Strontium Iodide, SrI₂

Strontium Iodide, SrI₂, is formed when metallic strontium is heated in iodine vapour. It may be prepared by the action of hydriodic acid, or iodine, and a reducing agent, on strontium hydroxide, carbonate, or sulphide, evaporation, and fusion of the residue in a current of gaseous hydriodic acid.

The heat of formation from its elements is 147.5 Cal. It is a white deliquescent compound of density 4.549 at 25° C., and melting-point 507° C., or probably higher, since most of Carnelley's results are rather low. It readily loses iodine if melted in contact with air, and in moist air at ordinary temperatures it is decomposed, forming iodine and strontium hydroxide.

Strontium iodide is even more soluble in water than either the bromide or the chloride. The heat of solution is 20.5 Cal. Etard found the following values for the solubility:

Temperature, ° C	-20	-10	-3	7	11	18	38	52	77	81	97	105	120	175
Grams SrI2 in 100 grams solution	60.0	60.3	62.2	63.0	63.4	63.5	64.8	66.0	70.5	74.0	79.2	79.4	80.8	86.5

Measurements of the densities of strontium iodide solutions have been made, and also of refractive indices and freezing-points, and of electrical conductivities. Electrical conductivity determinations of solutions in methyl, ethyl, and propyl alcohol, and in mixtures of these with water, have been made at 0° C. and 25° C.

From solutions at ordinary temperatures, hexagonal crystalline plates of the hexahydrate separate out. According to Tassilly, the salt obtained at 60° C. contains 7 molecules of water, and the heat of solution is -4.47 Cal. The heat of hydration is, therefore, 25.24 Cal.

Etard separated a duodecahydrate at low temperatures, and concluded that a dihydrate, which may be isomorphous with barium chloride dihydrate, BaCl₂.2H₂O, is probably the stable form above 90° C.

Strontium iodide forms addition compounds containing 1, 2, 4, and 6 molecules of ammonia. The compound, Sr(NH₃)₆I₂, has a dissociation pressure of 50 mm. at 62° C., and the heat of formation from strontium iodide and ammonia at the same temperature is 13.40 Cal. Huttig has found compounds with 1, 2, 6, and 8 molecules of ammonia which attain a dissociation pressure of 100 mm. At 204°, 134°, 74.5°, and 31° C. respectively, the heats of formation being 11.0, 12.6, 15.5, and 18.3 Cal.