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SELENIUM [symbol Se, atomic weight 79...

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Originally appearing in Volume V24, Page 603 of the 1911 Encyclopedia Britannica.
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SELENIUM [See also:symbol Se, atomic See also:weight 79.2 (O=16)] , a non-metallic chemical See also:element, discovered in 1817 by J. J. See also:Berzelius, who called it selenium (Gr. veAitvrt, the See also:moon) on See also:account of its See also:close See also:analogy with See also:tellurium (See also:Lat. tellus, the See also:earth). It is occasionally found in the native See also:condition, but more frequently in See also:combination with metals in the See also:form of selenides, the more important seleniferous minerals being euchairite, crookesite, clausthalite, naumannite and zorgite. It is also found as a constituent of various See also:pyrites and galenas, and in some specimens of native See also:sulphur. The element is usually obtained from the flue dust or chamber deposits of sulphuric-See also:acid See also:works in which a seleniferous pyrites is burned. In this See also:process, the residues are boiled with a dilute sulphuric acid to which nitric acid and See also:potassium chlorate are added in See also:order to transform the element into selenic acid, H2SeO4, which is then reduced to selenious acid, H2SeO3, by boiling with hydrochloric acid, and finally to selenium by sulphur dioxide. L. F. Nilson (Ber., 1874, 7, p. 1719) digests the well-washed chamber mud with a moderately concentrated See also:solution of potassium See also:cyanide, whereby the element goes into solution in the form of potassium selenocyanide, ISe(CN), from which it is precipitated by hydrochloric acid. As alternative methods, F.

WShler (See also:

Ann., 1859, 109, p. 375) heats the well-washed chamber residues with potassium nitrate and carbonate in order to obtain an alkaline selenate, which is then boiled with hydrochloric acid, yielding selenious acid, from which the element is obtained as above; whilst H. See also:Rose (Pogg. Ann., 1828, 90, p. 471) by the See also:action of See also:chlorine obtains selenium tetrachloride, which is converted into selenious acid by See also:water, and the acid so prepared is finally reduced to selenium by treatment with See also:sodium sulphite (see also G. See also:Magnus, Pogg. Ann., 1830, 96, p. 165; O. Pettersson, Ber., 1873, 6, p. 1477; H. See also:Koch, See also:German Patent 167457, 1903). It is obtained from zorgite by See also:heating the See also:mineral with aqua regia; the excess of acid is evaporated, and the resulting syrupy liquid diluted, filtered and decomposed by sulphur dioxide, when the selenium is precipitated (Billandot, Ency. chimique, 1883, 5, p.

198). The commercial element usually contains a certain amount of sulphur, and some tellurium, and various methods have been devised for its See also:

purification. L. See also:Oppenheim (Jour. prakt. Chem., 1857% 71, p. 279) fuses the commercial selenium with potassium cyanide in a stream of See also:hydrogen, takes up the melt in water and passes See also:air through the solution; the precipitated tellurium is filtered off, and the solution then supersaturated with hydrochloric acid, when selenium is gradually deposited. E. See also:Divers, (Chem. See also:News, 1885, 51, p. 199) dissolves the element in boiling concentrated sulphuric acid and reduces the resulting selenious acid with sulphur dioxide, filters off the precipitate and washes it with water and See also:alcohol. The resulting product, however, still contains traces of sulphur. C.

Hugot (Ann. chim. phys., 1900 (7), 2I, p. 34) converts the element by dilute nitric acid into selenium dioxide which is then sublimed, and dissolved in water. Any sulphuric acid See also:

present is removed by baryta water, the precipitated See also:barium sulphate filtered off, the solution acidified by hydrochloric acid and reduced by sulphur dioxide. Several allotropic forms of selenium have been described, but the See also:work of A. P. Saunders (Jour. Phys. Chem., ',goo, 4, p. 423) seems to establish that the element exists in three distinct forms, namely liquid selenium (which includes the vitreous, soluble and amorphous forms), crystalline red selenium (which includes, perhaps, two very closely allied forms), and crystalline, See also:grey or metallic selenium. Liquid selenium becomes more and more viscous in See also:character as its temperature falls from 2200 C. to 6o° C.; it is soft at about 6o°, but is hard and brittle between 30° and 4o°. It shows a conchoidal fracture. The amorphous variety, which only differs from the vitreous form in its See also:state of See also:aggregation, is obtained by reducing solutions of selenious acid with sulphur dioxide.

It is slightly soluble in See also:

carbon bisulphide. The red crystalline variety is obtained by See also:crystallization of selenium from carbon bisulphide, or by leaving the amorphous form in contact with the same solvent. The grey crystalline form is obtained by heating the other varieties, and is the most See also:stable form from See also:ordinary temperatures up to 217°. All varieties of selenium dissolve in concentrated sulphuric acid, forming a See also:green solution (see also R. Marc, Ber., 1906, 39, p. 697; and W. Oechsner de Coninck, Comptes rendus, 1906, 143, p. 682). A colloidal selenium was obtained by C. Paal and C. Koch (Ber., 1905, 38, p. 526) by reducing selenious acid dissolved in an aqueous solution of sodium protalbate with See also:hydrazine See also:hydrate and hydrochloric acid, the precipitate obtained being then dissolved in sodium carbonate.

