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= Thallium =
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Introduction
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Thallium is a chemical element; it has symbol Tl and atomic number 81.
It is a silvery-white post-transition metal that is not found free in
nature. When isolated, thallium resembles tin, but discolors when
exposed to air. Chemists William Crookes and Claude-Auguste Lamy
discovered thallium independently, in 1861, in residues of sulfuric
acid production. Both used the newly developed method of flame
spectroscopy, in which thallium produces a notable green spectral
line. Thallium, from Greek , , meaning "green shoot" or "twig", was
named by Crookes. It was isolated by both Lamy and Crookes in 1862,
Lamy by electrolysis and Crookes by precipitation and melting of the
resultant powder. Crookes exhibited it as a powder precipitated by
zinc at the International Exhibition, which opened on 1 May that year.
Thallium tends to form the +3 and +1 oxidation states. The +3 state
resembles that of the other elements in group 13 (boron, aluminium,
gallium, indium). However, the +1 state, which is far more prominent
in thallium than the elements above it, recalls the chemistry of
alkali metals and thallium(I) ions are found geologically mostly in
potassium-based ores and (when ingested) are handled in many ways like
potassium ions (K+) by ion pumps in living cells.
Commercially, thallium is produced not from potassium ores, but as a
byproduct from refining of heavy-metal sulfide ores. Approximately 65%
of thallium production is used in the electronics industry and the
remainder is used in the pharmaceutical industry and in glass
manufacturing. It is also used in infrared detectors. The radioisotope
thallium-201 (as the soluble chloride TlCl) is used in small amounts
as an agent in a nuclear medicine scan, during one type of nuclear
cardiac stress test.
Soluble thallium salts (many of which are nearly tasteless) are highly
toxic and they were historically used in rat poisons and insecticides.
Because of their nonselective toxicity, use of these compounds has
been restricted or banned in many countries. Thallium poisoning
usually results in hair loss. Because of its historic popularity as a
murder weapon, thallium has gained notoriety as "the poisoner's
poison" and "inheritance powder" (alongside arsenic).
Characteristics
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A thallium atom has 81 electrons, arranged in the electron
configuration [Xe]4f145d106s26p1; of these, the three outermost
electrons in the sixth shell are valence electrons. Due to the inert
pair effect, the 6s electron pair is relativistically stabilised and
it is more difficult to get these involved in chemical bonding than it
is for the heavier elements. Thus, very few electrons are available
for metallic bonding, similar to the neighboring elements mercury and
lead. Thallium, then, like its congeners, is a soft, highly
electrically conducting metal with a low melting point, of 304 °C.
A number of standard electrode potentials, depending on the reaction
under study, are reported for thallium, reflecting the greatly
decreased stability of the +3 oxidation state:
+0.73 Tl3+ + 3 e− ↔ Tl
−0.336 Tl+ + e− ↔ Tl
Thallium is the first element in group 13 where the reduction of the
+3 oxidation state to the +1 oxidation state is spontaneous under
standard conditions. Since bond energies decrease down the group, with
thallium, the energy released in forming two additional bonds and
attaining the +3 state is not always enough to outweigh the energy
needed to involve the 6s-electrons. Accordingly, thallium(I) oxide and
hydroxide are more basic and thallium(III) oxide and hydroxide are
more acidic, showing that thallium conforms to the general rule of
elements being more electropositive in their lower oxidation states.
Thallium is malleable and sectile enough to be cut with a knife at
room temperature. It has a metallic luster that, when exposed to air,
quickly tarnishes to a bluish-gray tinge, resembling lead. It may be
preserved by immersion in oil. A heavy layer of oxide builds up on
thallium if left in air. In the presence of water, thallium hydroxide
is formed. Sulfuric and nitric acids dissolve thallium rapidly to make
the sulfate and nitrate salts, while hydrochloric acid forms an
insoluble thallium(I) chloride layer.
