the fifteen-year-old Wilhelm Hisinger, from the family
owning the mine, sent a sample of it to Carl Scheele, who did not find
any new elements within. In 1803, after Hisinger had become an
ironmaster, he returned to the mineral with Jöns Jacob Berzelius and
isolated a new oxide which they named 'ceria' after the dwarf planet
Ceres, which had been discovered two years earlier. Ceria was
simultaneously independently isolated in Germany by Martin Heinrich
Klaproth. Between 1839 and 1843, ceria was shown to be a mixture of
oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who
lived in the same house as Berzelius and studied under him: he
separated out two other oxides which he named 'lanthana' and
'didymia'. He partially decomposed a sample of cerium nitrate by
roasting it in air and then treating the resulting oxide with dilute
nitric acid. That same year, Axel Erdmann, a student also at the
Karolinska Institute, discovered lanthanum in a new mineral from Låven
island located in a Norwegian fjord.
Finally, Mosander explained his delay, saying that he had extracted a
second element from cerium, and this he called didymium. Although he
did not realise it, didymium too was a mixture, and in 1885 it was
separated into praseodymium and neodymium.
Since lanthanum's properties differed only slightly from those of
cerium, and occurred along with it in its salts, he named it from the
Ancient Greek [] (lit. 'to lie hidden'). Relatively pure lanthanum
metal was first isolated in 1923.
Occurrence and production
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Lanthanum makes up 39 mg/kg of the Earth's crust, behind neodymium at
41.5 mg/kg and cerium at 66.5 mg/kg. Despite being among the so-called
"rare earth metals", lanthanum is thus not rare at all, but it is
historically so-named because it is rarer than "common earths" such as
lime and magnesia, and at the time it was recognized only a few
deposits were known. Lanthanum is also ruefully considered a 'rare
earth' metal because the process to mine it is difficult,
time-consuming, and expensive. Lanthanum is rarely the dominant
lanthanide found in the rare earth minerals, and in their chemical
formulas it is usually preceded by cerium. Rare examples of
La-dominant minerals are monazite-(La) and lanthanite-(La).
The ion is similarly sized to the early lanthanides of the cerium
group (those up to samarium and europium) that immediately follow in
the periodic table, and hence it tends to occur along with them in
phosphate, silicate and carbonate minerals, such as monazite
({{chem2|M^{III}PO4}}) and bastnäsite ({{chem2|M^{III}CO3F}}), where M
refers to all the rare earth metals except scandium and the
radioactive promethium (mostly Ce, La, and Y). Bastnäsite is usually
lacking in thorium and the heavy lanthanides, and the purification of
the light lanthanides from it is less involved. The ore, after being
crushed and ground, is first treated with hot concentrated sulfuric
acid, evolving carbon dioxide, hydrogen fluoride, and silicon
tetrafluoride: the product is then dried and leached with water,
leaving the early lanthanide ions, including lanthanum, in solution.
The procedure for monazite, which usually contains all the rare earths
as well as thorium, is more involved. Monazite, because of its
magnetic properties, can be separated by repeated electromagnetic
separation. After separation, it is treated with hot concentrated
sulfuric acid to produce water-soluble sulfates of rare earths. The
acidic filtrates are partially neutralized with sodium hydroxide to pH
3-4. Thorium precipitates out of solution as hydroxide and is removed.
After that, the solution is treated with ammonium oxalate to convert
rare earths to their insoluble oxalates. The oxalates are converted to
oxides by annealing. The oxides are dissolved in nitric acid that
excludes one of the main components, cerium, whose oxide is insoluble
in . Lanthanum is separated as a double salt with ammonium nitrate by
crystallization. This salt is relatively less soluble than other rare
earth double salts and therefore stays in the residue. Care must be
taken when handling some of the residues as they contain radium-228,
the daughter of , which is a strong gamma emitter. Lanthanum is
relatively easy to extract as it has only one neighbouring lanthanide,
cerium, which can be removed by making use of its ability to be
oxidised to the +4 state; thereafter, lanthanum may be separated out
by the historical method of fractional crystallization of , or by
ion-exchange techniques when higher purity is desired.
