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2 CHEMISTRY
Pages 3-9

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From page 3...
... It is located between the metals tellurium and polonium and the nonmetals oxygen and sulfur by group, and between the metal arsenic and the nonmetal bromine by period. The atomic properties and electronic configuration of selenium are summarized in Table 1.
From page 4...
... This phenomenon, like that demonstrated by amorphous sulfur, results from the formation at low temperatures of ring-shaped aggregates with lower viscosity; whereas selenium forms polymeric chains with greater viscosity at higher temperatures. Elemental selenium is, thus, vitreous at 31°C-230°C and is a hard and brittle glass below 31°C.
From page 5...
... Dissolved selenites are present as biselenite ions in aqueous solutions at pH 3.5 to pH 9. Selenite is readily reduced to elemental selenium at low pH by mild reducing agents such as ascorbic acid or sulfur dioxide.
From page 6...
... by addition reactions: from H2Se or alkali selenides by addition or nucleophilic displacement reactions, from potassium selenocyanate by nucleophilic displacement or electrophilic substitution reactions, from phosphorus pentaselenide by reactions with primary alcohols, and from selenium oxides by substitution reactions at carbon atoms or by electrophilic substitution reactions. Several reagents containing highly nucleophilic selenium anions are available.
From page 7...
... 2 As, Sb, Sn, Fe+2, MoO4 Pyrrole Se+4 pyrrole blue color 0.5 oxidizing elements, Se+6, Te+4 Te+6 Asymmetric Se+4 red color 2 oxidizing agents diphenyl hydrazine Methylene Se° decolorization 3 oxidizing agents blue and NaS2 Ammonium Se+4 molybdate 3,3' .
From page 8...
... The purest precipitates are formed when sulfurous acid is used as the reducing agent and when selenium is precipitated from concentrated hydrochloric acid. Other reducing agents (e.g., Fe+2, Sn+2, Cr+2 and v+2 salts, sodium hypophosphite, thiourea, glucose, lactose, ascorbic acid, thiosemicarbazide, sodium diethylthiocarbamate and mercaptobenzimidazole)
From page 9...
... While photometric methods with sulfur-containing organic reagents have been used, they are relatively less selective; the diazonium salt procedures require preliminary elimination of interfering elements and of oxidizing and reducing agents; procedures involving the formation of complexes with selenium of lower valence show relatively poor sensitivity. Selenium can be determined by atomic absorption spectroscopy or by neutron activation analysis.


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