Potassium Iodosmate, K₂OsI₆

Potassium Iodosmate, K₂OsI₆, is readily prepared by treating potassium osmyl oxynitrite with a fairly concentrated solution of hydriodic acid:

K₂(OsO₃)(NO₂)2 + 8HI = K₂OsI₆ + I₂ + 2HNO₂ + 3H₂O.

The reaction is vigorous, nitrous fumes being disengaged, as also iodine vapour. On filtering the brownish violet solution and allowing to stand, crystals of potassium iodosmate separate out as dark violet octahedra. The salt is not very soluble in water, but more so than the brom- and chlor-osmates, and the solution rapidly decomposes, although the presence of a little free hydriodic acid renders it stable. It may be heated to 200° C. in hydrogen without loss in weight, but in the neighbourhood of 300° C. hydrogen iodide is evolved.