Selenium Sulphoxide, SeSO₃

Selenium Sulphoxide or Sulphur - Selenium Sesquioxide, SeSO₃, has already been mentioned in connection with the solubility of selenium in sulphuric acid 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:

Se + H₂SO₄ = SeSO₃ + H₂O;
Se + SO₃ = SeSO₃.

It is described as a green liquid, solidifying to a green solid, which easily passes into a yellow modification. It is slightly more stable than sulphur sesquioxide, but decomposes on gentle heating. The compound SeO₂.SO₃ has already been mentioned.

Selenosulphuric Acid or Thioselenic Acid, H₂SSeO₃. - Salts of this acid are obtained when finely divided selenium is dissolved by warming with an aqueous solution of an alkali sulphite, potassium sulphite being the most satisfactory:

K₂SO₃ + Se = K₂SSeO₃.

The selenosulphates are very unstable and their solutions undergo partial decomposition with separation of red selenium even on mere dilution with water. The addition of acids immediately effects decomposition with formation of selenium, selenosulphuric acid itself being incapable of free existence:

K₂SSeO₃ + 2HCl = 2KCl + H₂SO₃ + Se.

The addition of iodine also brings about immediate decomposition with liberation of selenium.

The evidence supplied by the method of formation and the occurrence of isomerism as to analogous structures for the thiosulphates and selenosulphates, is amplified by the chemical behaviour of the potassium alkyl selenosulphates, obtained by treatment of potassium selenosulphates with alkyl halides. These, on electrolytic reduction and also on oxidation with hydrogen peroxide, yield the corresponding diselenides. The structure of the selenosulphates therefore involves a selenium atom directly attached to potassium, thus