THALLIUM

History and Uses

Thallium was discovered spectroscopically by Sir William Crookes in 1861. While searching for tellurium, he observed a beautiful green line in the spectrum of residues of a German sulfuric acid manufacturing plant. He named this element after the Latin word thallos meaning the budding green twig. In the following year, in 1862, both Crookes and Lamy independently isolated the metal.
Thallium occurs in nature in potash minerals and many sulfide ores. It is found in pyrites from which the metal is recovered. The metal also occurs in the minerals cooksite, lorandite, and hutchinsonite. The average concentration of thallium in the earth’s crust is estimated to be 0.85 mg/kg.
Thallium and its compounds have limited applications. It is used in insecticides and rodenticides. Thallium-mercury alloys are used for switches and closures for use at sub-zero temperatures. Another application is in making low melting glasses for electronic encapsulation. Thallium sulfide is used in photocells.

Physical Properties

Metallic luster when freshly cut but attains a bluish-gray tinge on exposure to air resembling lead in appearance; tetragonal crystals; density 11.85 g/cm3 at 20°C; melts at 303.5° C; vaporizes at 1473° C; electrical resistivity 18 microhm–cm at 0°C and 74 microhm–cm at 303°C; tensile strength 1300 psi; surface tension at 327°C, 401 dynes/cm; insoluble in water; soluble in nitric and sulfuric acids; slightly soluble in hydrochloric acid.

Sources

In the past, thallium was obtained as a by-product from smelting other metals. Thallium is a non-volatile heavy metal and, if released to the atmosphere by anthropogenic sources, may exist as an oxide (thallium oxide), hydroxide (TlOH), sulphate (thallium sulphate), or sulphide. Thallium exists in two chemical states (thallous and thallic). The thallous state is the more common and stable form. Thallous compounds are the most likely form to which common exposures occur in the environment. Thallium is present in air, water, and soil. Thallium is used mostly in the manufacture of electronic devices, switches, and closures. It also has limited use in the manufacture of special glasses and in medical procedures that evaluate heart disease. The levels of thallium in air and water are very low. The greatest exposure occurs from food, mostly home-grown fruits and green vegetables, contaminated by thallium. Small amounts of thallium are released into the air from coal-burning power plants, cement factories, and smelting operations. This thallium falls out of the air onto nearby fruit and vegetable gardens. Thallium enters food because it is easily taken up by plants through the roots. Very little is known on how much thallium is in specific foods grown or eaten. Cigarette smoking is also a source of thallium. People who smoke have twice as much thallium in their bodies than non-smokers. Although fish take up thallium from water, we do not know whether eating fish can increase thallium levels in our body. It has been estimated that the average person eats, on a daily basis, 2 parts thallium per billion parts (ppb) of food. Even though rat poison containing thallium was banned in 1972, accidental poisonings from old rat poison still occur, especially in children. Thallium is a heavy metallic element that exists in the environment mainly combined with other elements (primarily oxygen, sulphur, and halogens) in inorganic compounds. Thallium is quite stable in the environment, since it is neither transformed nor biodegraded.

Reactions

Thallium forms all its compounds in two valence states, +1 (thallous) and +3 (thallic). The metal oxidizes slowly in air at ambient temperature but rapidly on heating, forming thallous oxide, Tl2O. This oxide oxidizes further on heating to form thallic oxide, Tl2O3. When exposed to air at ambient temperatures for several days thallium forms a heavy oxide crust.
Thallium reacts with water containing oxygen to form thallous hydroxide, TlOH, which is a relatively strong base, absorbing carbon dioxide and attacking glass.
The metal dissolves in nitric and sulfuric acid. The solution on evaporation crystallizes to yield thallous nitrate and sulfate. Reaction with hydrochloric acid is very slow.
Thallium burns in fluorine with incandescence. Reactions with other halogens form halides. Thallium combines with several elements forming binary compounds.