Lanthanum metal is obtained from its oxide by heating it with ammonium
chloride or fluoride and hydrofluoric acid at 300-400 °C to produce
the chloride or fluoride:
: + + +
This is followed by reduction with alkali or alkaline earth metals in
vacuum or argon atmosphere:
: + +
Also, pure lanthanum can be produced by electrolysis of molten mixture
of anhydrous and or at elevated temperatures.
Applications
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The first historical application of lanthanum was in gas lantern
mantles. Carl Auer von Welsbach used a mixture of lanthanum oxide and
zirconium oxide, which he called 'Actinophor' and patented in 1886.
The original mantles gave a green-tinted light and were not very
successful, and his first company, which established a factory in
Atzgersdorf in 1887, failed in 1889.
Modern uses of lanthanum include:
hot cathode
* One material used for anodic material of nickel-metal hydride
batteries is . Due to high cost to extract the other lanthanides, a
mischmetal with more than 50% of lanthanum is used instead of pure
lanthanum. The compound is an intermetallic component of the type.
NiMH batteries can be found in many models of the Toyota Prius sold in
the US. These larger nickel-metal hydride batteries require massive
quantities of lanthanum for the production. The 2008 Toyota Prius NiMH
battery requires 10 to of lanthanum. As engineers push the technology
to increase fuel efficiency, twice that amount of lanthanum could be
required per vehicle.
* Hydrogen sponge alloys can contain lanthanum. These alloys are
capable of storing up to 400 times their own volume of hydrogen gas in
a reversible adsorption process. Heat energy is released every time
they do so; therefore these alloys have possibilities in energy
conservation systems.
* Mischmetal, a pyrophoric alloy used in lighter flints, contains 25%
to 45% lanthanum.
* Lanthanum oxide and the boride are used in electronic vacuum tubes
as hot cathode materials with strong emissivity of electrons. Crystals
of are used in high-brightness, extended-life, thermionic electron
emission sources for electron microscopes and Hall-effect thrusters.
* Lanthanum trifluoride () is an essential component of a heavy
fluoride glass named ZBLAN. This glass has superior transmittance in
the infrared range and is therefore used for fiber-optical
communication systems.
* Cerium-doped lanthanum bromide and lanthanum chloride are the recent
inorganic scintillators, which have a combination of high light yield,
best energy resolution, and fast response. Their high yield converts
into superior energy resolution; moreover, the light output is very
stable and quite high over a very wide range of temperatures, making
it particularly attractive for high-temperature applications. These
scintillators are already widely used commercially in detectors of
neutrons or gamma rays.
* Carbon arc lamps use a mixture of rare earth elements to improve the
light quality. This application, especially by the motion picture
industry for studio lighting and projection, consumed about 25% of the
rare-earth compounds produced until the phase out of carbon arc lamps.
* Lanthanum(III) oxide () improves the alkali resistance of glass and
is used in making special optical glasses, such as infrared-absorbing
glass, as well as camera and telescope lenses, because of the high
refractive index and low dispersion of rare-earth glasses. Lanthanum
oxide is also used as a grain-growth additive during the liquid-phase
sintering of silicon nitride and zirconium diboride.
* Small amounts of lanthanum added to steel improves its malleability,
resistance to impact, and ductility, whereas addition of lanthanum to
molybdenum decreases its hardness and sensitivity to temperature
variations.
* Small amounts of lanthanum are present in many pool products to
remove the phosphates that feed algae.
* Lanthanum oxide additive to tungsten is used in gas tungsten arc
welding electrodes, as a substitute for radioactive thorium.
* Various compounds of lanthanum and other rare-earth elements
(oxides, chlorides, triflates, etc.) are components of various
catalysis, such as petroleum cracking catalysts.
