34
  Se  
78.960000
Selenium

Name: Selenium
Symbol: Se
Atomic Number: 34
Atomic Weight: 78.960000
Family: Non Metals
CAS RN: 7782-49-2
Description: A grey solid.
State (25°C): Solid
Oxidation states: +4, +6, -2

Molar Volume: 16.42 cm3/mole
Valence Electrons: 4p4

Boiling Point:  958°K, 685°C, 1265°F
Melting Point:
494°K, 221°C, 430°F
Electrons Energy Level: 2, 8, 18, 6
Isotopes: 28 + 5 Stable
Heat of Vaporization: 37.7 kJ/mol
Heat of Fusion: 6.694 kJ/mol
Density: 4.79 g/cm3 @ 300°K
Specific Heat: 0.32 J/g°K
Atomic Radius: 1.22Å
Ionic Radius: 0.5Å
Electronegativity: 2.55 (Pauling); 2.48 (Allrod Rochow)
Vapor Pressure: 0.695 Pa @ 221°C
Selenium: (Greek, selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius after analyzing an impurity that was contaminating the Sulfuric Acid (H2SO4) being produced at a particular factory in Sweden. Originally believing the material was Tellurium, Berzelius eventually realized that it was actually a previously unknown element.  Selenium occurs in minerals such as Eucairite (CuAgSe), Crooksite (CuThSe) and Clausthalite (PbSe), but these minerals are too rare to use as a major source of selenium.  Today, most Selenium is obtained as a byproduct of refining copper, Cu.

Growth in Selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding, steel alloying, and Selenium rectifiers.  By 1970, Selenium in rectifiers had largely been replaced by Silicon, but its use as a photoconductor in plain paper copiers had become its leading application.  During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced.  Presently, the largest use of Selenium world-wide is in glass manufacturing, followed by uses in chemicals and pigments.  Electronic use, despite a number of continued applications, continues to decline.

In 1996, continuing research showed a positive correlation between selenium supplementation and cancer prevention in humans, but widespread direct application of this important finding would not add significantly to demand owing to the small doses required.   In the late 1990s, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards, became important.  At present, total world selenium production continues to increase modestly.

8
O
15.99
16
S
32.06
34
Se
78.96
52
Te
127.6
84
Po
210.0
116
Uuh
289.0

1s2 2s2p6 3s2p6d10 4s2p4

Most selenium is recovered from the electrolytic copper refining process. This is usually in the form of the red allotrope. There are at least two other allotropes of the element, including a semi-metallic state.

Selenium is an important semi-conductor which is particularly sensitive to light. Thus it finds applications that range from light sensors to xerography.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p4

Selenium occurs only rarely in the free state in nature. It is a nonmetal that is chemically related to sulfur and tellurium.  It is toxic in large amounts, but trace amounts of it, forming the active center of certain enzymes, are necessary for the function of all cells in (probably) all living organisms.

Isolated selenium occurs in several different forms, but the most stable of these is a dense purplish-gray semimetal (semiconductor) form that is structurally a trigonal polymer chain.  It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below). Selenium also exists in many nonconductive forms: a black glass-like substance, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter cousin sulfur.

Selenium is found in economic quantities partially replacing sulfur in sulfide ores such as Pyrite, FeS2.  Minerals that are selenide or selenate compounds are also known, but all are rare.

Occurrence

Selenium occurs naturally in a number of inorganic forms, selenide, selenate and selenite.  In soils, selenium most often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff.

Selenium has a biological role, and is found in organic compounds such as dimethyl selenide, selenomethionine and selenocysteine.  In these compounds selenium plays an analogous role to sulfur.

Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead.  It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of Sulfuric Acid plants.  These muds can be processed by a number of means to obtain free selenium.

Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain toxic plants such as locoweed.  Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.

