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Thallium

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Thallium (IPA: /ˈθaliəm/) is a chemical element in the periodic table that has the symbol Tl and atomic number 81.[1] This soft gray malleable poor metal resembles tin but discolors when exposed to air. Thallium is highly toxic and is used in rat poisons and insecticides but since it might also cause cancer (although the United States EPA does not class it as carcinogen), this use has been cut back or eliminated in many countries. It is also used in infrared detectors.[2] It has even been used in some murders, earning the nicknames "The Poisoner's Poison" and "Inheritance powder" (alongside arsenic).

Notable characteristics

File:Thallium 1.jpg
1 gram of Thallium

This metal is very soft and malleable and can be cut with a knife. When it is first exposed to air, thallium has a metallic luster but quickly tarnishes with a bluish-gray 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.

Applications

The odorless and tasteless thallium sulfate was widely used in the past as a rat poison and ant killer. In the United States and many other countries this use is no longer allowed due to safety concerns. Other uses:

  • 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.
  • thallium amalgam is used in thermometers for low temperature, because it freezes at -58 °C (pure mercury freezes at -38 °C).

In addition, 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.

History

Thallium (Greek θαλλός, thallos, meaning "a green shoot or twig")[7] was discovered by Sir William Crookes in 1861 in England while he was making spectroscopic determinations for tellurium on residues from a sulfuric acid plant. The name comes from Thallium's bright green spectral emission lines. In 1862 Crookes and Claude-Auguste Lamy isolated the metal independently of each other.

Occurrence

Although the metal is reasonably abundant in the Earth's crust at a concentration estimated to be about 0.7 mg/kg, 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.

Thallium is found in the minerals crookesite TlCu7Se4, hutchinsonite TlPbAs5S9, and lorandite TlAsS2. This metal is also contained in pyrites and is extracted as a by-product of sulfuric acid production when pyrite ore is roasted. Another way this element is obtained is 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. 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. See also: Category:Thallium minerals.

Isotopes

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.

Thallium-202 (half life 12.23 days) can be made in a cyclotron[8] while thallium-204 (half life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.[9]

Toxicity

 

Thallium and its compounds are highly 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.

Part of the reason for thallium's high toxicity is that, when present in aqueous solution as the univalent thallium(I) ion (Tl+), it exhibits some similarities with essential alkali metal cations, particularly potassium. It can thus enter the body via potassium uptake pathways. However other aspects of thallium's chemistry are very different from that of the alkali metals (e.g. its high affinity for sulfur ligands), and so this substitution disrupts many cellular processes (for instance thallium may attack sulphur-containing proteins such as cysteine residues and ferredoxins).

Thallium's toxicity has led to its use (now discontinued in many countries) as a rat and ant poison.

Amongst the distinctive effects of thallium poisoning are loss of hair (which ironically led it to its initial use as a depilatory before its toxicity was properly appreciated) and damage to peripheral nerves (victims may experience a sensation of walking on hot coals). Thallium was once an effective murder weapon before its effects became understood and an antidote (prussian blue) discovered.

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. The prussian blue is fed by mouth to the person, and it passes through their digestive system and comes out in the stool.[10]

Famous uses as a poison

  • The CIA is believed (by its Inspector General) to have conceived a scheme to poison Fidel Castro by exposure to thallium salts placed in his shoes while they were being polished. The goal was to discredit him by causing him to lose his characteristic hair and beard. The scheme progressed as far as testing on animals, but the trip during which the poison was to be administered fell through.[11]
  • In 1953, Australian Caroline Grills was sentenced to life in prison after three family members and a close family friend died. Authorities found thallium in tea that she had given to two additional family members.[12]
  • Félix-Roland Moumié, a leader of the Cameroonian anticolonial armed struggle against France was murdered by thallium poisoning on October 15, 1960. A French agent posing as a journalist was the main suspect of this murder.
  • In 1995, Zhu Ling, a student at Tsinghua University in Beijing, China, was reportedly poisoned twice by her roommate, over a period of a few months. The classmates of the victim asked for help through Usenet, to which access was very new in mainland China at the time. Joint efforts by physicians who responded through the web led to the diagnosis of thallium poisoning. The case was covered by news reports around the world.
 
Corroded Thallium rod
  • In June 2004, 25 Russian soldiers earned Honorable Mention Darwin Awards after becoming ill from thallium exposure when they found a can of mysterious white powder in a rubbish dump on their base at Khabarovsk in the Russian Far East. Oblivious to the danger of misusing an unidentified white powder from a military dump site, the conscripts added it to tobacco, and used it as a substitute for talcum powder on their feet.[13]
  • In 2005, a 17 year old girl in Numazu, Shizuoka, Japan, admitted to attempting to murder her mother by lacing her tea with thallium, causing a national scandal.[14]
  • In November 2006, a Russian critic of President Vladimir Putin, Alexander Litivenko was exposed to thallium at a sushi bar in London. He later died of thallium poisoning in a London hospital. Litivenko accussed President Putin of poisoning him on his deathbed. Three other people were admitted to hospitals in London. The case was covered by news agencies around the world.

See also

References

  1. ^ thallium, Los Alamos National Laboratory. Retrieved November 21, 2006.
  2. ^ Nayer, P. S. "Thallium selenide infrared detector". Smithsonian/NASA ADS Physics Abstract Service. Retrieved 2006-11-25. {{cite web}}: Cite has empty unknown parameters: |accessyear=, |month=, and |accessmonthday= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ Thallium Test from Walter Reed Army Medical Center
  4. ^ Thallium Stress Test from the American Heart Association
  5. ^ Abstract
  6. ^ Thallium-201 production from Harvard Medical School's Joint Program in Nuclear Medicine
  7. ^ Liddell & Scott, A Greek-English Lexicon, sub θαλλος
  8. ^ Thallium Research from Department of Energy
  9. ^ Manual for reactor produced radioisotopes from the International Atomic Energy Agency
  10. ^ Prussian blue fact sheet from the Centers for Disease Control and Prevention
  11. ^ Interim Report: Alleged Assassination Plots Involving Foreign Leaders, U.S. Senate report no. 94-465 ("Church Report"). From The Assassination Archives and Research Center. Retrieved November 21, 2006.
  12. ^ a b What is thallium?, BBC, November 19, 2006. Retrieved November 21, 2006.
  13. ^ White Russians at DarwinAwards.com
  14. ^ Girl admits trying to kill mom by lacing her tea, "GaijinPot", April 28, 2005. News Source from Mainichi Shimbun. Retrieved November 21, 2006.
 
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