Lanthanum is a soft, ductile, silvery-white
rare-earth metal element with symbol La and
atomic number 57. In the periodic table, it
is the first element of the lanthanide series.
It is usually found in combination with cerium
and other rare earth elements. Lanthanum oxidizes
rapidly when exposed to air. It is produced
from the minerals monazite and bastnäsite
using a complex multistage extraction process.
Lanthanum compounds have numerous applications
as catalysts, additives in glass, carbon lighting
for studio lighting and projection, ignition
elements in lighters and torches, electron
cathodes, scintillators, GTAW electrodes,
and others. Lanthanum carbonate3) has been
approved as a medicine for treating renal
failure.
Properties
Physical properties
Lanthanum has a hexagonal crystal structure
at room temperature. At 310°C, lanthanum
changes to a face-centered cubic structure.
At 865°C, it changes to a body-centered cubic
structure. Lanthanum is easily oxidized; a
centimeter-sized sample will completely oxidize
within a year. Therefore, it is used in elemental
form only for research purposes. Single lanthanum
atoms have been isolated by implanting them
into fullerene molecules; if carbon nanotubes
are filled with these lanthanum-encapsulated
fullerenes and annealed, metallic nanochains
of lanthanum are produced inside carbon nanotubes.
Chemical properties
Lanthanum exhibits two oxidation states, +3
and +2, the former being much more stable.
For example, LaH3 is more stable than LaH2.
Lanthanum burns readily at 150 °C to form
lanthanum(III) oxide:
4 La + 3 O2 → 2 La2O3
However, when exposed to moist air at room
temperature, lanthanum oxide forms a hydrated
oxide with a large volume increase. Lanthanum
is quite electropositive, reacting slowly
with cold water and quite quickly with hot
water to form lanthanum hydroxide:
2 La + 6 H2O → 2 La(OH)3 + 3 H2
Lanthanum metal reacts with all the halogens.
The reaction is vigorous if conducted above
200 °C:
2 La + 3 F2 → 2 LaF3
2 La + 3 Cl2 → 2 LaCl3
2 La + 3 Br2 → 2 LaBr3
2 La + 3 I2 → 2 LaI3
In dilute sulfuric acid, lanthanum readily
forms solutions containing the La(III) ions,
which exist as [La(OH2)9]3+ complexes:
2 La(s) + 3 H2SO4 → 2 La3+(aq) + 3 SO2−
4 + 3 H2
Lanthanum combines with nitrogen, carbon,
sulfur, phosphorus, boron, selenium, silicon
and arsenic at elevated temperatures, forming
binary compounds. The electron configuration
of the colourless La3+ ion is [Xe] 4f0.
Isotopes
Naturally occurring lanthanum is composed
of one stable and one radioactive isotope,
with 139La, being the most abundant. 38 radioisotopes
have been characterized - the most stable
is 138La with a half-life of 1.05×1011 years,
followed by 137La with a half-life of 60,000
years. Most other radioisotopes have half-lives
of less than 24 hours, and the majority of
these have half-lives less than 1 minute.
This element also has three meta states. Lanthanum
isotopes range in atomic weight from 117 u
to 155 u.
History
The word lanthanum comes from the Greek λανθανω
[lanthanō]. Lanthanum was discovered in 1839
by Swedish chemist Carl Gustav Mosander, who
partially decomposed a sample of cerium nitrate
by heating and treating the resulting salt
with dilute nitric acid. From the resulting
solution, he isolated a new rare earth he
called lantana. Lanthanum was isolated in
relatively pure form in 1923.
Lanthanum is the most strongly basic of all
the trivalent lanthanides, and it was this
property that allowed Mosander to isolate
and purify the salts of this element. Basicity
separation as operated commercially involved
the fractional precipitation of the weaker
bases from nitrate solution by the addition
of magnesium oxide or dilute ammonia gas.
Purified lanthanum remained in solution. The
alternative technique of fractional crystallization
was invented by Dmitri Mendeleev, in the form
of the double ammonium nitrate tetrahydrate,
which he used to separate the less-soluble
lanthanum from the more-soluble didymium in
the 1870s. This system was used commercially
in lanthanum purification until the development
of practical solvent extraction methods that
started in the late 1950s. As operated for
lanthanum purification, the double ammonium
nitrates were recrystallized from water. When
later adapted by Carl Auer von Welsbach for
the splitting of didymium, nitric acid was
used as a solvent to lower the solubility
of the system. Lanthanum is relatively easy
to purify, since it has only one adjacent
lanthanide, cerium, which itself is very readily
removed due to its potential tetravalency.
The fractional crystallization purification
of lanthanum as the double ammonium nitrate
was sufficiently rapid and efficient, that
lanthanum purified in this manner was not
expensive. The Lindsay Chemical Division of
American Potash and Chemical Corporation,
for a while the largest producer of rare earths
in the world, in a price list dated October
1, 1958 priced 99.9% lanthanum ammonium nitrate
at $3.15 per pound, or $1.93 per pound in
50-pound quantities. The corresponding oxide
was priced at $11.70 or $7.15 per pound for
the two quantity ranges. The price for their
purest grade of oxide was $21.60 and $13.20,
respectively.