Isotopes
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Thallium has 41 isotopes which have atomic masses that range from 176
to 216. 203Tl and 205Tl are the only stable isotopes and make up
nearly all of natural thallium. The five short-lived isotopes 206Tl
through 210Tl inclusive occur in nature, as they are part of the
natural decay chains of heavier elements. 204Tl is the most stable
radioisotope, with a half-life of 3.78 years. It is made by the
neutron activation of stable thallium in a nuclear reactor. The most
useful radioisotope, 201Tl (half-life 73 hours), decays by electron
capture, emitting X-rays (~70-80 keV), and photons of 135 and 167 keV
in 10% total abundance; therefore, it has good imaging characteristics
without an excessive patient-radiation dose. It is the most popular
isotope used for thallium nuclear cardiac stress tests.
Thallium(III)
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Thallium(III) compounds resemble the corresponding aluminium(III)
compounds. They are moderately strong oxidizing agents and are usually
unstable, as illustrated by the positive reduction potential for the
Tl3+/Tl couple. Some mixed-valence compounds are also known, such as
Tl4O3 and TlCl2, which contain both thallium(I) and thallium(III).
Thallium(III) oxide, Tl2O3, is a black solid which decomposes above
800 °C, forming the thallium(I) oxide and oxygen.
The simplest possible thallium compound, thallane (TlH3), is too
unstable to exist in bulk, both due to the instability of the +3
oxidation state as well as poor overlap of the valence 6s and 6p
orbitals of thallium with the 1s orbital of hydrogen. The trihalides
are more stable, although they are chemically distinct from those of
the lighter group 13 elements and are still the least stable in the
whole group. For instance, thallium(III) fluoride, TlF3, has the
β-BiF3 structure rather than that of the lighter group 13
trifluorides, and does not form the complex anion in aqueous
solution. The trichloride and tribromide disproportionate just above
room temperature to give the monohalides, and thallium triiodide
contains the linear triiodide anion () and is actually a thallium(I)
compound. Thallium(III) sesquichalcogenides do not exist.
Thallium(I)
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The thallium(I) halides are stable. In keeping with the large size of
the Tl+ cation, the chloride and bromide have the caesium chloride
structure, while the fluoride and iodide have distorted sodium
chloride structures. Like the analogous silver compounds, TlCl, TlBr,
and TlI are photosensitive and display poor solubility in water. The
stability of thallium(I) compounds demonstrates its differences from
the rest of the group: a stable oxide, hydroxide, and carbonate are
known, as are many chalcogenides.
The double salt has been shown to have hydroxyl-centred triangles of
thallium, , as a recurring motif throughout its solid structure.
The metalorganic compound thallium ethoxide (TlOEt, TlOC2H5) is a
heavy liquid (ρ , m.p. −3 °C), often used as a basic and soluble
thallium source in organic and organometallic chemistry.
Organothallium compounds
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Organothallium compounds tend to be thermally unstable, in concordance
with the trend of decreasing thermal stability down group 13. The
chemical reactivity of the Tl-C bond is also the lowest in the group,
especially for ionic compounds of the type R2TlX. Thallium forms the
stable [Tl(CH3)2]+ ion in aqueous solution; like the isoelectronic
Hg(CH3)2 and [Pb(CH3)2]2+, it is linear. Trimethylthallium and
triethylthallium are, like the corresponding gallium and indium
compounds, flammable liquids with low melting points. Like indium,
thallium cyclopentadienyl compounds contain thallium(I), in contrast
to gallium(III).
History
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Thallium (Greek , , meaning "a green shoot or twig") was discovered by
William Crookes and Claude Auguste Lamy, working independently, both
using flame spectroscopy (Crookes was first to publish his findings,
on March 30, 1861).* Crookes, William (March 30, 1861) "On the
existence of a new element, probably of the sulphur group," 'Chemical
News', vol. 3, pp. 193-194; reprinted in:
* Crookes, William (May 18, 1861) "Further remarks on the supposed new
metalloid," 'Chemical News', vol. 3, p. 303.
* Crookes, William (June 19, 1862) "Preliminary researches on
thallium," 'Proceedings of the Royal Society of London', vol. 12, pp.