Toxicity

Thallium and its compounds (particularly soluble salts) can cause serious or fatal poisoning from accidental ingestion or external application. Acute symptoms are nausea, vomiting, diarrhea, weakness, pain in extremities, convulsions, and coma. Chronic effects are weakness, pain in extremities, and rapid loss of hair. Thallium and its compounds are listed under Federal toxics regulations. It is listed by the US EPA as a priority pollutant metal in the environment.

Description

Thallium was discovered in 1861 by Sir William Crookes (and independently by Claude-Auguste Lamy a year later) and occurs in the lithosphere at 0.7 ppm. The name thallium is in reference to the particularly bright green spectral lines seen in the spectra resultant from a flame spectroscopy test (from Greek thallos, meaning a green shoot or twig), the one used in its discovery. Thallium is a heavy metallic element that exists in the environment mainly combined with other elements (primarily oxygen, sulfur, and the halogens) in inorganic compounds.

Chemical Properties

silver-grey metal, tarnishing quickly in air

Chemical Properties

Thallium is a soft, bluish-white, heavy, very soft metal insoluble in water and organic solvents. It turns gray on exposure to air.

Physical properties

Thallium has much the same look (silvery) and feel as lead and is just as malleable. Unlikelead, which does not oxidize readily, thallium will oxidize in a short time, first appearing as adull gray, then turning brown, and in just a few years or less turning into blackish corrodedchunks of thallium hydroxide. This oxide coating does not protect the surface of thalliumbecause it merely flakes off exposing the next layer to oxidation.
Thallium is just to the left of lead in period 6, and both might be considered extensionsof the period 6 transition elements. Thallium’s high corrosion rate makes it unsuitable formost commercial applications. Its melting point is 304°C, its boiling point is 1,473°C, andits density is 11.85 g/cm³.

Isotopes

There are a total of 55 isotopes for thallium. All are radioactive with relativelyshort half-lives, and only two are stable. The stable ones are Tl-203, which constitutes29.524% of the element’s existence in the Earth’s crust, and Tl-205, which makes up70.476% of the element’s natural abundance found in the Earth’s crust.

Origin of Name

From the Greek word thallos, meaning “young shoot” or “green twig.” Named for the green spectral line produced by the light from the element in a spectroscope.

Occurrence

Thallium is the 59th most abundant element found in the Earth’s crust. It is widely distributedover the Earth, but in very low concentrations. It is found in the mineral/ores ofcrooksite (a copper ore; CuThSe), lorandite (TlAsS₂), and hutchinsonite (lead ore, PbTl). Itis found mainly in the ores of copper, iron, sulfides, and selenium, but not in its elementalmetallic state. Significant amounts of thallium are recovered from the flue dust of industrialsmokestacks where zinc and lead ores are smelted.

Characteristics

Elemental thallium metal is rare in nature mainly because it oxidizes if exposed to air (oxygen)and water vapor, forming thallium oxide, a black powder. Although some compounds ofthallium are both toxic and carcinogenic, they have some uses in the field of medicine. Somecompounds have the ability to alter their electrical conductivity when exposed to infraredlight.

