Iridium Sesquisulphite, Ir₂(SO₃)₃

Iridium Sesquisulphite, Ir₂(SO₃)₃, may be obtained by exposing a suspension of the hydrated dioxide in water to a current of sulphur dioxide. The liquid becomes greenish in colour, and on standing deposits a yellow, crystalline precipitate of iridium sulphite. This salt is but slightly soluble in water. Alkalies convert it into the sesquioxide, which, upon exposure to air, oxidises to the dioxide. Acids liberate sulphur dioxide, converting the residue into the corresponding salt of iridium. When ignited, sulphur dioxide is expelled and a residue of iridium sesquioxide is obtained.

A second substance containing sulphur dioxide is described by Birnbaum as formed as a black, amorphous, insoluble residue during the preparation of the foregoing sesquisulphite. He regards it as a basic sulphite of tetravalent iridium, IrO₂.SO₂.4H₂O, but it may equally well be a basic sulphate.

Iridium sesquisulphite combines with sulphites of the alkali metals to yield double salts.