Chemical elements
    Physical Properties
    Chemical Properties
      Hydrogen Selenide
      Selenium Fluorides
      Selenium Monochloride
      Selenium Tetrachloride
      Selenium Monobromide
      Selenium Tetrabromide
      Selenium Chlorobromides
      Selenium Oxyfluoride
      Selenium Oxychloride
      Sulphur Selenium Oxytetrachloride
      Selenium Oxybromide
      Chloroselenic Acid
      Selenium Dioxide
      Selenious Acid
      Selenium Trioxide
      Selenic Acid
      Perselenic Acid
      Selenium Sulphoxide
      Selenotrithionic Acid
      Diselenotrithionic Acid
      Selenopentathionic Acid
      Selenium Nitride
      Nitrosylselenic Acid
      Phosphorus Subselenide
      Phosphorus Monoselenide
      Tetraphosphorus Triselenide
      Phosphorus Triselenide
      Phosphorus Pentaselenide
      Phosphorus Chloroselenide
      Selenophosphates and Oxyselenophosphates
      Carbon Diselenide
      Carbon Subselenides
      Carbon Oxyselenide
      Carbon Sulphidoselenide
      Cyanogen Monoselenide
      Cyanogen Diselenide or Selenocyanogen
      Cyanogen Triselenide
      Selenocyanic Acid
      Ammonium Selenocyanate
      Caesium Triselenocyanate
      Copper Selenocyanate
      Lead Selenocyanate
      Magnesium Selenocyanate
      Mercurous Selenocyanate
      Mercuric Selenocyanate
      Potassium Selenocyanate
      Silver Selenocyanate
      Sodium Selenocyanate
      Zinc Selenocyanate
      Silicon Selenide
    Detection and Estimation

Selenium Trioxide, SeO3

All attempts to prepare Selenium Trioxide or Selenic Anhydride, SeO3, by methods analogous to those used for sulphur trioxide have failed, and it has been found impossible to isolate it by dehydration of selenic acid.

When a solution of selenium in selenium oxychloride is subjected for about 36 hours to a stream of ozonised oxygen, the red solution gradually becomes colourless and a white or very pale yellow precipitate appears which can be obtained free, from oxychloride by washing first with carbon tetrachloride and then with ether. This solid was first obtained by Worsley and Baker, who concluded from analyses that it was selenium trioxide. It has a density of 3.6, is readily soluble in water with evolution of heat, forming selenic acid, and soluble also in alcohol. It is insoluble in benzene, carbon tetrachloride, chloroform and ether. On heating it decomposes at 120° C. without melting or subliming, yielding the dioxide. It combines with dry hydrogen chloride to form chloroselenic acid. The molecular weight, determined by cryoscopic methods using phosphorus oxychloride as solvent, corresponded with the formula SeO3.

A repetition of the foregoing work by Meyer and Pawletta, however, failed to confirm the production of selenium trioxide. According to these investigators, the dissolution of selenium in selenium oxychloride results in the formation of selenious chloride, and the precipitate obtained by the action of ozone is a mixture of this salt with selenium dioxide. Furthermore, substitution of carbon tetrachloride or glacial acetic acid for selenium oxychloride did not lead to the separation of the trioxide.

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