History

Thallium was discovered spectroscopically in 1861 by Crookes. The element was named after the beautiful green spectral line, which identified the element. The metal was isolated both by Crookes and Lamy in 1862 about the same time. Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from the roasting of this ore in connection with the production of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. Extraction is somewhat complex and depends on the source of the thallium. Manganese nodules, found on the ocean floor, contain thallium. When freshly exposed to air, thallium exhibits a metallic luster, but soon develops a bluish-gray tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydroxide is formed. The metal is very soft and malleable. It can be cut with a knife. Forty-seven isotopes of thallium, with atomic masses ranging from 179 to 210 are recognized. Natural thallium is a mixture of two isotopes. The element and its compounds are toxic and should be handled carefully. Contact of the metal with skin is dangerous, and when melting the metal adequate ventilation should be provided. Thallium is suspected of carcinogenic potential for man. Thallium sulfate has been widely employed as a rodenticide and ant killer. It is odorless and tasteless, giving no warning of its presence. Its use, however, has been prohibited in the U.S. since 1975 as a household insecticide and rodenticide. The electrical conductivity of thallium sulfide changes with exposure to infrared light, and this compound is used in photocells. Thallium bromide-iodide crystals have been used as infrared optical materials. Thallium has been used, with sulfur or selenium and arsenic, to produce low melting glasses which become fluid between 125 and 150°C. These glasses have properties at room temperatures similar to ordinary glasses and are said to be durable and insoluble in water. Thallium oxide has been used to produce glasses with a high index of refraction. Thallium has been used in treating ringworm and other skin infections; however, its use has been limited because of the narrow margin between toxicity and therapeutic benefits. A mercury–thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at –60°C, some 20° below the freezing point of mercury. Thallium metal (99.999%) costs about $2/g.

Uses

Thallium is used in photoelectric cells, insemiconductor studies, and in low-rangeglass thermometers. It is alloyed with manymetals. Many of its salts are used as rodent poisons. Radioactive thallium-201 is used fordiagnostic purposes in nuclear medicine inpatients with coronary artery disease.

Uses

Thallium is used as an alloy with mercury and other metals. One main use is in photoelectricapplications and for military infrared radiation transmitters.
It is also used to make artificial gemstones and special glass and to make green colors infireworks and flares. It formerly was used as a rat poison, but is no longer used for this purposebecause it is very toxic to humans.
Another main use is the radioisotope TlCl-201, with the relatively short half-life of about73 hours, in cardiac stress tests to identify potential heart abnormalities. TlCl-201 has anability to bind with the heart muscle, but only if the heart is receiving an adequate supply of blood. Restricted blood flow by blocked or narrow arteries in the heart limits the supply ofTlCl-201 absorbed. First, a small dose of TlCl-201 is injected into the patient, and the patientthen engages in a strenuous workout on a treadmill. Both before and after the test, the patientis scanned by a “gamma” detector that sends the results to a computer where the physiciancan compare the uptake of TlCl-201 before and after the treadmill stress test to determine thecondition of the patient’s heart. An area where the heart’s muscle is weak and the blood flowis limited will show up as a darkish spot on the computer screen. Since the radioisotope TlCl-201 has such a short half-life, it is soon excreted from the body. Thus, there are no long-termdetriments to the body.

Uses

In semi-conductor industry; alloyed with mercury for switches and closures which operate at subzero temperetures. In manufacture of highly refractive optical glass. Has been used in admixture with 97-98% of inert substances as poison for rats and other rodents.

Definition

A soft malleable grayish metallic element belonging to group 13 of the periodic table. It is found in lead and cadmium ores, and in pyrites (FeS₂). Thallium is highly toxic and was used previously as a rodent and insect poison. Various compounds are now used in photocells, infrared detectors, and lowmelting glasses. Symbol: Tl; m.p. 303.5°C; b.p. 1457°C; r.d. 11.85 (20°C); p.n. 81; r.a.m. 204.3833.

Production Methods

Thallium sulfide is insoluble in alkaline solution, but soluble in acid, allowing its separation from group I elements. Thallium chloride is only slightly soluble in cold water, which permits its separation from chlorides of cadmium, copper, tellurium, and zinc.
Thallium metal may be obtained from the compounds in several ways: by electrolysis of carbonates, sulfates, or perchlorates; by precipitation of metallic thallium with zinc; and by reduction of thallous oxalate or chloride. A number of industrial processes for the recovery of thallium have been described in the literature. Several of them depend on the extraction of thallium from flue dust by boiling it in acidified water.

Definition

ChEBI: A metallic element first identified and named from the brilliant green line in its flame spectrum (from Greek thetaalphalambdalambdaomicronsigma, a green shoot).

