By Combustion of Selenium in Oxygen and Nitrogen Dioxide

When all of the air and moisture have been displaced from the tube, the selenium is strongly heated with a Bunsen flame. A white deposit of selenium dioxide forms on the surface of the selenium but as soon as the temperature has become high enough, it sublimes away. At the same time, the remaining selenium melts to a viscous mass and eventually burns with a pale-blue flame. The sublimate collects on the gas-delivery tube and on the sides of 

A yield of 114 g. (80 per cent) is readily obtained by this method. There is always some loss of selenium due, perhaps, to the formation of the suboxide. Naturally, the presence of tellurium as an impurity in the starting material also decreases the yield. Tellurium remains in place, presumably as the oxide, as it is not readily volatilized. 

The selenium dioxide is obtained in the form of a snow-white product, which may be kept in a tightly stoppered bottle for an indefinite length of time. On exposure to air, it may turn pink as the result of reduction (by dust). 

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