Magnesium Iodide, MgI₂

Anhydrous MgI₂ is similar to MgBr₂, though it is more easily oxidised and melts at a higher temperature. It has not been prepared, apparently, by passing iodine vapour over a heated mixture of carbon and magnesium oxide, but its methods of preparation are similar to those of the bromide.

A saturated solution contains 148 parts of MgI₂ in every 100 parts of water. MgI₂.8H₂O separates at ordinary temperatures from a solution either of the anhydrous salt in water or of magnesia in hydriodic acid, or, in colourless prismatic needles, from the product of the action of iodine on magnesium powder and water. At lower temperatures MgI₂.10H₂O separates.

Magnesium iodide occurs in sea-water and saline springs; it forms double salts with mercuric chloride.

The heat of combination of MgI₂ is 84.800 Cal., and the heat of solution 49.800 Cal.

MgI₂,6H₂O and NH₄I,MgI₂,6H₂O.Double Iodides, KI.MgI₂.6H₂O and NH₄I.MgI₂.6H₂O, have been prepared by evaporating solutions of the mixed salts over sulphuric acid in a vacuum. They crystallise in flattened prisms of densities 2.547 and 2.346 at 15° C. MgI₂.2BiI₃.12H₂O has been obtained.