Periodides of Calcium

By the spontaneous decomposition of the double iodide of lead and calcium when left in contact with its saturated aqueous solution, red crystals are obtained, corresponding in composition to a tri-iodide, CaI₃.15H₂O.

By fusing together at 70°-80° C. an equimolecular mixture of calcium iodide and iodine, a black melt is obtained which gives crystals with a greenish lustre when cooled. Even at 100° C. there is practically no pressure of iodine vapour, which seems to justify the assumption that a compound, calcium tetraiodide, CaI₄, has been formed. By fusing with 2, 3, and 4 molecules of iodine, crystalline masses are obtained of composition CaI₆, CaI₈, and CaI₁₀ respectively, and similar in appearance to the tetraiodide. These also have a very small iodine vapour pressure.

The existence of a tetraiodide in aqueous solution is confirmed by freezing-point determinations. The lowering is, in fact, somewhat less than for an equivalent quantity of iodide, which would indicate either that the degree of dissociation or the degree of hydration of the tetraiodide is less than that of the iodide. 8.062 grams of iodine dissolve in 100 c.c. of a 10 per cent, solution of calcium iodide at 13.5° C. Carbon bisulphide or chloroform will remove iodine from either the solid or the solution. The dissociation constant of the poly-iodides of calcium has been determined by finding the distribution of iodine between carbon tetrachloride and aqueous solutions of calcium iodide. The dissociation constant is much less than in the case of the bromide and tends to diminish with increasing quantities of iodine. For an approximately 1/10 molar solution of calcium iodide, the value of "k," the dissociation constant of the tetraiodide, is in the neighbourhood of 0.0008 at 25° C., when all the concentrations are expressed in gram-molecules per litre.