Thallium
Overview
 
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

 but discolors when exposed to air. The two chemists William Crookes
William Crookes
Sir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...

 and Claude-Auguste Lamy
Claude-Auguste Lamy
Claude Auguste Lamy was a French chemist who discovered the element thallium independently from William Crookes in 1862.-Life:...

 discovered thallium independently in 1861 by the newly developed method of flame spectroscopy. Both discovered the new element in residues of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 production.

Approximately 60–70% of thallium production is used in the electronics industry, and the remainder is used in the pharmaceutical industry and in glass manufacturing
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

.
Encyclopedia
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

 but discolors when exposed to air. The two chemists William Crookes
William Crookes
Sir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...

 and Claude-Auguste Lamy
Claude-Auguste Lamy
Claude Auguste Lamy was a French chemist who discovered the element thallium independently from William Crookes in 1862.-Life:...

 discovered thallium independently in 1861 by the newly developed method of flame spectroscopy. Both discovered the new element in residues of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 production.

Approximately 60–70% of thallium production is used in the electronics industry, and the remainder is used in the pharmaceutical industry and in glass manufacturing
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. It is also used in infrared detector
Infrared detector
An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....

s. Thallium is highly toxic and was used in rat poison
Rat poison
Rodenticides are a category of pest control chemicals intended to kill rodents.Single feed baits are chemicals sufficiently dangerous that the first dose is sufficient to kill....

s and insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...

s. Its use has been reduced or eliminated in many countries because of its nonselective toxicity. Because of its use for murder
Murder
Murder is the unlawful killing, with malice aforethought, of another human being, and generally this state of mind distinguishes murder from other forms of unlawful homicide...

, thallium has gained the nicknames "The Poisoner's Poison" and "Inheritance Powder" (alongside arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

).

Characteristics

Thallium is extremely 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-gray tinge that resembles lead. It may be preserved by immersing it in oil. A heavy layer of oxide builds up on thallium if left in air. In the presence of water, thallium hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 is formed. Sulfuric and nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

 dissolve thallium rapidly to make the sulfate
Thallium(I) sulfate
Thallium sulfate , archaically known as thallous sulfate, is the sulfate salt of thallium. It is colorless, odorless, and tasteless, but highly toxic.-Uses:...

 and nitrate salts, while hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 forms an insoluble thallium(I) chloride
Thallium(I) chloride
Thallium chloride is the chemical compound with the formula TlCl. This colourless solid is an intermediate in the isolation of thallium from its ores. Typically, an acidic solution of thallium sulfate is treated with hydrochloric acid to precipitate insoluble thallium chloride...

 layer. Its standard electrode potential
Standard electrode potential
In electrochemistry, the standard electrode potential, abbreviated E° or E , is the measure of individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 mol dm−3, and gases at a pressure of 1 atm...

 is -0.34, slightly higher than the potential for iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 (at -0.44).

Isotopes

Thallium has 25 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s which have atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....

es 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
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 3.78 years.

202Tl (half-life 12.23 days) can be made in a cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

, while 204Tl is made by the neutron activation
Neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...

 of stable thallium in a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

.

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; therefore it has good imaging characteristics without excessive patient radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress test
Cardiac stress test
Cardiac stress test is a test used in medicine and cardiology to measure the heart's ability to respond to external stress in a controlled clinical environment....

s.

208Tl (half-life 3.05 minutes) is generated in the naturally-occurring thorium decay chain. It's prominent 2615 keV gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 is the dominant high-energy feature observed in natural background radiation
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....

.

