Sulphur Sesquioxide, S₂O₃

Buchholz in 1804 observed that fuming sulphuric acid could dissolve finely divided sulphur, giving an unstable blue solution. In 1812 Vogel obtained the coloured substance by the direct addition of sulphur to liquid sulphur trioxide at the ordinary temperature; drops of a bluish-green liquid separated, which solidified in crusts. The main excess of trioxide was drained away, complete removal being effected by careful evaporation near 38° C. By the addition of liquid sulphur trioxide to finely powdered sulphur, instead of vice versa, the product is more easily obtainable.

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.

Nitric acid or bromine causes rapid oxidation of the oxide, which may be accompanied by explosion. Pure dry ether also reacts violently, with separation of sulphur and formation of a yellow solution which, on evaporation, yields a yellow oil. Absolute alcohol acts similarly.

The composition of the oxide suggests that it may be an acid anhydride, but all attempts to convert it into hydrosulphurous acid, H₂S₂O₄, or its derivatives, have been unsuccessful.

Analogous oxides are known containing one atom of sulphur replaced by selenium or tellurium. The tellurium compound, tellurium sulphoxide, STeO₃, decomposes with production of tellurium monoxide, TeO, when gradually heated in a vacuum to 225° C. Attempts to produce sulphur monoxide similarly from sulphur sesquioxide have not been successful.

An application has been suggested for this sesquioxide in the preparation of certain organic dyes containing sulphur.