Iridium Sesquisulphate, Ir₂(SO₄)₃

Iridium Sesquisulphate, Ir₂(SO₄)₃.xH₂O, is obtained by dissolving the hydrated sesquioxide in dilute sulphuric acid and allowing it to crystallise in the absence of air.

Unless special precautions are taken, the product has a violet colour due to the presence of iridium in a higher state of oxidation. By effecting the preparation of the salt in vacuo yellow crystals of sesquisulphate are obtained, which dissolve in water to a bright yellow solution. Exposure to air causes the solution to turn violet immediately, particularly if the solution is dilute.