Definition

thallium: Symbol Tl. A greyishmetallic element belonging togroup 13 (formerly IIIB) of the periodictable; a.n. 81; r.a.m. 204.39; r.d.11.85 (20°C); m.p. 303.5°C; b.p.1457±10°C. It occurs in zinc blendeand some iron ores and is recoveredin small quantities from lead andzinc concentrates. The naturally occurringisotopes are thallium–203and thallium–205; eleven radioisotopeshave been identified. It has fewuses – experimental alloys for specialpurposes and some minor uses inelectronics. The sulphate has beenused as a rodenticide. Thallium(I)compounds resemble those of the alkalimetals. Thallium(III) compounds are easily reduced to the thallium(I)state and are therefore strong oxidizingagents. The element was discoveredby Sir William Crookes in1861.

General Description

Bluish-white soft malleable metal or gray granules. Density 11.85 g / cm3. Emits toxic fumes when heated. May be packaged under water.

Air & Water Reactions

Flammable in the form of powder or dust. Insoluble in water.

Reactivity Profile

THALLIUM is a reducing agent. Reacts so vigorously with fluorine that the metal becomes incandescent [Mellor 5:421 1946-47].

Hazard

Forms toxic compounds on contact with moisture; keep from skin contact. Gastrointestinal damage and peripheral neuropathy.

Hazard

In all forms, thallium is very toxic if inhaled, when in contact with the skin, and in particular,if ingested. Mild thallium poisoning causes loss of muscle coordination and burningof the skin, followed by weakness, tremor, mental aberration, and confusion.
Thallium disease (thallotoxicosis) results from the ingestion of relatively large doses (morethan a few micrograms). The severity may vary with the age and health of the patient. Nervesbecome inflamed, hair is lost, the patient experiences stomach pain, cramps, hemorrhage,rapid heartbeat, delirium, coma, and respiratory paralysis. The disease has the potential tocause death in about one week. In the past thallium was one of the poisons of choice used bymurderers because it acts slowly and makes victims suffer. In 1987 then Iraqi dictator SaddamHussein’s agents mixed thallium powder in orange juice or yogurt and fed it to people heperceived to be his enemies. There were at least 40 thallium poisonings, mostly of Kurdishleaders. (William Langewiesche, “The Accuser,” Atlantic, March 2005, 56.)

Health Hazard

Thallium and its soluble compounds arehighly toxic in experimental animals. Theacute toxic symptoms in humans are nausea,vomiting, diarrhea, polyneuritis, convulsion,and coma. Ingestion of 0.5 g can be fatalto humans. Severe chronic toxicity can leadto kidney and liver damage, deafness, andloss of vision. Other signs of toxicity fromchronic exposure include reddening of theskin, abdominal pain, polyneuritis, loss ofhair, pain in legs, and occasionally cataracts.Ingestion of thallium salts in children hascaused neurological abnormalities, mentalretardation, and psychoses.
Hoffman (2000) reviewed thallium poisoningin women during pregnancy and cited acase that began in the first trimester of pregnancyresulting in fetal demise. John Peter andViraraghavan (2005) have reviewed toxicityof thallium and public health risk and discussedenvironmental concerns and variousremoval technologies from aquatic system.
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Safety Profile

Human poison by unspecified route. Human systemic effects by ingestion: nerve or sheath structural changes, extra-ocular muscle changes, sweating, and other effects. Flammable in the form of dust when exposed to heat or flame. Violent reaction with F2. When heated to decomposition it emits toxic fumes of Tl. Used as a rodenticide and fungicide, and in lenses and prisms, in highdensity liquids. See also THALLIUM COMPOUNDS and POWDERED METALS.