The specific gravity of selenium is 4.8; the specific See also:

heat varies from 0.0716 to 0.1147, depending upon the particular form. Selenium combines directly with hydrogen when heated in the See also:gas, and with See also:fluorine in the See also:cold. It See also:burns with a See also:blue See also:flame when heated in the air or in See also:oxygen, at the same See also:time giving a characteristic See also:smell of rotten See also:horse-See also:radish, a reaction which serves for the recognition of the element. It combines directly with See also:nitrogen, See also:phosphorus, See also:antimony and carbon, and with all the metals (except See also:gold) to form selenides, of which those of the See also:alkali and alkaline earth metals are soluble in water. Metallic selenium is a conductor of See also:electricity, and its conductivity is increased by See also:light; this See also:property has been utilized in apparatus for transmitting photographs by telegraphy (See See also:TELEGRAPH). Seleniuretted Hydrogen, H2Se, is obtained by the See also:direct See also:union of its constituent elements in the heat; by the decomposition of various selenides with mineral acids; by the decomposition of See also:aluminium selenide, or phosphorus selenide with water; by the action of selenium on a concentrated solution of hydriodic acid; and by heating selenium with colophene (H. See also:Moissan), or better with See also:paraffin See also:wax (H. Wuyts and A. See also:Stewart, See also:Bull. See also:Soc. chim. Belg., 1909, 23, p. 9).

It is a colourless gas which possesses a characteristic smell, more unpleasant than sulphuretted hydrogen. Its physiological effects are much more persistent and injurious than sulphuretted hydrogen, producing temporary See also:

paralysis of the olfactory nerves and inflammation of the mucous membrane. It may be liquefied, the liquid boiling at – 41 ° to — 42° C. and becoming solid at — 68° C. (K. Olszewski). It is somewhat soluble in water and forms a hydrate. It is decomposed by heat, burns with a blue flame, and behaves as a reducing See also:agent. It - precipitates many of the heavy metals as selenides when passed into solutions of their salts. Its aqueous solution is unstable, gradually depositing red selenium on See also:standing. Selenium fluoride, SeFa, is obtained as a colourless liquid by the direct action of fluorine or selenium (P. See also:Lebeau, Comptes rendus, 1907, 144, p. 1042).

It boils at about too° C., attacks See also:

glass readily, is decomposed by water, and dissolves See also:iodine. Selenium dichloride, Se2Cl2, is obtained by the action of chlorine on selenium; by the action of phosphorus pentachloride on selenium or the dioxide; by the action of hydrochloric acid on seleno-sulphur trioxide (E. Divers, Chem. News, 1884, 49, p. 212): 2S•SeO3+2HCI=H2SO4+ S.Se03•SeCl2(+H20)->SeiC12+SOi(OH)Cl; and by heating selenium and selenium tetrachloride to too° C. in a sealed See also:tube. It is a yellowish-See also:brown oily liquid which commences to distil at 13o° C. with partial decomposition into selenium and the tetrachloride. It is decomposed by water with formation of selenium and selenious acid: 2Se2C12+3H2O = H2SeO3+35e+4HC1. Selenium tetrachloride, SeC14, is obtained by passing excess of chlorine over selenium; by the action of phosphorus pentachloride on selenium dioxide: SeO2 +PC14=SeOCl2+POC13; 3SeOCl2+2POC13=3SeCl4+P20s; and by the action of thionyl chloride on selenium oxychloride. It is a See also:white solid which can be obtained crystalline by sublimation in a current of chlorine. It dissociates when heated, and is decomposed by water with See also:production of selenious acid. It dissolves selenium. Similar bromides and iodides are known.

Selenyl chloride, SeOC12, is formed when selenium tetrachloride is heated with the dioxide to 15o° C. (R. See also:

Weber, Pogg. Ann., 1859, 184, p. 615), or when the dioxide is heated with See also:common See also:salt: 2SeO2+2NaCI=SeOC12+Na2SeO3. It is a yellow-coloured liquid which solidifies at o° C., and fumes on exposure to air. It combines with See also:titanium and See also:tin bichlorides and with antimony trichloride, and it is decomposed by water. Selenium dioxide, SeO2, is prepared by burning selenium in oxygen, or by oxidizing selenium with nitric acid and heating the See also:residue. It may also be prepared by the action of selenium on sulphur oxyfluoride (H. Moissan, Bull. Soc. See also:chin., 1902 (3), 27, p. 251): 2SO2F2+Se+SiO2=SeO2+2SO2+SiF4.