Non-Biological Applications

Chemistry

Selenium is a catalyst in many chemical reactions and is widely used in various industrial and laboratory syntheses.  Selenium's resistance to the flow of electricity is greatly affected by the amount of light shining on it.  The brighter the light, the better Selenium conducts electricity.  This property has made Selenium useful in devices that respond to the intensity of light, such as electric eyes, photo cells, light meters for cameras and copiers.  Selenium can also produce electricity directly from sunlight and is used in solar cells.  Selenium is also a semiconductor and is used in some types of solid-state electronics as well as in rectifiers, devices which convert alternating current electricity into direct current electricity.  In addition to its use in electrical devices, Selenium is also used to make a ruby-red color in glasses and enamels, as a photographic toner and as an additive to stainless steel.

Manufacturing and Materials Use

The largest use of selenium world-wide is in glass and ceramic manufacturing, where it is used to give a red color to glasses, enamels and glazes as well as to remove color from glass by counteracting the green tint imparted by ferrous impurities.

Selenium is used with bismuth in brasses to replace more toxic lead.  It is also used to improve the abrasion resistance in vulcanized rubbers.

Electronics

Because of its photovoltaic and photoconductive properties, selenium is used in photocopying, photocells, light meters and solar cells.  It was once widely used in rectifiers.  These uses have mostly been replaced by silicon-based devices, or are in the process of being replaced.

Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state flat panel x-ray cameras.

Photography

Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed.  Its use intensifies and extends the tonal range of black and white photographic images as well as improving the permanence of prints.

Biological Applications

Nutrition

Selenium is used widely in vitamin preparations and other dietary supplements, in small doses (typically 50 to 200 micrograms per day for adult humans).  Some livestock feeds are fortified with selenium as well.

Production and Allotropic Forms

Selenium is a common byproduct of copper refining, or the production of Sulfuric Acid.   Isolation of selenium is often complicated by the presence of other compounds and elements.  Commonly, production begins by oxidation with Sodium Carbonate to produce Sodium Selenite.  The Sodium Selenite is then acidified with Sulfuric Acid producing Selenous Acid.  The Selenous Acid is finally bubbled with Sulfur Dioxide producing elemental red amorphous Selenium.

Na2CO3 rarrow.gif (63 bytes) Na2SeO3 + H2SO4 rarrow.gif (63 bytes) H2SeO3 + SO2 rarrow.gif (63 bytes) Se
Copper-Selenium Ore + Sodium Carbonate + Oxidizer rarrow.gif (63 bytes) Sodium Selenite + Sulfuric Acid rarrow.gif (63 bytes) Selenous Acid + Sulfur Dioxide rarrow.gif (63 bytes) Selenium

Selenium produced in chemical reactions invariably appears as the amorphous red form-- an insoluble brick red powder. When this form is rapidly melted, it forms the black, vitreous form which is usually sold industrially as beads. The most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms. The conductivity of this form is notably light sensitive. Selenium also exists in three different deep red crystalline monoclinic forms, which are composed of Se8 molecules, similar to many allotropes of sulfur.

Compounds

  • Mercury Selenide (HgSe)
  • Hydrogen Selenide (H2Se)
  • Selenium Dioxide (SeO2)
  • Selenic Acid (H2SeO4)
  • Selenium Dichloride (SeCl2)
  • Selenous Acid (H2SeO3)
  • Selenium Sulfides: Se4S4, SeS2, Se2S6
  • Sodium Selenite (Na2SeO3)
  • Zinc Selenide (ZnSe)
  • Selenium Oxychloride (SeOCl2)

