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Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray malleable poor metal resembles tin but discolors when exposed to air. Approximately 60-70% of thallium production is used in the electronics industry, and the rest is used in the pharmaceutical industry and in glass manufacturing.[2] It is also used in infrared detectors. Thallium is highly toxic and is used in rat poisons and insecticides, but its use has been cut back or eliminated in many countries. Because of its use for murder, thallium has gained the nicknames "The Poisoner's Poison" and "Inheritance Powder" (alongside arsenic).[3]


Thallium is very soft and malleable and can be cut with a knife at room temperature. It has a metallic luster, but when exposed to air, it quickly tarnishes with a bluish-grey tinge that resembles lead. (It is preserved by keeping it under oil). A heavy layer of oxide builds up on thallium if left in air. In the presence of water, thallium hydroxide is formed.
Main article: isotopes of thallium

Thallium has 25 isotopes which have atomic masses that range from 184 to 210. 203Tl and 205Tl are the only stable isotopes, and 204Tl is the most stable radioisotope, with a half-life of 3.78 years.[4]

202Tl (half life 12.23 days) can be made in a cyclotron,[5] while 204Tl (half life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.[4][6][4]

201Tl (half-life 73 hrs), decays by electron capture, emitting Hg x-rays (~ 70-80 keV), and photons of 135 and 167 keV in 10% total abundance;[4] therefore it has good imaging characteristics without excessive patient radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress tests.[7]
See also: Category:Thallium compounds

The two main oxidation states of thallium are +1 and +3. In the oxidation state +1 most compounds closely resemble the potassium or the silver compounds (The ionic radius of thallium(I) is 1.47 Å while that of potassium is 1.33 Å and that of silver is 1.26 Å). For example the water soluble and very basic thallium(I) hydroxide reacts with carbon dioxide forming water soluble thallium carbonate This carbonate is the only water soluble heavy metal carbonate. . The similarity with silver compounds is observed with the halides the oxides and the sulfides compounds. Thallium(I) bromide is a photosensitive yellow compound very similar to the silver bromide, while the black thallium(I) oxide and thallium(I) sulfide are very similar to the silver oxide and silver sulfide. The compounds with oxidation state +3 resemble the aluminium(III) compounds. The thallium(III) oxide is a black solid which decomposes at temperatures above 800°C forming the thallium(I) oxide and oxygen.[8]

Thallium (Greek θαλλός, thallos, meaning "a green shoot or twig")[9] was discovered by flame spectroscopy in 1862. The name comes from thallium's bright green spectral emission lines.[10]

After the publication of the improved method of flame spectroscopy by Robert Bunsen and Gustav Kirchhoff[11] 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. William Crookes and Claude-Auguste Lamy both started to use the new method. William 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.[12][13] By 1862 Crookes was able to isolate small quantities of the element and determine the properties of a few compounds.[14] Claude-Auguste Lamy used a similar spectrometer 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 Fréd Kuhlmann and this by-product was available in large quantities. Lamy started to isolate the new element from that source.[15] 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 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.[16][17]
Occurrence and production

Although the metal is reasonably abundant in the Earth's crust at a concentration estimated to be about 0.7 mg/kg,[18] mostly in association with potassium minerals in clays, soils, and granites, it is not generally considered to be commercially recoverable from those forms. The major source of commercial thallium is the trace amounts found in copper, lead, zinc, and other sulfide ores.[19][20]
Crystals of Hutchinsonite (TlPbAs5S9)

Thallium is found in the minerals crookesite TlCu7Se4, hutchinsonite TlPbAs5S9, and lorandite TlAsS2.[21] It also occurs as trace in pyrite and extracted as a by-product of roasting this ore for sulfuric acid production.[2][22] The metal can be obtained from the smelting of lead and zinc rich ores. Manganese nodules found on the ocean floor also contain thallium, but nodule extraction is prohibitively expensive and potentially environmentally destructive.[23] In addition, several other thallium minerals, containing 16% to 60% thallium, occur in nature as sulfide or selenide complexes with antimony, arsenic, copper, lead, and silver, but are rare, and have no commercial importance as sources of this element.[18] The Allchar deposit in southern Macedonia is the only area where thallium was mined. The deposit contains still 500 t of thallium and is a source for several rare thallium minerals for example lorandite and others.[24]