Chemistry

The two main oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

s of thallium are +1 and +3. In the oxidation state +1 most compounds closely resemble the corresponding potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 or silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 compounds (The ionic radius
Ionic radius
Ionic radius, rion, is the radius of an atom's ion. Although neither atoms nor ions have sharp boundaries, it is important to treat them as if they are hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice...

 of thallium(I) is 1.47 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

 while that of potassium is 1.33 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

 and that of silver is 1.26 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

). For example, the water-soluble and very basic thallium(I) hydroxide
Thallium(I) hydroxide
Thallium hydroxide, also called thallous hydroxide, TlOH, is a hydroxide of thallium, with thallium in oxidation state +1. Thallous hydroxide is a strong base; it is changed to thallous ion, Tl+, except in strongly basic conditions.-References:...

 reacts with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 forming water-soluble thallium carbonate This carbonate is the only water soluble heavy metal carbonate. The similarity with silver compounds is observed with the halide, oxide, and sulfide compounds. Thallium(I) bromide
Thallium(I) bromide
Thallium bromide , a chemical compound, available in an ultra-pure state is a compound semiconductor; used in room temperature X- and gamma-ray detectors and blue sensitive photodetectors; used as a real-time x-ray image sensor; also used as a standard for elemental thallium.The crystalline...

 is a photosensitive yellow compound very similar to the silver bromide
Silver bromide
Silver bromide , a soft, pale-yellow, water insoluble salt well known for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. AgBr is widely used in photographic films and is believed by some to have been used for making...

, while the black thallium(I) oxide and thallium(I) sulfide
Thallium(I) sulfide
Thallium sulfide, Tl2S, is a chemical compound of thallium and sulfur.It was used in some of the earliest photo-electric detectors by T. Case who developed the so-called thalofide cell, used in early film projectors...

 are very similar to the silver oxide and silver sulfide
Silver sulfide
Silver sulfide, Ag2S, is the sulfide of silver. This dense black solid constitutes the tarnish that forms over time on silverware and other silver objects. Silver sulfide is insoluble in all solvents, but is degraded by strong acids. Silver sulfide features a covalent bond, as it is made up of...

.

The compounds with oxidation state +3 resemble the corresponding aluminium(III) compounds. They are moderately strong oxidizing agents, as illustrated by the reduction potential of +0.72 volts for Tl3+ + 3 e → Tl(s). The thallium(III) oxide is a black solid which decomposes above 800 °C, forming the thallium(I) oxide and oxygen.

History

Thallium (Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 , , meaning "a green shoot or twig") was discovered by flame spectroscopy in 1861. The name comes from thallium's bright green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

 spectral emission lines.

After the publication of the improved method of flame spectroscopy by Robert Bunsen
Robert Bunsen
Robert Wilhelm Eberhard Bunsen was a German chemist. He investigated emission spectra of heated elements, and discovered caesium and rubidium with Gustav Kirchhoff. Bunsen developed several gas-analytical methods, was a pioneer in photochemistry, and did early work in the field of organoarsenic...

 and Gustav Kirchhoff
Gustav Kirchhoff
Gustav Robert Kirchhoff was a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects...

 and the discovery of caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

 and rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...

 in the years 1859 to 1860, flame spectroscopy became an approved method to determine the composition of minerals and chemical products. William Crookes
William Crookes
Sir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...

 and Claude-Auguste Lamy
Claude-Auguste Lamy
Claude Auguste Lamy was a French chemist who discovered the element thallium independently from William Crookes in 1862.-Life:...

 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
Lead chamber process
The lead chamber process was an industrial method used to produce sulfuric acid in large quantities. It has been largely supplanted by the contact process....

 of a sulfuric acid production plant near Tilkerode
Abberode
Abberode is a village and a former municipality in the Mansfeld-Südharz district, Saxony-Anhalt, Germany. Since 6 March 2009, it is part of the town Mansfeld....

 in the Harz mountains
Harz
The Harz is the highest mountain range in northern Germany and its rugged terrain extends across parts of Lower Saxony, Saxony-Anhalt and Thuringia. The name Harz derives from the Middle High German word Hardt or Hart , latinized as Hercynia. The legendary Brocken is the highest summit in the Harz...

. He had obtained the samples for his research on selenium cyanide from August Hofmann
August Wilhelm von Hofmann
August Wilhelm von Hofmann was a German chemist.-Biography:Hofmann was born at Gießen, Grand Duchy of Hesse. Not intending originally to devote himself to physical science, he first took up the study of law and philology at Göttingen. But he then turned to chemistry, and studied under Justus von...