Isotopes

Selenium has at least 28 isotopes, of which 5 are stable, and 6 are nuclear isomers.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Se65 64.965 <50 ms
Se66 65.955  
Se67 66.95 60 ms
Se68 67.942 35.5 seconds
Se69 68.9396 27.4 seconds
Se70 69.934 41.1 minutes
Se71 70.932 4.74 minutes
Se72 71.9271 8.4 days
Se73 72.9268 7.15 hours
Se74 73.9225 Stable
Se75 74.9225 119.779 days
Se76 75.9192 Stable
Se77 76.9199 Stable
Se78 77.9173 Stable
Se79 78.9185 1.13E 6 years
Se80 79.9165 Stable
Se81 80.918 18.45 minutes
Se82 81.9167 1.08E 20 years
Se83 82.9191 22.3 minutes
Se84 83.9185 3.1 minutes
Se85 84.9222 31.7 seconds
Se86 85.9243 15.3 seconds
Se87 86.9285 5.29 seconds
Se88 87.9314 1.53 seconds
Se89 88.936 0.41 seconds
Se90 89.939 >150 ns
Se91 90.945 0.27 seconds
Se92 91.949  
Se93    
Se94    

Precautions

Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life.  It is a component of the unusual amino acids selenocysteine and selenomethionine.  In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and thioredoxin reductase.  It also plays a role in the functioning of the thyroid gland by participating as a cofactor for thyroid hormone deiodinase.  Dietary selenium comes from nuts, cereals, meat, fish, and eggs.  Brazil nuts are the richest source, with high levels also in kidney, crab and lobster, in that order.  The recommended dietary allowance for adults is 55 micrograms per day.

Toxicity

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Although selenium is an essential trace element it is toxic if taken in excess.  Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. 

Symptoms of selenosis include a garlic odour on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema and death.

Elemental Selenium and most metallic selenides have relatively low toxicities because of their low bioavailability.  By contrast, selenate and selenite are very toxic, and have modes of action similar to that of Arsenic.  Hydrogen Selenide, H2Se, is an extremely toxic, corrosive gas.  Selenium also occurs in organic compounds such as Dimethyl Selenide, Selenomethionine and Selenocysteine, all of which have high bioavailability and are toxic in large doses.

Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally-dry undeveloped lands.  This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as it evaporates.  High Selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.

Deficiency

Selenium deficiency is relatively rare in healthy well-nourished individuals.  It can occur in patients with severely comprimised intestinal function, or those undergoing total parenteral nutrition.  Alternatively, people dependent on food grown from selenium-deficient soil are also at risk.  The Dietary Reference Intake for adults is 55 micrograms per day.

Selenium deficiency can lead to Keshan disease, which is potentially fatal. Selenium deficiency also contributes (along with iodine deficiency) to Kashin-Beck disease.   The primary symptom of Keshan disease is myocardial necrosis, leading to weakening of the heart. Kashin-Beck disease results in atrophy, degeneration and necrosis of cartilage tissue.  Keshan disease also makes the body more susceptible to illness caused by other nutritional, biochemical, or infectious diseases.  These diseases are most common in certain parts of China where the soil is extremely deficient in selenium. Studies in Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements.

Selenium is also necessary for the conversion of the thyroid hormone Thyroxine (T4) into its more active counterpart, Triiodothyronine, and as such a deficiency can cause symptoms of hypothyroidism, including extreme fatigue, mental slowing, goiter, cretinism and recurrent miscarriage.

Controversial Health Effects

Cancer

Several studies have suggested a link between cancer and selenium deficiency.  A study conducted on the effect of selenium supplementation on the recurrence of skin cancers did not demonstrate a reduced rate of recurrence of skin cancers, but did show a significantly reduced occurrence of total cancers.  Dietary Selenium prevents chemically induced carcinogenesis in many rodent studies.  In these studies, organic seleno-compounds are more potent and less toxic than selenium salts (e.g., Selenocyanates, Selenomethionine, Selenium-rich Brazil nuts, or Selenium-enriched garlic or broccoli).   Selenium may help prevent cancer by acting as an antioxidant or by enhancing immune activity.  Not all studies agree on the cancer-fighting effects of Selenium.   One long-term study of Selenium levels in over 60,000 participants did not show any correlation between Selenium levels and risk of cancer.  The SU.VI.MAX study concluded that low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of Vitamin E, 6 mg of beta carotene, 100 µg of Selenium, and 20 mg of zinc) resulted in a 31% reduction in the incidence of cancer and a 37% reduction in all cause mortality in males, but did not get a significant result for females.  The SELECT study is currently investigating the effect of selenium and Vitamin E supplementation on incidence of prostate cancer.  However, Selenium has been proved to help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs.  One of the studies showed that in just 72 hours, the efficacy of treatment using chemotherapeutic drugs, such as Taxol and Adriamycin, with selenium yeast is significantly higher than the treatment using the drugs alone.  The finding was shown in various cancer cells (breast, lung, small intestine, colon, liver).

