Selenium

SELENIUM, a chemical element closely allied in physical and chemical properties with sulphur (symbol Se, atomic number 34, atomic weight 79.2o). It is a complex element having six isotopes (q.v.) with atomic weights ranging from 74 to 80. It was discovered in 1817 by J. J. Berzelius who called it selenium (Greek creXipm, the moon) because of the close analogy between the new element and the recently discovered tellurium (Latin, tellus, the earth).

Distribution.

Selenium is widely distributed throughout the world, although in small quantities, being the 4oth element in order of plentifulness, between bismuth and gold. It is present in meteoric iron. It rarely occurs native ; occasionally it accom panies native sulphur, but is usually found in combination with the heavy metals to form selenides, such as clausthalite, PbSe (29% Se); eucairite, crookesite naumannite, and zorgite (a complex selenide with 31% Se). It occurs also in various pyrites and galenas. Whenever any of the foregoing seleniferous minerals are subjected to manufac turing operations, the selenium appears as a by-product.

In the U.S.A. selenium is obtained in considerable quantities together with some tellurium from the anode slime of the copper refineries. This material is fused with sodium nitrate and sand or oxidised with nitric acid. In either case the aqueous extract is treated with hydrochloric acid and sulphur dioxide to set free the selenium.

Physical Properties.

Selenium exists in several allotropic forms (see ALLOTROPY), four of which are definitely crystalline.

Crystalline Red Selenium a and 13 are two varieties separating in monoclinic crystals obtained from solutions of amorphous or vitreous selenium in hot carbon disulphide. At IIO--12o° C the a-form, which is isomorphous with monoclinic sulphur, changes into grey metallic selenium, while with the 13-f orm the transfor mation temperature is 125-130° C. By rapid heating the melting points of a and 13 forms have been found to be about 180° C. The density of crystalline red selenium is 4.45.

Crystalline Grey Selenium A is obtained by warming vitreous selenium to C. It is only sparingly soluble in hot carbon disulphide, is a bad conductor of the electric current, and is metastable, changing into the following second grey variety.

Crystalline Grey Selenium B (Metallic Selenium), formed by warming the other modifications to 200° C, has a metallic lustre, is malleable and, unlike variety A, is in certain circumstances a good electrical conductor. This metallic selenium (m.p. C, sp.gr. 4.8) crystallises in rhombohedral crystals of the hexagonal system and is isomorphous with tellurium. Metallic selenium is a non-conductor in the dark, but on exposure to light its electrical conductivity is proportional to the intensity of the light falling upon it. This remarkable property has led to the selenium cell, the basic principle of which depends on coating with a thin film of selenium a conducting metal such as copper, brass, platinum, silver or gold. This metal is either in the form of plates insulated from each other by mica or other insulating material, or two metallic wires wound closely on a cylinder of glass or other non conductor and covered with selenium. As it is only metallic selenium (variety B) which is sensitive to light, the cell contents after coating with the element must be carefully annealed at 200° C to insure that the selenium has been transformed into the photo-sensitive modification.

United States Production.

The output of selenium in the United States has increased very considerably in recent years. In 1914 22,867 lb. were produced, whereas in 1924 the production reached 130,000 lb., the average price being about $2.10 per pound. This supply goes chiefly to the glass industry, the makers having displaced manganese by selenium as a glass decolorizer, the pink tint produced by this element being complementary to the objectionable green shade arising from the presence of iron in the glass. Moreover, the addition of further quantities of selen ium, either as such or as sodium selenite, imparts to glass a clear red colour useful in railway work. Selenium is used in the ceramic industries for the production of red enamels and red enamelled steelwares. The use of 1-3% of selenium in vul canised India rubber articles increases their resistance to abrasion by 5o%.