150-159.
* Lamy, A. (May 16, 1862) "De l'existencè d'un nouveau métal, le
thallium," 'Comptes Rendus', vol. 54,
[
http://gallica2.bnf.fr/ark:/12148/bpt6k30115.image.r=Comptes+Rendus+Hebdomadaires.f1254.langFR
pp. 1255-1262]. . The name comes from thallium's bright green spectral
emission lines derived from the Greek 'thallos', meaning a green twig.
After the publication of the improved method of flame spectroscopy by
Robert Bunsen and Gustav Kirchhoff and the discovery of caesium and
rubidium in the years 1859 to 1860, flame spectroscopy became an
approved method to determine the composition of minerals and chemical
products. Crookes and Lamy both started to use the new method. Crookes
used it to make spectroscopic determinations for tellurium on selenium
compounds deposited in the lead chamber of a sulfuric acid production
plant near Tilkerode in the Harz mountains. He had obtained the
samples for his research on selenium cyanide from August Hofmann years
earlier. By 1862, Crookes was able to isolate small quantities of the
new element and determine the properties of a few compounds.
Claude-Auguste Lamy used a spectrometer that was similar to Crookes'
to determine the composition of a selenium-containing substance which
was deposited during the production of sulfuric acid from pyrite. He
also noticed the new green line in the spectra and concluded that a
new element was present. Lamy had received this material from the
sulfuric acid plant of his friend Frédéric Kuhlmann and this
by-product was available in large quantities. Lamy started to isolate
the new element from that source. The fact that Lamy was able to work
ample quantities of thallium enabled him to determine the properties
of several compounds and in addition he prepared a small ingot of
metallic thallium which he prepared by remelting thallium he had
obtained by electrolysis of thallium salts.
As both scientists discovered thallium independently and a large part
of the work, especially the isolation of the metallic thallium was
done by Lamy, Crookes tried to secure his own priority on the work.
Lamy was awarded a medal at the International Exhibition in London
1862: 'For the discovery of a new and abundant source of thallium' and
after heavy protest Crookes also received a medal: 'thallium, for the
discovery of the new element.' The controversy between both scientists
continued through 1862 and 1863. Most of the discussion ended after
Crookes was elected Fellow of the Royal Society in June 1863.
The dominant use of thallium was the use as poison for rodents. After
several accidents the use as poison was banned in the United States by
Presidential Executive Order 11643 in February 1972. In subsequent
years several other countries also banned its use.
Occurrence and production
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Thallium concentration in the Earth's crust is estimated to be 0.7
mg/kg, mostly in association with potassium-based minerals in clays,
soils, and granites. The major source of thallium for practical
purposes is the trace amount that is found in copper, lead, zinc, and
other heavy-metal-sulfide ores.
Thallium is found in the minerals crookesite TlCu7Se4, hutchinsonite
TlPbAs5S9, and lorándite TlAsS2. Thallium also occurs as a trace
element in iron pyrite, and thallium is extracted as a by-product of
roasting this mineral for the production of sulfuric acid.
Thallium can also be obtained from the smelting of lead and zinc ores.
Manganese nodules found on the ocean floor contain some thallium. In
addition, several other thallium minerals, containing 16% to 60%
thallium, occur in nature as complexes of sulfides or selenides that
primarily contain antimony, arsenic, copper, lead, and silver. These
minerals are rare, and have had no commercial importance as sources of
thallium. The Allchar deposit in southern North Macedonia was the only
area where thallium was actively mined. This deposit still contains an
estimated 500 tonnes of thallium, and it is a source for several rare
thallium minerals, for example lorándite.
