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Solid Sulfur Allotropes 7 sulfur samples while pure a-S8 is greenish-yellow. Sulfur samples from vol- canic areas sometimes also contain traces of S7 but in addition minute con- centrations of selenium may be present (as determined by neutron activa- tion analysis), most probably as S7Se heterocycles [23]. To remove these im- purities the material is dissolved in toluene or CH2Cl2, and after filtration the solution is cooled to 50 C. Carbon disulfide is not a good solvent for this purpose since traces of it tend to remain in the product. However, even after this treatment most sulfur samples still contain traces of carbon com- pounds which can best be tested for by carefully heating the sulfur in a clean test tube for 2–3 min to the boiling point (445 C) avoiding ignition of the vapor! After cooling of the sample to room temperature black spots will be seen on the walls of the glass and the color of the sulfur itself may have changed to darker hues or even to black, caused by the formed carbon-sul- fur polymer. The organic impurities can be removed by heating the sulfur for 10 h to 300 C (with addition of 1% magnesium oxide) followed by re- fluxing for 1 h [24] which causes these impurities to decompose to H2S and CS2 which both escape; in addition, a black precipitate is formed which looks like carbon-black but is in fact a sulfur-rich polymer. After slow cool- ing to 125 C and decantation from the black sludge the liquid is filtered through glass-wool. If necessary, this procedure is repeated several times. An improved method uses an immersed electrical heater to keep the sulfur boiling [25]. The purified liquid sulfur is then distilled in a vacuum resulting in a bright-yellow, odorless product. Commercial “high-purity sulfur” (99.999%) often still contains organic impurities since the purity claimed on the label applies to the metal content only. Many contradictory reports about the physical properties of elemental sulfur possibly can be explained by the differing purity of the samples investigated, especially but not exclusively in the older literature. S8 can also be highly purified by zone melting (carbon content then <2.4104%) [26]. From most solvents S8 crystallizes as orthorhombic a-S8. Monoclinic b-S8 is stable above 96 C and is usually obtained by cooling liquid sulfur slowly below the triple-point temperature of 115 C. At 25 C crystals of b-S8 con- vert to polycrystalline a-S8 in less than 1 h but are stable for several weeks at temperatures below 20 C [27]. g-S8 is metastable at all temperatures and occasionally crystallizes by chance, for example from ethanolic solutions of ammonium polysulfide [28], by decomposition of copper ethylxanthate [29] or in the preparation of bis(dialkylthiophosphoryl)disulfane [30]. Surpris- ingly, g-S8 occurs also naturally as the mineral rosickyite. Furthermore, g-S8 is a component of stretched “plastic sulfur” which is obtained by quenching liquid sulfur from 350 C to 20 C (in cold water) and stretching the fibers obtained in the direction of their axes. According to an X-ray diffraction study, this “fibrous” sulfur consists of helical polymeric sulfur chains (Sw, see below) which form pockets filled with S8 molecules as the monoclinic g- allotrope [31].PDF Image | Topics in Current Chemistry
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