Potassium Iridium Alum, K₂SO₄.Ir₂(SO₄)₃

Potassium Iridium Alum, K₂SO₄.Ir₂(SO₄)₃.24H₂O, is obtained by mixing the requisite quantities of solutions of potassium sulphate and iridium sesquisulphate in vacuo and concentrating over sulphuric acid. After several days the salt crystallises out from the syrupy solution as yellow octahedra. These begin to melt at about 96° C., and completely melt at 102° to 103° C. In a hot-air bath they lose all their combined water, becoming green and then violet in colour. The product, however, remains soluble in water, unless the temperature of desiccation exceeds 400° C. At red heat a basic sulphate is obtained as well as metallic iridium.