 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 similar spectrometer to Crookes' to determine the composition of a selenium-containing substance which was deposited during the production of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 from pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

. 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. 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.
The dominant use of thallium was the use as poison for rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s. After several accidents the use as poison was banned in the United States by the Presidential Executive Order 11643 in February 1972. In the subsequent years several other countries also banned the use.

Occurrence and production

Although thallium is a modestly abundant element in the Earth's crust, with a concentration estimated to be about 0.7 mg/kg, mostly in association with potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

-based mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s in clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

s, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

s, and granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

s, thallium is not generally economically recoverable from these sources. The major source of thallium for practical purposes is the trace amount that is found in copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

, and other heavy-metal-sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

s.
Thallium is found in the minerals crookesite
Crookesite
Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver.-Characteristics:Its chemical formula is reported either as Cu7Se4 or 2Se...

 TlCu7Se4, hutchinsonite
Hutchinsonite
Hutchinsonite is a sulfosalt mineral of thallium, arsenic and lead with formula 2As5S9. Hutchinsonite is a rare hydrothermal mineral.It was first discovered in Binnental, Switzerland in 1904 and named after Cambridge mineralogist Arthur Hutchinson, F.R.S. ....

 TlPbAs5S9, and lorandite
Lorandite
Lorandite is a thallium arsenic sulfosalt with the chemical formula: TlAsS2. Though rare, it is the most common thallium-bearing mineral. Lorandite occurs in low-temperature hydrothermal associations and in gold and mercury ore deposits...

 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
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

.

Thallium can also be obtained from the smelting
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...

 of lead and zinc ores. Manganese nodule
Manganese nodule
Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization...

s found on the ocean floor also contain some thallium, but the collection of these nodules has been and continues to be prohibitively expensive. There is also the potential for damaging the environment of the oceans. In addition, several other thallium minerals, containing 16% to 60% thallium, occur in nature as complexes of sulfides or selenides that primarily contain antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...

, arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, copper, lead, and/or silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

. However, these minerals are rare, and they have had no commercial importance as sources of thallium. The Allchar deposit
Allchar deposit
Allchar deposit is a low-temperature hydrothermal gold–arsenic–antimony–thallium deposit in southern Macedonia. For some time, the thallium-rich part of the deposit was mined. The Crven Dol mine yielded thallium and the ore body still holds estimated amount of 500 t of thallium...

 in southern Macedonia
Republic of Macedonia
Macedonia , officially the Republic of Macedonia , is a country located in the central Balkan peninsula in Southeast Europe. It is one of the successor states of the former Yugoslavia, from which it declared independence in 1991...

 was the only area where thallium was ever actively mined. This deposit still contains a loosely estimated 500 tonnes of thallium, and it is a source for several rare thallium minerals, for example lorandite
Lorandite
Lorandite is a thallium arsenic sulfosalt with the chemical formula: TlAsS2. Though rare, it is the most common thallium-bearing mineral. Lorandite occurs in low-temperature hydrothermal associations and in gold and mercury ore deposits...

.

The United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...

 (USGS) estimates that the annual worldwide production of thallium is about 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
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

 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 a base or sulfuric acid from the material. The thallium is several times precipitated from the solution and to remove further impurities. At the end it is converted to thallium sulfate and the thallium is extracted by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 on platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 or stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

 plates. The production of thallium decreased by about 33% in the period from 1995 to 2009 – from about 15 metric tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s 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 of it if a new application, such as a hypothetical thallium-containing high-temperature superconductor
High-temperature superconductivity
High-temperature superconductors are materials that have a superconducting transition temperature above . From 1960 to 1980, 30 K was thought to be the highest theoretically possible Tc...

, becomes practical for widespread use outside of the laboratory.

