Separation selenium

Extraction.—(1) From Pyrites.—In the oxidation of the pyrites (or other sulphur mineral) for the formation of sulphur dioxide in the manufacture of sulphuric acid, foreign elements like arsenic and selenium also undergo oxidation and pass ofC as vapours with the sulphur dioxide. The selenium dioxide produced in this manner their suffers more or less complete reduction by the sulphur dioxide, when finely divided selenium separates, mainly in the lead chambers, as a red, amorphous powder, accompanied possibly by some of the greyish-black form a portion of the dioxide is also found in the Glover tower acid. The amount of selenium in the chamber mud depends, of course, on the nature of the pyrites relatively large quantities of compounds of arsenic, zinc, tin, lead, iron, copper or mercury are always present, arising almost entirely from impurities in the pyrites. 

Small quantities of selenium in solution may be detected by the test described by Meunier,1 which is similar to Marsh s test for arsenic. If there is a relatively large amount of selenium present it is readily detected by its red colour, but if the amount of the element be small the deposit closely resembles that of arsenic, and in this case the following process may be adopted to detect the selenium. A current of hydrogen sulphide is passed into the hot solution containing a little sulphurous acid. The precipitate of finely divided sulphur carries down with it any selenium present and arsenic as arsenious sulphide, and it is clotted by stirring. In the presence of selenium the clots are brown they may be dried and the sulphur and selenium separated by careful sublimation in a closed tube. 

As regards solubility in water, the general behaviour of the selenides is analogous to that of the sulphides. Aqueous solutions of the alkali and alkaline earth selenides gradually undergo oxidation by atmospheric oxygen and selenium separates, generally as the amorphous red variety, but sometimes as the crystalline modification. 

Dichloroselenoacetone, (CHg.CO.CH SeCla.1—Two molecular proportions of pure acetone in a large volume of absolute ether are treated with selenium tetrachloride (1 mol.) in small quantities. The resulting product crystallises in white needles, M.pt. 82° C., which emit a vapour tending to attack the mucous membrane. The compound decomposes on keeping and also on boiling with water, selenium separating. 

Place a little powdered thiourea on quantitative filter paper and moisten it with a drop of the test solution. Orange-red selenium separates out. 

Selenites are likewise reduced to selenium. If, however, the solution of the selenite in concentrated sulphuric acid is treated with sodium sulphite, selenium separates but the tellurite is unaffected the latter can be detected in the solution after eliminating the sulphur dioxide. Salts of silver, copper, gold, and platinum must be absent for they are reduced to the metal by the reagent. 

Iron Selenites.—Although metallic iron does not appear to be soluble in selenous acid, yet selenites of iron are readily obtained in a variety of ways. When sodium selenite is added to ferrous sulphate solution, a white precipitate of ferrous selenite, FeSe03, is obtained.4 This becomes darker on exposure to air in consequence of oxidation. If the white precipitate is dissolved in hydrochloric acid, a portion of the selenium separates out, whilst ferric chloride and selenous acid remain in solution. Thus —... 

The selenium separates in the form of an orange-yellow suspension. Selenious acid in sulphuric acid solution has been proposed by Yablich as a reaction for the detection of dichloroethyl sulphide (see p. 247),... 

Selenium dioxide, Seienious acid, SeOj, HsSeOa. Mol. wts. 110.96,128.98. Suppliers Fairmont Chemical Co. K and K Laboratories Matheson, Coleman and Bell (SeOJ Fisher (HjSeO.,). Review and procedures for conversion of recovered Se into SeOa, In oxidations and dehydrogenations with either preformed seienious acid or selenium dioxide and water, usually in acetic acid, ethanol, or dioxane, the solution turns yellow and then red, and later some of the selenium separates in a red form difficult to Alter, some is retained in colloidal solution, and some is bound to an oraanic lubitrita. Elimination of all loleniuin fVom a reaction mixture often oreienti... 

Trimethylphosphine selenide, (CH3)gPSe.—This is obtained on replacing the sulphur in the preceding preparation by selenium. The product forms prisms, M.pt, 84° C., which blacken on exposure to the air, selenium separating out. 

Colloidal selenium is sensitive to light. If a vessel is partially covered with black paper and allowed to stand in the light for a long time a lustrous precipitate of selenium is seen in the illuminated part of the liquid. Electrolytes precipitate colloidal selenium, while boiling causes the color to change to blue. When this blue liquid is shaken with carbon disulfide selenium separates out in the layer between the water and the carbon disulfide but does not appear to be dissolved. If the red liquid is shaken with carbon disulfide the selenium separates in the dividing layer but finally dissolves in the carbon disulfide. Especially beautiful red preparations that are rather stable toward electrolytes are obtained by the method of Paal. ... 

Procedure To one drop of the tested solution one drop of hydrochloric acid and a crystal of Se02 are added. Red selenium separates, which is filtered off after the addition of water and 1—2 min standing. A few drops of 1-naphthylamine solution are added to the filtrate, followed by a few crystals of sodium acetate. The test is positive if a red color appears which becomes more pronounced on addition of hydrochloric acid. 

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