Calcium Iodide, CaI₂

Molten calcium will combine directly with iodine vapour to form the iodide. The heat of formation is 141.0 Cal. The salt may be prepared by the action of hydriodic acid on lime or pure marble, or, instead of the acid, iodine and a reducing agent may be used - for example, iron filings or phosphorus. It can also be obtained by reduction of calcium sulphate to sulphide with carbon and addition of iodine to the aqueous extract until the colour persists. To obtain the salt in the anhydrous state it must be dried in a current of dry hydrogen. When heated in air it gives up all its iodine and the oxide is left.

The density of the fused salt is 3.956 at 25° C. The melting-point is 740° C.

Calcium iodide is very hygroscopic and dissolves in water and alcohol. The heat of solution in water is 28.12 Cal.

Etard found the following values for the solubility at different temperatures: -

Temperature, ° C.	-22	+7	+10	+19	+51	+64	+130	+248
Grams CaI2 in 100 grams solution	66.6	65.0	65.1	66.3	69.4	75.9	81.3	87.1

The deliquescent nature of the hydrated salt has led to some doubt as to the exact composition of the hydrate stable at the ordinary temperature. Tassilly gave the formula CaI₂.8H₂O, but it is generally considered to be a hexahydrate. It crystallises in long deliquescent needles which melt at 42° C., forming a lower hydrate. In this connection it might be noted that Kremers observed a break in the solubility curve between 40° and 43° C., indicating a transition-point. The boiling-point of the hexahydrate is about 160° C. It dissolves in water with the evolution of 1.730 Cal.

Kusnetzoff mentions a tetrahydrate, CaI₂.4H₂O, transition temperature 65° C., and also a heptahydrate, CaI₂.7H₂O, which passes into the hexahydrate between - 1° and + 2° C.

A trihydrate, CaI₂.3H₂O, melting below 100° C., has also been described.

Addition compounds, containing 1, 2, 4, and 6 molecules of ammonia respectively, are known. The hexammonia compound exerts a dissociation pressure of 50 mm. at 96° C., and the heat of formation from ammonia and calcium iodide is 13.63 Cal. According to Htittig, however, the ammonia compounds contain 1, 2, 6, and 8 molecules of ammonia per molecule of calcium iodide.

Like the chloride and bromide, calcium iodide also forms addition compounds with various organic derivatives of ammonia. Tombeck combined it with aniline, and Okuda and Fujiwaka with carbamide and asparagine. Spitz also obtained a compound with carbamide as well as compounds with glycine, glycyl glycine, and alanine.

Several double iodides of calcium and mercury have been described; for example, CaI₂.HgI₂.8H₂O, CaI₂.5HgI₂.8H₂O, and 3CaI₂.4HgI₂.24H₂O. The first dissolves in water and many organic solvents without decomposition, but the others are readily decomposed by many solvents with the separation of mercuric iodide.

With zinc iodide the compound CaI₂.ZnI₂.8H₂O has been obtained, and with lead the salt CaI₂.2PbI₂.7H₂O.