Aluminium iodide, AlI₃

Aluminium iodide, AlI₃, may be prepared by heating aluminium with iodine in a sealed tube, by passing iodine vapour over heated aluminium, or by adding aluminium to iodine dissolved in carbon disulphide. It separates from carbon disulphide in colourless crystals of density 2.63, melts at 125°, and boils at 350°; its critical temperature is 955°. At 444° the vapour density is 27.0 (air = 1), the formula Al₂I₆ requiring 28.2 (Devilleand Troost). The vapour forms an exclusive mixture with air. The molecular formula is Al₂I₆ in molten iodine, and in carbon disulphide solution.

Aluminium iodide is very soluble in liquid ammonia, from which the compound AlI₃.2ONH₃ (?) may be crystallised at -33° C. At 8-13° C. the crystals lose ammonia and leave the compound AlI₃.6NH₃. The liquid ammonia solution reacts with potassium amide to form a soluble aluminium ammonobasic iodide, Al(NH₂)₃.AlI₃, which crystallises with six molecules of ammonia at the ordinary temperature and about twenty at low temperatures. This compound reacts with more potassium amide to produce an insoluble ammonobasic iodide, Al(NH₂)₃.Al(NH₂)₂I.NH₃, which loses two molecules of ammonia at 160° C.

Aluminium iodide is very soluble in water; an aqueous solution can be readily prepared from aluminium or its hydroxide and hydriodic acid. Like the chloride and bromide, it forms a hexahydrate, AlI₃.6H₂O; it also forms a hydrate, 2AlI₃.15H₂O.

The double salts AlI₃.KI, AlI₃.NaI and AlI₃.HgI₂.8H₂O are known.