Historic uses

The odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

less and taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

less thallium sulfate
Thallium(I) sulfate
Thallium sulfate , archaically known as thallous sulfate, is the sulfate salt of thallium. It is colorless, odorless, and tasteless, but highly toxic.-Uses:...

 was once widely used as rat poison
Rat poison
Rodenticides are a category of pest control chemicals intended to kill rodents.Single feed baits are chemicals sufficiently dangerous that the first dose is sufficient to kill....

 and ant
Ant
Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

 killer. Since 1972 this use has been prohibited in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 due to safety concerns. Many other countries followed this example in the following years. Thallium salts were used in the treatment of ringworm, other skin infection
Skin infection
A skin infection is an infection of the skin. Infection of the skin is distinguished from dermatitis, which is inflammation of the skin, but a skin infection can result in skin inflammation...

s and to reduce the night sweating of tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

 patients. However this use has been limited due to their narrow therapeutic index
Therapeutic index
The therapeutic index is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes death or toxicity ....

, and the development of more-advanced medicines for these conditions.

Optics

Thallium(I) bromide
Thallium(I) bromide
Thallium bromide , a chemical compound, available in an ultra-pure state is a compound semiconductor; used in room temperature X- and gamma-ray detectors and blue sensitive photodetectors; used as a real-time x-ray image sensor; also used as a standard for elemental thallium.The crystalline...

 and thallium(I) iodide
Thallium(I) iodide
Thallium iodide is a chemical compound of formula TlI. It is unusual in being one of the few water-insoluble metal iodides, along with AgI, PbI2 and HgI2.-Chemistry:...

 crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s 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 oxide
Thallium oxide
Thallium oxide is the inorganic compound of thallium and oxygen with the formula Tl2O in which thallium is in its +1 oxidation state. It is black and produces a basic yellow solution of thallium hydroxide when dissolved in water. It is formed by heating solid TlOH or Tl2CO3 in the absence of air....

 has been used to manufacture glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

es that have a high index of refraction
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

. Combined with sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 or selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 and arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

, thallium has been used in the production of high-density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 glasses that have low melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

s in the range of 125 and 150 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

. These glasses have room temperature properties that are similar to ordinary glasses and are durable, insoluble in water and have unique refractive indices.

Electronics

Thallium(I) sulfide
Thallium(I) sulfide
Thallium sulfide, Tl2S, is a chemical compound of thallium and sulfur.It was used in some of the earliest photo-electric detectors by T. Case who developed the so-called thalofide cell, used in early film projectors...

's electrical conductivity changes with exposure to infrared light therefore making this compound useful in photoresistor
Photoresistor
A photoresistor or light dependent resistor is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor or CdS device, from "cadmium sulfide," which is the material from which the device is made and that actually exhibits the...

s. Thallium selenide has been used in a bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...

 for infrared detection. Doping selenium semiconductors with thallium improves their performance, and therefore it is used in trace amounts in selenium rectifiers. Another application of thallium doping is the sodium iodide
Sodium iodide
Sodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....

 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
Scintillation (physics)
Scintillation is a flash of light produced in a transparent material by an ionization event. See scintillator and scintillation counter for practical applications.-Overview:...

 generators. Some of the electrodes in dissolved oxygen analyzers contain thallium.

High-temperature superconductivity

Research activity with thallium is ongoing to develop high-temperature superconducting
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...

 materials for such applications as magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...

, 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
Thallium barium calcium copper oxide
Thallium barium calcium copper oxide, or TBCCO , is a family of high-temperature superconductors having the generalized chemical formula TlmBa2Can−1CunO2n+m+2....

 superconductor in 1988.

Medical

Before the widespread application of technetium-99m
Technetium-99m
Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e., that its half-life of 6 hours is considerably longer than most nuclear isomers that undergo gamma decay...

 in nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...

, 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
Technetium-99m generator
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope 99mTc of technetium from a source of decaying molybdenum-99...

. The generator contains lead-201 (half-life 9.33 hours) which decays by electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

 to the thallium-201. The lead-201 can be produced in a cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

 by the bombardment of thallium with proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s or deuterons by the (p,3n) and (d,4n) reactions.

Thallium stress test

A thallium stress test is a form of scintigraphy
Scintigraphy
Scintigraphy is a form of diagnostic test used in nuclear medicine, wherein radioisotopes are taken internally, and the emitted radiation is captured by external detectors to form two-dimensional images...