Occurrence
Although lanthanum belongs to the element
group called rare earth metals, it is not
rare at all. Lanthanum is available in relatively
large quantities. "Rare earths" got their
name because they were indeed rare as compared
to the "common" earths such as lime or magnesia,
and historically only a few deposits were
known. Lanthanum is taken into consideration
as a rare earth metal because the process
to mine is difficult, time consuming and expensive.
MonazitePO4, and bastnäsiteCO3F, are the
principal ores in which lanthanum occurs,
in percentages of up to 25 to 38 percent of
the total lanthanide content. In general,
there is more lanthanum in bastnäsite than
in monazite. Until 1949, bastnäsite was a
rare and obscure mineral, not even remotely
contemplated as a potential commercial source
for lanthanides. In that year, the large deposit
at the Mountain Pass rare earth mine in California
was discovered. This discovery alerted geologists
to the existence of a new class of rare earth
deposit, the rare-earth bearing carbonatite,
other examples of which soon surfaced, particularly
in Africa and China.
Production
Lanthanum is most commonly obtained from monazite
and bastnäsite. The mineral mixtures are
crushed and ground. 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 HNO3. 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.
The most efficient separation routine for
lanthanum salt from the rare-earth salt solution
is, however, ion exchange. In this process,
rare-earth ions are adsorbed onto suitable
ion-exchange resin by exchange with hydrogen,
ammonium or cupric ions present in the resin.
The rare earth ions are then selectively washed
out by a suitable complexing agent, such as
ammonium citrate or nitrilotriacetate. Lanthanum
can also be separated from a solution of rare
earth nitrates by liquid-liquid extraction
with a suitable organic liquid, such as tributyl
phosphalate. Currently, the most widely used
extractant for the purification of lanthanum
and the other lanthanides is the 2-ethylhexyl
ester of 2-ethylhexylphosphonic acid; this
has better handling characteristics than the
previously used bis-2-ethylhexyl phosphate.
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:
La2O3 + 6 NH4Cl → 2 LaCl3 + 6 NH3 + 3 H2O
This is followed by reduction with alkali
or alkaline earth metals in vacuum or argon
atmosphere:
LaCl3 + 3 Li → La + 3 LiCl
Also, pure lanthanum can be produced by electrolysis
of molten mixture of anhydrous LaCl3 and NaCl
or KCl at elevated temperatures.
Applications
The first historical application of lanthanum
was in gas lantern mantles. Carl Auer von
Welsbach used a mixture of 60% magnesium oxide,
20% lanthanum oxide, and 20% yttrium oxide
which he called Actinophor, and patented in
1885. 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:
One material used for anodic material of nickel-metal
hydride batteries is La(Ni3.6Mn0.4Al0.3Co0.7).
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 AB5 type.
As most hybrid cars use nickel-metal hydride
batteries, massive quantities of lanthanum
are required for the production of hybrid
automobiles. A typical hybrid automobile battery
for a Toyota Prius requires 10 to 15 kg of
lanthanum. As engineers push the technology
to increase fuel economy, 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 LaB6 are used in high brightness, extended
life, thermionic electron emission sources
for electron microscopes, and Hall effect
thrusters.
Lanthanum fluoride 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 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
to absorb excess phosphate in cases of end-stage
renal failure.
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 is used to remove
phosphates from water in lake treatments.
Biological role
Lanthanum has no known biological role. The
element is very poorly absorbed after oral
administration and when injected its elimination
is very slow. Lanthanum carbonate was approved
as a medication named Fosrenol to absorb excess
phosphate in cases of end-stage renal failure.
While lanthanum has pharmacological effects
on several receptors and ion channels, its
specificity for the GABA receptor is unique
among divalent cations. Lanthanum acts at
the same modulatory site on the GABA receptor
as zinc- a known negative allosteric modulator.
The lanthanum cation La3+ is a positive allosteric
modulator at native and recombinant GABA receptors,
increasing open channel time and decreasing
desensitization in a subunit configuration
dependent manner.
Precautions
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,
sometimes led to pneumoconiosis.
See also
References
Books
The Industrial Chemistry of the Lanthanons,
Yttrium, Thorium and Uranium, by R. J. Callow,
Pergamon Press, 1967
Extractive Metallurgy of Rare Earths, by C.
K. Gupta and N. Krishnamurthy, CRC Press,
2005
Nouveau Traite de Chimie Minerale, Vol. VII.
Scandium, Yttrium, Elements des Terres Rares,
Actinium, P. Pascal, Editor, Masson & Cie,
1959
Chemistry of the Lanthanons, by R. C. Vickery,
Butterworths 1953
External links
WebElements.com – Lanthanum
