Palladous Iodide, PdI₂

Palladium is stained by iodine solution and tarnished by iodine vapour, whilst a certain amount of combination between the two elements takes place when they are heated together, if the metal is in a finely divided condition. Palladium is slowly attacked by a mixture of nitric and hydriodic acid. In each of these cases a little palladous iodide is produced.

A convenient method of preparing the salt consists in adding potassium iodide to a slight excess of palladous chloride solution, when it is precipitated as a black mass, readily soluble in excess of potassium iodide. When dried in air it contains one molecule of water; heated to 300-360° C. it decomposes, evolving iodine. In order to remove the last traces of iodine, however, it is necessary to heat to redness in a current of hydrogen.

Ammonia converts palladous iodide into the diammoniate, PdI₂.2NH₃. Although insoluble in water, alcohol, and ether, it dissolves slightly in hydriodic acid, and easily in aqueous potassium iodide, yielding:

Potassium Iodopalladite, K₂PdI₄, which, upon concentration, crystallises out as black, hygroscopic cubes.