Selenium, red and gray

Samarium (III) nitrate, analysis of anhydrous, 6 41 Selenic acid, crystalline, 3 137 Selenides, precipitation of pure metallic, from solutions of hydrogen selenide, 2 185 Selenium, red and gray, 1 119 Selenium (II) chloride, formation of, by selenium(IV) chloride, 6 127... 

Selenium with four forms, amorphous, two crystalline munoclinic forms (red), and the stable, crystalline gray metallic form. 

Procedure. A drop of the alkaline test solution is stirred with a drop of yellow ammonium polysulfide in a depression of a spot plate. A pinch (tip of knife blade) of solid sodium sulfite is then stirred in. The spot plate is placed on a heating block at 120 C. The yellow (or in the presence of selenium, red-brown) solution loses its color, and if tellurium is present, a black precipitate or gray coloration appears in 1-2 minutes. This procedure will reveal 2.5 y tellurium. Greater sensitivity can be attained by evaporating to dryness and stirring the residue with a drop or two of water. Any red selenium, that may have come down, redissolves while the free tellurium remains unchanged. 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

Another chemist who just missed discovering bromine was J. R. Joss, who in 1824, and again in January, 1826, had recorded in his laboratory notes the appearance of a red color in some hydrochloric acid prepared from gray Hungarian rock salt and Bohemian fuming sulfuric acid. At the time, he attributed this color to the possible presence of selenium from the sulfuric acid. After Balard s discovery, however, he made further experiments with the same materials and became convinced that the red color must be due to bromine. His attempts to obtain more of the bromine-containing rock salt were unsuccessful (147). 

The selenium filtered from the reaction mixture is refluxed with 31. of 95% ethanol for 1 hour, which converts the red amorphous form to the gray hexagonal form and frees it of organic matter. This metallic selenium is removed by filtration, washed with water, and converted to the dioxide by the method of Baker and Maxson.2... 

Selenium, which is just under sulfur in the periodic tabic, crystallizes as red crystals containing Scg molecules, and also as semi-metallic gray crystals which contain long staggered chains, stretching from one end of the crystal to the other. Tellurium crystals also contain long chains. 

Selenium is quite rare (9 X 10 % of the earth s crust). It occurs mainly as an impurity in sulfur, sulfide, and sulfate deposits. It is obtained from the flue dusts that result from roasting sulfide ores and from the anode mud formed in the electrolytic refining of copper. It is used as a red coloring in glass. The gray crystalline allotropic form of selenium has an electrical conductivity that is very light-sensitive, so it is used in photocopy machines and in solar cells. 

Some of the group 16 elements have several common allotropic forms. Oxygen can exist as either O2 or O3 (ozone). Sulfur has many allotropes. Selenium has three common allotropes amorphous gray, red crystalline, and red/black powder. 

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