, where the amount of thallium in tissues correlates with tissue blood supply. Viable cardiac cells have normal Na+/K+ ion exchange pumps
Na+/K+-ATPase
Na+/K+-ATPase is an enzyme located in the plasma membrane in all animals.- Sodium-potassium pumps :Active transport is responsible for cells containing relatively high...

. The Tl+ cation binds the K+ pumps and is transported into the cells. Exercise or dipyridamole
Dipyridamole
Dipyridamole is a drug that inhibits thrombus formation when given chronically and causes vasodilation when given at high doses over a short time.-Mechanism and effects:...

 induces widening (vasodilation
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...

) of normal coronary arteries. This produces coronary steal from areas where arteries are maximally dilated. Areas of infarct or ischemic tissue
Ischemia
In medicine, ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue. It may also be spelled ischaemia or ischæmia...

 will remain "cold". Pre- and post-stress thallium may indicate areas which will benefit from myocardial revascularization
Revascularization
Revascularization is "a surgical procedure for the provision of a new, additional, or augmented blood supply to a body part or organ." The term derives from the prefix re-, in this case meaning "restoration" and vasculature, which refers to the circulatory structures of an organ.Revascularization...

. Redistribution indicates the existence of coronary steal and the presence of ischemic coronary artery disease.

Other uses

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 performing different transformations in aromatics, ketones, olefins, among others. Thallium is a constituent of the alloy in the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 plates in magnesium seawater batteries. Soluble thallium salts are added to gold plating
Gold plating
Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver , by chemical or electrochemical plating...

 baths to increase the speed of plating and to reduce grain size within the gold layer.

The saturated solution of equal parts of thallium(I) formate
Formate
Formate or methanoate is the ion CHOO− or HCOO− . It is the simplest carboxylate anion. It is produced in large amounts in the hepatic mitochondria of embryonic cells and in cancer cells by the folate cycle Formate or methanoate is the ion CHOO− or HCOO− (formic acid minus one hydrogen ion). It...

 (Tl(CHO2)) and thallium(I) malonate
Malonate
The malonate or propanedioate ion is CH222− . Malonate compounds include salts and esters of malonic acid, such as*diethyl malonate, 2,*dimethyl malonate, 2,...

 (Tl(C3H3O4)) in water is known as Clerici solution
Clerici solution
Clerici solution is a solution of equal parts of thallium formate and thallium malonate in water. It is a freely flowing, odorless liquid which changes from yellowish to colorless upon decreasing the concentration of the thallium salts. At a density of 4.25 g/cm3 at 20 °C, the saturated Clerici...

. 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
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

 method, but the use is discontinued due to the high toxicity and corrosiveness of the solution.

Toxicity

Thallium and its compounds are extremely toxic, and should be handled with great care. There are numerous recorded cases of fatal thallium poisoning.
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
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

. For a long time thallium compounds were easily available as rat poison. This fact and that it is water soluble and nearly tasteless led to frequent intoxications caused by accident or criminal intent.

Treatment and internal decontamination

One of the main methods of removing thallium (both radioactive and normal) from humans is to use Prussian blue
Prussian blue
Prussian blue is a dark blue pigment with the idealized formula Fe718. Another name for the color Prussian blue is Berlin blue or, in painting, Parisian blue. Turnbull's blue is the same substance but is made from different reagents....

, which is a material which absorbs thallium. 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
Human feces
Human feces , also known as a stool, is the waste product of the human digestive system including bacteria. It varies significantly in appearance, according to the state of the digestive system, diet and general health....

. Hemodialysis
Hemodialysis
In medicine, hemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. Hemodialysis is one of three renal replacement therapies .Hemodialysis can be an outpatient or inpatient therapy...

 and hemoperfusion
Hemoperfusion
Hemoperfusion is a medical process used to remove toxic substances from a patient's blood. The technique involves passing large volumes of blood over an adsorbent substance. The adsorbent substance most commonly used in hemoperfusion are resins and activated carbon...

 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.

Thallium pollution

According to the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 (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.

See also

External links

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