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.

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).