HIV/AIDS

Some research has indicated a geographical link between regions of selenium deficient soils and peak incidences of HIV/AIDS infection  infection. For example, much of sub-Saharan Africa is low in selenium.  However, Senegal is not, and also has a significantly lower level of AIDS infection than the rest of the continent.  AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in Selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.

Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death.  Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system.  Others have argued that HIV encodes for the human Selenoenzyme Glutathione Peroxidase, which depletes the victim's Selenium levels.  Depleted Selenium levels in turn lead to a decline in CD4 helper-T-cells, further weakening the immune system.

Regardless of the cause of depleted Selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.  Selenium supplementation may help mitigate the symptoms of AIDS and reduce the risk of mortality.  It should be emphasized that the evidence to date does not suggest that selenium can reduce the risk of infection or the rate of spread of AIDS, but rather treat the symptoms of those who are already infected.

atom.gif (700 bytes)

Selenium Data

Atomic Radius (Å): 1.22Å
Atomic Volume cm3/mol : 16.45cm3/mol
Covalent Radius: 1.16Å
Crystal Structure: Hexagonal
Ionic Radius: 0.5Å

Chemical Properties

Electrochemical Equivalents: 0.7365 g/amp-hr
Electron Work Function: 5.9eV
Electronegativity: 2.55 (Pauling); 2.48 (Allrod Rochow)
Heat of Fusion: 6.694 kJ/mol
Incompatibilities: Acids, strong oxidizers, chromium trioxide, potassium bromate, cadmium
First Ionization Potential: 9.752
Second Ionization Potential: 21.19
Third Ionization Potential: 30.82
Valence Electron Potential: 120
Ionization Energy (eV): 9.752 eV

Physical Properties

Atomic Mass Average: 78.96
Boiling Point: 958°K, 685°C, 1265°F
Melting Point: 494°K, 221°C, 430°F
Heat of Vaporization: 37.7 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: 36.9E-6
Electrical Conductivity: 1.0E-12 106/cm
Thermal Conductivity: 0.0204 W/cm°K
Density: 4.79 g/cm3 @ 300°K
Enthalpy of Atomization: 205.9 kJ/mole @ 25°C
Enthalpy of Fusion: 6.69 kJ/mole
Enthalpy of Vaporization: 26.3 kJ/mole
Flammability Class: Combustible Solid
Molar Volume: 16.42 cm3/mole
Optical Refractive Index: 1.000895
Relative Gas Density (Air=1): unknown
Specific Heat: 0.32 J/g°K
Vapor Pressure: 0.695 Pa @ 221°C
Estimated Crustal Abundance: 5×10-2 milligrams per kilogram
Estimated Oceanic Abundance: 2×10-4 milligrams per liter


(Gr. Selene, moon) Discovered by Berzelius in 1817, who found it associated with tellurium, named for the earth. Selenium is found in a few rare minerals such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode metal from electrolytic copper refineries now provide the source of most of the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray. Natural selenium contains six stable isotopes. Fifteen other isotopes have been characterized. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. It is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby-colored glasses and enamels. It is also used as a photographic toner, and as an additive to stainless steel. Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soil in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils. Exposure to selenium compounds (as Se) in air should not exceed 0.2 mg/m3 (8-hour time-weighted average - 40-hour week). Selenium is priced at about $300/lb. It is also available in high-purity form at a somewhat higher cost.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond