Osmium Hydriodide, OsI₂.2HI

Osmium Hydriodide, OsI₂.2HI, is stated by Alvarez to result as an emerald-green substance by the interaction of hydrochloric acid, potassium iodide, and osmium tetroxide:

OsO₄ + 10HCl + 10KI = OsI₂.2HI + 4H₂O + 10KCl + 3I₂.

It is soluble in water, readily reducing acidulated permanganate and chromate solutions. It is bleached in contact with oxidising acids, but is very stable in concentrated solutions of salts, with which it forms iodosmites. The green colour of the iodide is of such intensity that its formation serves as a delicate colorimetric test for the presence of osmium. The test is carried out as follows:

The solution supposedly containing osmium tetroxide is treated with a saturated solution of calcium chloride, a little potassium iodide and phosphoric acid, and the colour noted.