The United States Geological Survey (USGS) estimates that the annual
worldwide production of thallium is 10 metric tonnes as a by-product
from the smelting of copper, zinc, and lead ores. Thallium is either
extracted from the dusts from the smelter flues or from residues such
as slag that are collected at the end of the smelting process. The raw
materials used for thallium production contain large amounts of other
materials and therefore a purification is the first step. The thallium
is leached either by the use of an alkali or sulfuric acid from the
material. The thallium is precipitated several times from the solution
to remove impurities. At the end it is converted to thallium sulfate
and the thallium is extracted by electrolysis on platinum or stainless
steel plates. The production of thallium decreased by about 33% in the
period from 1995 to 2009 - from about 15 metric tonnes to about 10
tonnes. Since there are several small deposits or ores with relatively
high thallium content, it would be possible to increase the production
if a new application, such as a thallium-containing high-temperature
superconductor, becomes practical for widespread use outside of the
laboratory.
Historic uses
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The odorless and tasteless thallium sulfate was once widely used as
rat poison and ant killer. Since 1972 this use has been prohibited in
the United States due to safety concerns. Many other countries
followed this example. Thallium salts were used in the treatment of
ringworm, other skin infections and to reduce the night sweating of
tuberculosis patients. This use has been limited due to their narrow
therapeutic index, and the development of improved medicines for these
conditions.
Optics
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Thallium(I) bromide and thallium(I) iodide crystals have been used as
infrared optical materials, because they are harder than other common
infrared optics, and because they have transmission at significantly
longer wavelengths. The trade name KRS-5 refers to this material.
Thallium(I) oxide has been used to manufacture glasses that have a
high index of refraction. Combined with sulfur or selenium and
arsenic, thallium has been used in the production of high-density
glasses that have low melting points in the range of 125 and 150
Celsius°. These glasses have room-temperature properties that are
similar to ordinary glasses and are durable, insoluble in water and
have unique refractive indices.
Electronics
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Thallium(I) sulfide's electrical conductivity changes with exposure to
infrared light, making this compound useful in photoresistors.
Thallium selenide has been used in bolometers for infrared detection.
Doping selenium semiconductors with thallium improves their
performance, thus it is used in trace amounts in selenium rectifiers.
Another application of thallium doping is the sodium iodide and cesium
iodide crystals in gamma radiation detection devices. In these, the
sodium iodide crystals are doped with a small amount of thallium to
improve their efficiency as scintillation generators. Some of the
electrodes in dissolved oxygen analyzers contain thallium.
High-temperature superconductivity
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Research activity with thallium is ongoing to develop high-temperature
superconductors for such applications as magnetic resonance imaging,
storage of magnetic energy, magnetic propulsion, and electric power
generation and transmission. The research in applications started
after the discovery of the first thallium barium calcium copper oxide
superconductor in 1988. Thallium cuprate superconductors have been
discovered that have transition temperatures above 120 K. Some
mercury-doped thallium-cuprate superconductors have transition
temperatures above 130 K at ambient pressure, nearly as high as the
world-record-holding mercury cuprates.
Nuclear medicine
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Before the widespread application of technetium-99m in nuclear
medicine, the radioactive isotope thallium-201, with a half-life of 73
hours, was the main substance for nuclear cardiography. The nuclide is
still used for stress tests for risk stratification in patients with
coronary artery disease (CAD). This isotope of thallium can be
generated using a transportable generator, which is similar to the
technetium-99m generator. The generator contains lead-201 (half-life
9.33 hours), which decays by electron capture to thallium-201. The
lead-201 can be produced in a cyclotron by the bombardment of thallium
with protons or deuterons by the (p,3n) and (d,4n) reactions.
Thallium stress test
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A thallium stress test is a form of scintigraphy in which the amount
of thallium in tissues correlates with tissue blood supply. Viable
cardiac cells have normal Na+/K+ ion-exchange pumps. The Tl+ cation
binds the K+ pumps and is transported into the cells. Exercise or
dipyridamole induces widening (vasodilation) of arteries in the body.
This produces coronary steal by areas where arteries are maximally
dilated. Areas of infarct or ischemic tissue will remain "cold". Pre-
and post-stress thallium may indicate areas that will benefit from
myocardial revascularization. Redistribution indicates the existence
of coronary steal and the presence of ischemic coronary artery
disease.
