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HIGH-SCHOOL CHEMISTRY
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Experiments with Selenium CHARLES H. STONE Vermont Junior College, Montpelier, Vermont
HE chemical world owes much to Sweden. When modem chemistry was in its infancy, Karl Wilhelm Scheele (1742-1786), a Swedish apothecary, discovered chlorine, proved fluorine to be an element, and, before Priestley, discovered oxygen. To these discoveries we may add scandium by Nilsson; molybdenum by Hjelm; lithium by Mvedson; terbium by Mosander; vanadium by Sefstrom. Arrhenius gave us the first successful attempt to clarify the mystery of ionization in solution, later to be much questioned through the researches of Debye and others; Svedberg won the Nobel prize for his investigation of the chemistry of colloids; von Euler shared the Nobel prize with Arthur Harden of England: the first for his work on enzymes and vitamins, and the second for an investigation of fermentation, sugar, and vitamins. Surely there is a record of which any country may be proud. Outstanding among the chemists of his native land stands that giant among chemists, Jons Jakob Berzelius (1779-1848). To him we owe our present system of writing symbols and formulas. Also, he discovered the elements cerium, thorium, zirconium, and finally, in the sludge of the sulfuric acid plants a new substance which he named selenium, an element that receives little attention in our preparatory schools. Selenium is one of the elements in the sulfur family together with tellurium. The element is not a t all rare and occurs mostly in the form of a black solid that has a conchoidal fracture; two of its forms are soluble in carbon disulfide and sulfuric acid, but the amorphous form is not soluble in the disulfide. The molecular formulas fcr these forms are the same, Sea. Compared with the sister element, sulfur, there are marked resemblances. It will be remembered that the two crystalline forms of sulfur are soluble in carbon disulfide while the amorphous form is not. Sulfur boils at 444.7'C., while selenium boils at 688°C. Both substances are brittle, and both form oxides when burned, respectively SO2 and Se02. The first is a gas while the second is a solid. Both of these oxides dissolve in water to form the respective acids, sulfurous and selenious, H2SOsand H9eOa. Each of these acids may be oxidized, to HzSOl and HaSeOrrespectively. When the element selenium is treated with warm nitric acid
T
4HNOa
+ Se
-
SeOz
+ 4NO9 + 2H20
the oxide Se02 is formed, which dissolves in water to form the selenious acid. Heated with metals,
the element combines to form selenides just as sulfur forms sulfides. So we may prepare lead selenide, copper selenide, iron selenide, etc. These selenides, like the sulfides, are insoluble in water. Selenides are notably toxic, and must be handled with care. The selenides of the alkali metals, like the corresponding sulfides, are soluble, but with decomposition. Like sulfurous acid, selenious acid is unstable; also i t can be oxidized or reduced. When sulfur dioxide is bubbled into a water solution of selenious acid, red amorphous selenium is formed and sulfuric acid. The amorphous selenium may be filtered off, washed, and dried. When the red variety is heated it reverts to the more stable black form. When a solution of selenious acid is treated with hydrogen sulfide, yellow selenium sulfide is formed. HBeO.
+ ZHIS
-
3 8 0
+ SeSl
Treated with oxidizing agents, selenious acid is converted into selenic acid. H&Os
-
+ (H202) 0 -HdeO
