Selenium

Uses as a Solvent, etc.

Selenium oxychloride was shown by V. Lenher to be a liquid having many valuable properties. It is a solvent for natural and synthetic resins, fish oils, etc. ; it assists in distinguishing between saturated and unsaturated hydro carbons, attacking the latter but not the former. In a mixture of benzene and heptane, it dissolves the former whereas the latter floats unchanged. That this liquid is now a commercial product is largely due to Lenher, who established a research school at Wisconsin University for the study of selenium and tellurium.

The remarkable electrical properties of metallic selenium have led to the development of the selenium cell, whereby one can over the telephone "hear a ray of light falling upon a metallic plate." The following electrical contrivances are rendered possible by this selenium cell : (i.) phototelegraph for transmitting photographs or sketches along a wire; (ii.) photographophone for making synchronous records of sounds with moving pictures; (iii.) optophone, invented by E. E. Fournier d'Albe in 1914, for enabling the blind to read ordinary type by ear; (iv.) self-lighting buoys, whereby buoys and beacons in inaccessible spots may light and extinguish themselves automatically, depending only on sun light; (v.) photometry; (vi.) military uses in wireless telephony and in locating enemy batteries by light.

The fire-proofing of cables with selenium has proved success ful. Diethyl selenide has been recommended as a warning stench for introduction into inodorous poisonous commercial gases such as carbon monoxide.

Chemical Proper

ties.—Hydrogen selenide (seleniuretted hydrogen) is a colourless gas with repulsive odour, which when liquefied boils at —41° C/760 mm. and melts at —68° C (Olzewski). It burns with a blue flame giving either red selenium or white selenium dioxide depending on the amount of oxygen present. Its solubility in water is somewhat greater than that of hydrogen sulphide. It is obtained by the action of hydrochloric acid on ferrous selenide, but is better prepared by dropping water on aluminium selenide produced by the direct combination of selenium and powdered aluminium.

Metallic Selenides are produced by direct combination of the metals with selenium or, in the case of selenides insoluble in water, by precipitation of solutions of metallic salts by hydrogen selenide. The alkali selenides are colourless but become red on taking up more selenium to form polyselenides such as Na2Se2.

Selenium tetrafluoride, was obtained in a state of purity (1928) by E. B. R. Prideaux and C. B. Cox by the interaction of selenium tetrachloride and silver fluoride as a colourless liquid boiling at 93° C (m.p. —13.2° C) and having density 2.77. It attacks glass and is completely hydrolysed by water to selenious and hydrofluoric acids.

Diselenium dichloride, Se2C12, a brownish-yellow liquid with pungent odour, is decomposed by water into hydrochloric and selenious acids with elimination of red selenium. Selenium tetra chloride, a colourless crystalline mass, which vaporises without melting, is produced by burning selenium in chlorine. Selenium oxychloride, a pale yellow liquid boiling at 177.2° C (m.p. 8.5° C), is prepared by mixing the tetrachloride with selenious oxide under carbon tetrachloride, or by partially hydrolysing the tetrachloride.

Selenium forms two oxides : The long-known dioxide, crystallising in colourless, lustrous needles, obtained by burning selenium in oxygen, readily sublimes but under pressure it melts at 34o° C; it dissolves in water to form selenious acid. Selenium trioxide, recently described by R. R. Worsley and H. B. Baker (1923) as a solid substance resulting from the action of ozone on selenium dissolved in dry selenium oxychloride, dissolves in water to give selenic acid. This acid, the analogue of sulphuric acid, is usually seen as a syrupy liquid, but has been obtained solid (m.p. 58° C) ; it has the noteworthy property of dissolving gold.