Other uses
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A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is
reported to freeze at −60 °C, some 20 °C below the freezing point of
mercury. This alloy is used in thermometers and low-temperature
switches. In organic synthesis, thallium(III) salts, as thallium
trinitrate or triacetate, are useful reagents for performing different
transformations in aromatics, ketones and olefins, among others.
Thallium is a constituent of the alloy in the anode plates of
magnesium seawater batteries. Soluble thallium salts are added to gold
plating baths to increase the speed of plating and to reduce grain
size within the gold layer.
A saturated solution of equal parts of thallium(I) formate (Tl(HCO2))
and thallium(I) malonate (Tl(C3H3O4)) in water is known as Clerici
solution. It is a mobile, odorless liquid which changes from yellowish
to colorless upon reducing the concentration of the thallium salts.
With a density of 4.25 g/cm3 at 20 °C, Clerici solution is one of the
heaviest aqueous solutions known. It was used in the 20th century for
measuring the density of minerals by the flotation method, but its use
has discontinued due to the high toxicity and corrosiveness of the
solution.
Thallium iodide is frequently used as an additive in metal-halide
lamps, often together with one or two halides of other metals. It
allows optimization of the lamp temperature and color rendering, and
shifts the spectral output to the green region, which is useful for
underwater lighting.
Toxicity
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Thallium and its compounds are extremely toxic, with numerous recorded
cases of fatal thallium poisoning. The Occupational Safety and Health
Administration (OSHA) has set the legal limit (permissible exposure
limit) for thallium exposure in the workplace as 0.1 mg/m2 skin
exposure over an eight-hour workday. The National Institute for
Occupational Safety and Health (NIOSH) also set a recommended exposure
limit (REL) of 0.1 mg/m2 skin exposure over an eight-hour workday. At
levels of 15 mg/m2, thallium is immediately dangerous to life and
health.
Contact with skin is dangerous, and adequate ventilation is necessary
when melting this metal. Thallium(I) compounds have a high aqueous
solubility and are readily absorbed through the skin, and care should
be taken to avoid this route of exposure, as cutaneous absorption can
exceed the absorbed dose received by inhalation at the permissible
exposure limit (PEL). Exposure by inhalation cannot safely exceed 0.1
mg/m2 in an eight-hour time-weighted average (40-hour work week). The
Centers for Disease Control and Prevention (CDC) states, "Thallium is
not classifiable as a carcinogen, and it is not suspected to be a
carcinogen. It is unknown whether chronic or repeated exposure to
thallium increases the risk of reproductive toxicity or developmental
toxicity. Chronic high level exposure to thallium through inhalation
has been reported to cause nervous system effects, such as numbness of
fingers and toes." For a long time, thallium compounds were readily
available as rat poison. This, and that it is water-soluble and nearly
tasteless, led to frequent intoxication caused by accident or criminal
intent.
One of the main methods of removing thallium, both radioactive and
stable, from humans is to use Prussian blue, a material which absorbs
thallium. Up to 20 grams per day of Prussian blue is fed by mouth to
the patient, and it passes through the patient's digestive system and
comes out in the patient's stool. Hemodialysis and hemoperfusion are
also used to remove thallium from the blood serum. At later stages of
the treatment, additional potassium is used to mobilize thallium from
the tissues.
According to the United States Environmental Protection Agency (EPA),
artificially-made sources of thallium pollution include gaseous
emission of cement factories, coal-burning power plants, and metal
sewers. The main source of elevated thallium concentrations in water
is the leaching of thallium from ore processing operations.
See also
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*
* Myocardial perfusion imaging
External links
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* [
http://www.periodicvideos.com/videos/081.htm Thallium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.emedicine.com/emerg/topic926.htm Toxicity, thallium]
* [
https://www.nlm.nih.gov/toxnet/index.html NLM hazardous substances
databank - Thallium, elemental]
*
[
https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsDetails.aspx?faqid=308&toxid=49
ATSDR - ToxFAQs]
* [
https://www.cdc.gov/niosh/npg/npgd0608.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
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Original Article:
http://en.wikipedia.org/wiki/Thallium
