Iodo-platinic Acid, H₂PtI₆

Iodo-platinic Acid, H₂PtI₆.9H₂O, exists in solution when platinum tetra-iodide is dissolved in aqueous hydriodic acid. Upon concentration separates out in reddish black, monoclinic crystals. These are easily soluble in water; they deliquesce upon exposure to air, whilst in vacuo they lose hydrogen iodide. When heated to 100° C. platinum tetra-iodide is obtained.

The aqueous solution is unstable, a deposit of the tetra-iodide being obtained on prolonged standing, particularly with exposure to sunlight.