The United States Geological Survey estimates the world wide production of 10 metric tonnes as a by-product of smelting of copper, zinc, and lead ores. It is either extracted from the flue dusts or residues collected of the smelting.[18] The production of thallium decreased by 50% in the time between 1995 and 2009 from 15 metric tonnes to 10 t. As there are several deposits with relative high thallium content it is possible to increase production if new application, for example the thallium containing high-temperature superconductor, will enter the mass market [18][18]
Historic uses

The odorless and tasteless thallium sulfate was once widely used as rat poison and ant killer. Since 1975, this use in the United States and many other countries is prohibited due to safety concerns.[2] Thallium salts were used in the treatment of ringworm, other skin infections and to reduce the night sweating of tuberculosis patents. However this use has been limited due to the narrow therapeutic index.[25][26][27]

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.[28] Thallium 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 °C. These glasses have room temperature properties that are similar to ordinary glasses and are durable, insoluble in water and have unique refractive indices.[29]
Corroded thallium rod

Thallium(I) sulfide's electrical conductivity changes with exposure to infrared light therefore making this compound useful in photocells.[25][30] Doping selenium semiconductors with thallium improves their performance and therefore it is used in selenium rectifiers.[25] Another application of thallium doping is the [sodium iodide]] crystals in gamma radiation detection equipment, the crystals are doped with small amounts of thallium to improve the efficency for scintillation counters.[31] Some of the electrodes in dissolved oxygen analyzers contain thallium.[2]
High-temperature superconducting

Research activity with thallium is ongoing to develop high-temperature superconducting materials for such applications as magnetic resonance imaging, storage of magnetic energy, magnetic propulsion, and electric power generation and transmission. After the discovery of the first thallium barium calcium copper oxide superconductor in 1988 the research in applications started.[32]

Before the widespread application of technetium-99m in nuclear medicine radioactive thallium-201 with a half-life of 73 hours was the main substance for nuclear diagnostic. The nuclide is still used for stress tests for risk stratification in patients with coronary artery disease A(CAD).[33] This isotope of thallium can be generated using a transportable generator which is similar to the technetium cow.[34] The generator contains lead-201 (half life 9.33 hours) which decays by electron capture to the 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.[35][36]
Other use

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.[25] In organic synthesis thallium(III) salts, as thallium trinitrate or triacetate, are useful reagents performing different transformations in aromatics, ketones, olefins, among others.[37] Thallium is a constituent of the alloy in the anode plates in magnesium seawater batteries.[2]

The saturated solution of equal parts of thallium(I) formate (Tl(CHO2)) and thallium(I) malonate (Tl(C3H3O4)) in water is known as Clerici solution. It is a mobile odorless liquid whose color changes from yellowish to clear upon reducing the concentration of the thallium salts. With the 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 density of minerals by the flotation method, but the use is discontinued due to the high toxicity and corrosiveness of the solution.[38][39]
Main article: Thallium poisoning
Skull and crossbones.svg

Thallium and its compounds are extremely toxic, and should be handled with great care. Contact with skin is dangerous, and adequate ventilation should be provided when melting this metal. Thallium(I) compounds have a high aqueous solubility and are readily absorbed through the skin. Exposure to them should not exceed 0.1 mg per m² of skin in an 8-hour time-weighted average (40-hour work week). Thallium is a suspected human carcinogen.[40] For a long time thallium compounds where easily available as rat poison. This fact and that it is water soluble and nearly tasteless led to frequent intoxications by accident or by criminal intent.[17]
Treatment and internal decontamination

One of the main methods of removing thallium (both radioactive and normal) from humans is to use Prussian blue, which is a solid ion exchange material which absorbs thallium and releases potassium. Up to 20 g per day of Prussian blue is fed by mouth to the person, and it passes through their digestive system and comes out in the stool. Hemodialysis and hemoperfusion are also used to remove thallium from the blood serum. At later stage of the treatment additional potassium is used to mobilize thallium from the tissue.[41][42]
Thallium pollution

According to the United States Environmental Protection Agency (EPA), man-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.[43][20]

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External links

* — Thallium
* pure Thallium >=99,99% picture in the element collection from Heinrich Pniok
* Toxicity, Thallium
* NLM Hazardous Substances Databank – Thallium, Elemental

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