Sulphur sesquioxide 581 trioxide

Reduction of sulphur trioxide by hydrazine gives rise to sulphur sesquioxide,6 possibly by primary formation of sulphur, which combines with unaltered trioxide (see p. 224). 

Sulphur trioxide is immediately reduced by hydrazine under all conditions, sulphur sesquioxide, S203, being formed.7... 

Selenium Sulphoxide or Sulphur - Selenium Sesquioxide, SeS03.—This has already been mentioned in connection with the solubility of selenium in sulphuric acid (pp. 291, 300) as the probable cause of the green colour produced. It is also obtainable, similarly to sulphur sesquioxide, from fluid sulphur trioxide by the addition of powdered amorphous selenium, the sulphoxide being insoluble in the sulphur trioxide, so that the latter can be decanted off ... 

The bluish-green crystalline mass is very unstable and decomposes fairly rapidly even at the ordinary temperature, giving sulphur dioxide and sulphur. The pure substance may be preserved for several hours in a vacuum. It is hygroscopic, and on absorbing moisture first becomes brown and then deposits sulphur, with concurrent formation of sulphur dioxide, sulphuric acid and possibly other sulphur acids. It is soluble in fuming sulphuric acid, an acid rich in trioxide giving a bluish solution, whilst with less rich acids the solution is brown ordinary pure sulphuric acid causes decomposition. The colour of the brown solution is regarded by some as due to colloidal sulphur formed by partial decomposition of the sesquioxide.6... 

The affinity of selenium for oxygen is less than that of sulphur. The dioxide, SeQ2, is well defined. The trioxide, Se03, is much less stable than its sulphur analogue, and until recently all attempts to isolate it have been unsuccessful (see p. 331). Selenium sesquioxide, Se203, has not been definitely isolated.1 An oxide of composition Se304 has been described,2 but its existence needs confirmation. The existence of a lower oxide, SeO, postulated to explain the characteristic odour of selenium when vaporised in air, is improbable.3... 

Electrolytic Methods.—3Iolvbdenum mav be accurately deter-mined in solutions of molybdates by means of electrolysis. In presence of free sulphuric acid the metal is completely precipitated on the cathode as the hydrated sesquioxide, HyChoehlorie acid may be used instead of sulphuric acid to accelerate the deposition. The precipitate after washing may be converted by gentle ignition to molybdenum trioxide, and weighed as such. 

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