* Lanthanum-barium radiometric dating is used to estimate age of rocks
and ores, though the technique has limited popularity.
* Lanthanum carbonate was approved as a medication (Fosrenol, Shire
Pharmaceuticals) to absorb excess phosphate in cases of
hyperphosphatemia seen in end-stage kidney disease.
* Lanthanum fluoride is used in phosphor lamp coatings. Mixed with
europium fluoride, it is also applied in the crystal membrane of
fluoride ion-selective electrodes.
* Like horseradish peroxidase, lanthanum is used as an electron-dense
tracer in molecular biology.
* Lanthanum-modified bentonite (or phoslock) is used to remove
phosphates from water in lake treatments.
* Lanthanum telluride () is considered to be applied in the field of
radioisotope power system (nuclear power plant) due to its significant
conversion capabilities. The transmuted elements and isotopes in the
segment will not react with the material itself, thus presenting no
harm to the safety of the power plant. Though iodine, which can be
generated during transmutation, is suspected to react with segment,
the quantity of iodine is small enough to pose no threat to the power
system.
Biological role
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Lanthanum has no known biological role in humans. The element is very
poorly absorbed after oral administration and when injected its
elimination is very slow. Lanthanum carbonate (Fosrenol) was approved
as a phosphate binder to absorb excess phosphate in cases of end stage
renal disease.
While lanthanum has pharmacological effects on several receptors and
ion channels, its specificity for the GABA receptor is unique among
trivalent cations. Lanthanum acts at the same modulatory site on the
GABA receptor as zinc, a known negative allosteric modulator. The
lanthanum cation is a positive allosteric modulator at native and
recombinant GABA receptors, increasing open channel time and
decreasing desensitization in a subunit configuration dependent
manner.
Lanthanum is a cofactor for the methanol dehydrogenase of the
methanotrophic bacterium 'Methylacidiphilum fumariolicum' SolV,
although the great chemical similarity of the lanthanides means that
it may be substituted with cerium, praseodymium, or neodymium without
ill effects, and with the smaller samarium, europium, or gadolinium
giving no side effects other than slower growth.
Precautions
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Lanthanum has a low to moderate level of toxicity and should be
handled with care. The injection of lanthanum solutions produces
hyperglycemia, low blood pressure, degeneration of the spleen and
hepatic alterations. The application in carbon arc light led to the
exposure of people to rare earth element oxides and fluorides, which
sometimes led to pneumoconiosis. As the ion is similar in size to the
ion, it is sometimes used as an easily traced substitute for the
latter in medical studies. Lanthanum, like the other lanthanides, is
known to affect human metabolism, lowering cholesterol levels, blood
pressure, appetite, and risk of blood coagulation. When injected into
the brain, it acts as a painkiller, similarly to morphine and other
opiates, though the mechanism behind this is still unknown. Lanthanum
meant for ingestion, typically as a chewable tablet or oral powder,
can interfere with gastrointestinal (GI) imaging by creating opacities
throughout the GI tract; if chewable tablets are swallowed whole, they
will dissolve but present initially as coin-shaped opacities in the
stomach, potentially confused with ingested metal objects such as
coins or batteries.
Prices
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The price for a (metric) ton [1000 kg] of 'Lanthanum oxide 99% (FOB
China in USD/Mt)' is given by the Institute of Rare Earths Elements
and Strategic Metals (IREESM) as below $2,000 for most of the period
from early 2001 to September 2010 (at $10,000 in the short term in
2008); it rose steeply to $140,000 in mid-2011 and fell back just as
rapidly to $38,000 by early 2012. The average price for the last six
months (April-September 2022) is given by the IREESM as follows:
'Lanthanum Oxide - 99.9%min FOB China - 1308 EUR/mt' and for
'Lanthanum Metal - 99%min FOB China - 3706 EUR/mt'.
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Lanthanum
