Thallium

Thallium is the chemical elementin the periodic tablethat has the symbol Tl and atomic number81. This soft gray malleable poor metalresembles tinbut discolors when exposed to air. Thallium is highly toxicand is used in rat poisonsand insecticidesbut since it might also cause cancer(although the EPAdoes not class it as carcinogen), this use has been cut back or eliminated in many countries. It is also used in infrared detectors.

Notable characteristics

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 lusterbut quickly tarnishes with a bluish-gray tinge that resembles lead(it is preserved by keeping it under water). A heavy layer of oxidebuilds up on thallium if left in air. In the presence of water, thallium hydroxideis formed.

Applications

The odorlessand tastelessthallium sulfate was widely used in the past as a rat poisonand antkiller. In the United Statesand many other countries this use is no longer allowed due to safety concerns. Other uses:

• thallium sulfide's electrical conductivitychanges with exposure to infrared lighttherefore making this compound useful in photocells.
• thallium bromide-iodidecrystalshave been used as infrared optical materials.
• thallium oxidehas been used to manufacture glassesthat have a high index of refraction.
• used in semiconductormaterials for selenium rectifiers,
• in gamma radiationdetection equipment,
• high-density liquid used for sink-float separation of minerals,
• used in the treatment of ringwormand other skin infections. However this use has been limited due to the narrow margin that exists between toxicity and therapeutic benefit.
• radioactivethallium-201 is used for diagnostic purposes in nuclear medicine, particularly in stress tests used for risk stratification in patients with coronary artery disease(CAD).
• combined with sulfuror seleniumand arsenic, thallium has been used in the production of high-densityglasses that have low melting pointsin 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 indexes.
• 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 generationand transmission.

History

Thallium (Greekthallos meaning "a green shoot or twig") was discovered by Sir William Crookesin 1861in Englandwhile he was making spectroscopic determinations for tellurium on residues from a sulfuric acidplant. The name comes from Thallium's bright greenspectral emission lines. In 1862 Crookes and Claude-Auguste Lamyisolated the metal independent 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, it exists mostly in association with potassiumminerals in clays, soils, and granitesand, thus, 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 sulfideores.

Thallium is found in the minerals crookesite(TlCu₇Se₄), hutchinsonite(TlPbAs₅S₉), and lorandite(TlAsS₂). This metal is also contained in pyritesand is extracted as a by-product of sulfuric acid production when pyrite ore is roasted. Another way this element is obtained is from the smeltingof lead and zinc rich ores. Manganese noduleswhich are 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 silverbut are rare and have no commercial importance as sources of this element.

Isotopes

Thallium has 25 isotopeswhich have atomic massesthat range from 184 to 210. Tl–203 and Tl–205 are the only stable isotopes and Tl–204 is the most stable radioisotopewith a half-lifeof 3.78 years.

Precautions

Thallium and its compounds are highly toxic and should be handled with great care. The toxicity derives from its ability to replace important alkali metalcations such as sodiumand potassiumin the body; all these metals share a common oxidation state of +1. This substitution disrupts many cellular processes. The toxicity has led to its use (now discontinued in many countries) as a ratpoison. Amongst the distinctive effects of thallium poisoning are loss of hair, and damage to peripheral nerves. Contact with skin is dangerous and adequate ventilation should be provided when melting this metal. Exposure to soluble compounds of thallium shouldn't exceed 0.1 mgper m³of skin in an 8-hour time-weighted average (40-hour work week). Thallium is a suspected human carcinogen. Thallium was once an effective murder weapon before its effects became understood and an antidote (prussian blue) discovered.

Famous uses

Image:Thallium rod corroded.jpg In June 2004, at least 25 Russian soldiers became sick after thallium exposure when they found a can of Thallium powder in a rubbish dump near their base at Khabarovskin the Russian Far East. Unaware of the danger, the conscripts added it to the tobacco they smoked and used it as a substitute for talcum powderon their feet.

The detective fictionwriter, Agatha Christie, who had worked as a pharmacist, used thallium as the agent of murder in her novel The Pale Horse — the first clue to the murder method coming from the hair loss of the victims.

The 1995 film The Young Poisoner's Handbook was based on the activities of Graham Frederick Youngwho killed at least three people with thallium in the 1960s and 1970s.

The CIAis believed (by its Inspector General) to have conceived (but not executed) a scheme to poison Fidel Castroby 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.

References

• Los Alamos National Laboratory — Thallium
• CIA Inspector General's Report on Plots to Assassinate Castro

External links

• WebElements.com — Thallium

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