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.