Potential Exposure

Thallium is usually obtained as a byproduct from the flue dust generated during the roasting of pyrite ores in the smelting and refining of lead and zinc. Thallium has not been produced in the United States since 1984, but is imported for use in the manufacture of electronics, optical lenses, and imitation precious jewels. It also has use in some chemical reactions and medical procedures. Thallium and its compounds are used as a rodenticide and fungicide; in the manufacture of plates and prisms, high-density liquids; as insecticides, catalysts; in certain organic reactions, in phosphor activators; in bromoiodide crystals for lenses, plates, and prisms in infrared optical instruments; in photoelectric cells; in mineralogical analysis; alloyed with mercury in low-temperature thermometers, switches and closures; in high-density liquids; in dyes and pigments; in fire-works; and imitation precious jewelry. It forms a stainless alloy with silver and a corrosion-resistant alloy with lead. Its medicinal use for epilation has been almost discontinued. Highly persistent in the environment. Note: Thallium was used in the past as a rodenticide, it has been banned in the United States due to its toxicity from accidental exposure. In some countries, thallium(I)sulfate(2:1) is still used as a rat poison and ant bait.

Carcinogenicity

Female mice treated orally or cutaneously with high doses of thallium showed a degenerative process in the genital tract similar to that found in castrated animals or after uterine denervation. The diagnoses were papilloma, precancerous lesions, and cancer. The control mice did not develop cancer.

Environmental Fate

Metallic thallium (TI) is bluish white or gray; it is very soft and malleable. The element can exist in the environment mainly combined with other elements (primarily oxygen, sulfur, and the halogens) in inorganic compounds. Thallium exists in monovalent (thallous, thallium (I), Tl⁺¹) and trivalent (thallic, thallium (III), Tl⁺³) states. Monovalent thallium ions also are more stable in aqueous solution, but trivalent thallium (Tl⁺³) can be stabilized by complexing agents. Monovalent thallium is similar to potassium (K⁺) in ionic radius and electrical charge, which contribute to its toxic nature.
Compounds of thallium, however, are generally soluble in water and the element is found primarily as the monovalent ion (Tl⁺). Thallium tends to adsorb to soils and sediments, and to bioconcentrate in aquatic plants, invertebrates, and fish. Terrestrial plants can also absorb thallium from soil. Thallium is quite stable in the environment because it is neither transformed nor biodegraded. However, thallium may be bioconcentrated by organisms from water. The US Environmental Protection Agency has identified several National Priorities List sites polluted by thallium.

Shipping

Thallium: UN3288 Toxic solids, inorganic, n.o.s., Hazard Class: 6.1; Labels: 6.1-Poisonous materials, Technical Name Required. UN1707 Thallium compounds, n.o.s., Hazard Class: 6.1; Labels: 6.1-Poisonous materials, Technical Name Required.

Toxicity evaluation

Several mechanisms have been postulated for the toxic action of thallium; however, the exact mechanism or mechanisms of toxicity are unknown. Thallium’s mechanism of toxicity is related to its ability to interfere with potassium ion functions because both obtain similar ionic radii. In addition, there is evidence that thallium interferes with energy production at essential steps in glycolysis, the Kreb’s cycle, and oxidative phosphorylation that adversely affects protein synthesis. Other effects include inhibition of sodium–potassium–adenosine triphosphatase and binding to sulfhydryl groups.

Incompatibilities

Varies. Cold thallium ignites on contact with fluorine. Thallium metal reacts violently with strong acids (such as hydrochloric, sulfuric, and nitric) and strong oxidizers (such as chlorine, bromine, and fluorine). Cold thallium ignites on contact with fluorine. Reacts with other halogens at room temperature.

Waste Disposal

Consult with environmental regulatory agencies for guidance on acceptable disposal practices. Generators of waste containing this contaminant (≥100 kg/mo) must conform with EPA regulations governing storage, transportation, treatment, and waste disposal. Dilute thallium solutions may be disposed of in chemical waste landfills. When possible, thallium should be recovered and returned to the suppliers.