It crystallizes in needles or prisms and volatilizes when heated, giving a See also:

pale yellow vapour. It is very hygroscopic, and dissolves in water and alcohol. It reacts with the See also:caustic alkalis to form selemtes, and combines directly with hydrocyanic acid. It is decomposed by hydriodic acid with liberation of selenium and iodine, and by See also:ammonia with formation of selenium and nitrogen. Selenious acid, H2SeO3, is obtained in the crystalline form when a solution of selenium dioxide in water is concentrated over sulphuric acid. It effloresces on exposure to air. Oxidizing agents readily convert it into selenic acid, whilst reducing agents transform it into selenium. It yields normal, acid and super-acid salts (e.g. KHSeO3.H2SeO3)• It is decomposed by many acids with liberation of selenium. Selenic acid, H2SeO4, was discovered by E. See also:Mitscherlich (Pogg. Ann., 1827, 85, p.

623). Its salts, the selenates, are obtained by the oxidation of the selenites, and the See also:

free acid may be obtained by the decomposition of the See also:lead or barium salt. It is also obtained in the See also:electrolysis of solutions of selenious acid (C. Manuelli and G. Lazzarini, See also:Gaza., 1909, 39, 1, p. 50). The acid crystallizes in hexagonal prisms and melts at 58° C. It dissolves in water and yields a hydrate of See also:composition H2SeO4•See also:H2O. It is very, hygroscopic, dissolves sulphur readily and acts on organic compounds in a manner similar to sulphuric acid. It decomposes when strongly heated. The selenates are isomorphous with the chromates and sulphates. A See also:compound of selenium and sulphur has been described as resulting from the action of sulphuretted hydrogen on selenious acid, but A.

Gutbier (Zeit. anorg. Chem., 1905, 43, p. 384) is of the See also:

opinion that in this reaction, at ordinary temperature, a See also:simple reduction takes See also:place, leading to the formationof a mixtureof sulphur and selenium. Selenium sulphoxide, SeSO3, is formed as a yellowish crystalline See also:mass when selenium is warmed with sulphur trioxide. It decomposes when heated above 35° C., and also in the presence of water. A compound of composition, SeSO6, has been obtained by the addition of selenium dioxide to sulphuric acid saturated with sulphur trioxide (R. Metznen, Ann. chim. phys., 1898, (7), 15, p. 203): It crystallizes in colourless needles. Selenosulphuric acid, H2SeSO3, is only known in the form of its salts, which are usually obtained by the action of selenium on solutions of the metallic sulphites, a selenotrithionate being simultaneously produced. The salts are unstable ' and readily decompose when heated. Selenotrithionic acid, H2SeS2O6, is also obtained in the form of its potassium salt by the action of potassium hydrogen sulphite on a selenosulphate. It is readily decomposed by acids with liberation of sulphur dioxide and selenium.

Nitrogen selenide, N2Se2, is formed by the decomposition of selenium chloride with ammonia (A See also:

Verneuil, Bull. soc. chim., 1882, 38, p. 548). It crystallizes readily from See also:benzene or acetic acid and explodes when subjected to See also:shock or when heated. It is also obtained when dry ammonia gas is passed into a dilute solution of selenyl chloride in benzene, the precipitate produced being digested with potassium cyanide to remove any selenium (V. Lenher and E. Wolesensky, Jour. Amer. Chem. Soc.; 1907, 29, p. 215). It is a See also:brick-red See also:powder which explodes when heated to 130° C. Selenium cyanide, Se(CN)2, is obtained by decomposing See also:silver selenocyanide with See also:cyanogen iodide, or by the action of silver cyanide on a solution of selenium bromide in carbon bisulphide.

It crystallizes in tables and is very soluble in water. A more complex cyanide, Se3(CN)2, is obtained by passing a current of chlorine and air'into an aqueous solution of potassium selenocyanide (A. Verneuil, Ann. chim. phys., 1886 (6), 9, p. 289). It crystallizes in See also:

golden yellow needles and is decomposed by boiling water: 2Se3(CN)2+2H20=4HCN+SeO2+ 5Se. When heated to 18o° C. in vacuo it yields the simple cyanide Se(CN)2. Potassium selenocyanide, KSeCN, is obtained by the action of selenium on a concentrated aqueous solution of potassium cyanide, or by heating selenium with anhydrous potassium ferrocyanide (W. See also:Crookes, Ann., 1851, 78, p. 177). It crystallizes in needles, possesses an alkaline reaction, and is readily decomposed by acids with Iiberation of selenium. It forms numerous See also:double salts. Numerous determinations of the atomic weight of selenium have been made.

The earlier results of J. J. Berzelius from an See also:

analysis of the chloride gave values from 79'2 to 79'35. Later determinations by V. Lenher (Jour. Amer. Chem. Soc., 1898, 20, p. 595), from the analysis of silver selenite and the reduction of the double selenium ammonium bromide, give values from 79.277 to 79.367; whilst J. See also:Meyer (See also:Bee., 1902, 35, p. 1591) by the electrolysis of silver selenite in the presence of potassium cyanide obtained the value 79.22.

End of Article: SELENIUM [symbol Se, atomic weight 79.2 (O=16)]

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