Selenium & compounds: Overview
Description
Industry uses selenium to manufacture photocells, photographic exposure meters, solar cells, and rectifiers for home entertainment equipment. Further uses are in metal alloys, xerography, red or black glass, anti-dandruff shampoos, pigments in plastics, paints, dyes, enamels, inks, textiles, rubber (accelerator and vulcanising agent), photographic emulsions, and petroleum. It is also used in veterinary medicine, in medical therapeutic agents, as a nutritional feed additive for poultry and livestock, and as a fungicide and insecticide. Uses in electronics and photography account for the majority of selenium use, followed by the glass industry, then pigments. Some applications for specific selenium compounds follow. Selenium dioxide is the most widely used selenium compound in industry. It is used as an oxidising agent in drug and other chemical manufacture, a catalyst in organic syntheses, and an anti-oxidant in lubricating oils. Selenium sulfide is used in anti-dandruff shampoo. Selenous acid can be found in gun blueing solution which is used to clean the metal parts of a gun.
Substance details
Substance name: Selenium & compounds
CASR number: 7782-49-2
Molecular formula: Se
Synonyms: No synonyms.
Selenium compounds include selenium (IV) oxide or selenium dioxide (CAS# 7446-08-4), selenium (VI) fluoride or selenium hexafluoride (CAS# 7783-79-1), selenium (II) hydride or hydrogen selenide (CAS# 7783-07-5), selenium (IV) oxychloride (CAS# 7791-23-3), selenium (IV) sulfide or selenium disulfide (CAS# 7488-56-4), selenic acid (CAS# 7783-08-6), and selenous acid (CAS# 7783-00-8).
Physical properties
Selenium is an odourless metalloid (an element which has both metallic and non-metallic properties). It can be a grey (the 'metallic' and most stable form), red or black solid.
Atomic Number: 34
Atomic Mass: 79.0
Melting Point: 221°C (grey selenium)
Boiling Point: 685°C
Specific Gravity: 4.4-4.8
Properties vary widely depending on the particular compound. Some physical properties for selected selenium compounds follow.
- Selenium disulfide is a light orange to red powder. Its melting point is less than 100°C and it decomposes when heated.
- Selenium hydride is an extremely poisonous gas with revolting odour. Its melting point is -66°C, its boiling point is -41°C, and the compound is thermodynamically stable to 280°C.
- Selenium dioxide is hygroscopic and melts at 315°C. It has a specific gravity of 3.95.
- Selenous acid forms hygroscopic, colourless crystals that melt at 70°C. Its boiling point is 315°C, with a specific gravity 3.0.
- Selenic acid forms white crystals that melt at 58°C. Its boiling point is 260°C.
- Selenium hexafluoride is a very toxic colourless gas (melting point -39°C, boiling point -35°C).
- Selenium oxychloride has a vapour density 5.7 and a melting point of 9°C.
Chemical properties
Selenium can exist in four different oxidation states, (II-), (0), (IV) and (VI), forming various salts. In aquatic environments, the (IV) and (VI) oxidation states are predominant. Selenium (IV) is known as selenite and selenium (VI) is known as selenate. Various chemical properties for selenium and selected selenium compounds follow.
- Selenium burns in air when heated to give selenium dioxide. It reacts when heated with halogens, most metals and non-metals. Non-oxidising acids do not affect selenium.
- Selenium dioxide can be obtained by treating selenium with hot nitric acid. It dissolves in water to give solutions of selenous acid.
- Selenic acid evolves oxygen when heated above 200°C. It is a strong oxidising agent. Selenous acid and selenic acid are both water soluble.
- Selenium dioxide is slightly water soluble.
- Selenium disulfide and selenium hexafluoride are both insoluble in water.
- Selenium hydride has a water solubility of 0.70 g/100 mL.
Further information
The National Pollutant Inventory (NPI) holds data for all sources of selenium and compounds in Australia.
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