Chemical Properties
Brownish red crystalline powder
Physical properties
Exists in two isomorphous forms, α- and ?-forms; both modifications highly hygroscopic. The α-form is black hexagonal crystal; density 5.58 g/cm3; turns dark green in air; melts at 560°C; disolves in water giving pink coloration. The α-forms sublimes in vacuo, partly forming an isomorous yellow modification-the anhydrous β-form.
The β-modification is a yellow powder; density 5.45 g/cm3; converts to the α-form when heated to 400°C; absorbs moisture from air, the yellow powder becoming green droplets; dissolves readily in water forming a colorless solution which turns pink on heating.
The hexahydrate is red hexagonal crystals; density 2.90 g/cm3; loses water at 130°C giving anhydrous iodide; soluble in water, ethanol, acetone, chloroform and ether, forming colored solutions, (while the aqueous solution is red below 20°C and green above this temperature; the salt forms blue solution in ethanol, chloroform and ether).
Preparation
Cobalt(II) iodide is prepared by heating cobalt powder in a stream of hydrogen iodide at 400 to 450°C:
Co + 2HI → CoI2 + H2
The product obtained is the black crystalline α-form.
Cobalt(II) iodide also may be made by heating cobalt powder with iodine vapor.
Structure and conformation
Cobalt(II) iodide crystallizes in two polymorphs, the α- and β-forms. The α-polymorph comprises black hexagonal crystals, which turn dark green when exposed to air. Under vacuum at 500℃, samples of α-CoI2 sublime, forming the β-polymorph as yellow crystals. β-CoI2 also easily absorbs moisture from the air, converting into green hydrate. Below 400℃, β-CoI2 reverts back to the α-form. The anhydrous salt has the cadmium halide structure. The hexaaquo salt comprises separated [Co(H2O)6] 21 and iodide ions.
