Cadmium Iodide, CdI₂

The anhydrous salt of Cadmium Iodide, CdI₂, has been prepared by heating equivalent quantities of cadmium and iodine in an evacuated tube. It also crystallises, in transparent hexagonal crystals, from solutions obtained by (a) the action of iodine upon cadmium under water; (b) the action of aqueous hydriodic acid upon cadmium carbonate or metallic cadmium, or a mixture of these two, or cadmium oxide; (c) interaction between potassium iodide and cadmium sulphate.

It has been prepared pure by dissolving cadmium carbonate in hydriodic acid, evaporating to dryness with excess of the acid, and heating in a current of hydrogen iodide.

Determinations of its density have varied greatly and Clarke and Kebler inferred that the salt existed in both an α-form and a β-form of lower density that was brownish and gradually transformed into the α-variety at 50° C. According to Snell, the so-called β-cadmium iodide, of low density, obtained by crystallising the salt from its solution in hydriodic acid, contained hydrogen iodide and water, and he concluded that there is no valid evidence for a cadmium iodide of lower density than 5.6. Cohen and Moesveld affirm that between ordinary temperatures and its melting-point (about 400° C.) cadmium iodide exists in a stable α-form of density 5.670 at 30°/4° C, and an unstable β-form of considerably less density. If this is so, the properties of cadmium iodide, as usually described, refer to a mixture of the two forms in unknown proportions.

Cadmium iodide is said to melt at 385° C. and boil at 708°-719° C. It is readily oxidised by nitrogen peroxide at ordinary temperatures and by oxygen when heated. It dissociates slightly when heated in nitrogen, and is incompletely reduced by heating in a current of hydrogen. Its heat of formation is 48.5 Cal.

The salt is very soluble in water - a solution saturated at 18° C. contains 46.02 per cent, of cadmium iodide - and no hydrates have been crystallised from its solutions. Its heat of solution is –9.60 Cal.

Aqueous solutions of cadmium iodide are somewhat strongly ionised.

The experimental data indicate the presence of complex ions, and, according to McBain, dilute solutions contain the ions Cd^•• and I', and concentrated solutions the ions Cd^•• and CdI'3.

Solubility curves of the cadmium halides.

Cadmium iodide dissolves in alcohol, ether, and amyl alcohol. Its heats of solution in methyl, ethyl, and propyl alcohols are respectively 6.65, 4.31, and 2.66 Cal. Neither methylene iodide nor arsenic tribromide has a solvent action on it. Conductivity and osmotic data indicate that complex ions exist in its solutions in the alcohols.

The boiling-points of its solutions in quinoline indicate the formula CdI₂.

Cd₁₂I₂₃ is analogous to Cd₄Cl₇, and is prepared in a similar manner.

Cadmium iodide absorbs ammonia gas, at a gentle heat, to form the hexammoniate CdI₂.6NH₄. It is a white powder, decomposed by water, that loses its ammonia when heated. When a solution of the anhydrous salt in warm ammonia solution is cooled, colourless crystals of the di-ammoniate CdI₂.2NH₄ separate. They melt, on heating, with the loss of their ammonia, and are decomposed by water. When ammonia solution is added to a solution of cadmium iodide, the product is usually a mixture of this ammoniacal and a basic salt. By using dilute solutions the basic salt only is precipitated. The diammoniate can then be crystallised from the mother-liquor. By severely limiting the quantity of ammonia any formation of the ammoniacal salt is prevented. Some oxyiodides of cadmium have been described.

Dry cadmium iodide absorbs hydrogen iodide. It is also readily soluble in concentrated hydriodic acid solution. This solution, on cooling to - 25° C., deposits a crystalline precipitate which is probably an unstable acid cadmium iodide. Cryoscopic data indicate the presence of CdI₂.2HI in the solution. Colourless needles of CdI₂.HI.3H₂O have been obtained by saturating a pasty mass of cadmium iodide and its saturated solution with hydrogen iodide and cooling to -4.7° C. They are only stable in contact with the mother-liquor or in an atmosphere of hydrogen iodide.

The following salts have been prepared: 2NaI.CdI₂.6H₂O, 2NaI.2CdI₂.5H₂O; 2KI.CdI₂, KI.CdI₂.H₂O, 2KI.CdI₂.2H₂O; CsI.CdI₂.H₂O, 2CsI.CdI₂, 3CsI.CdI₂; NH₄I.CdI₂.½H₂O, NH₄I.CdI₂.H₂O, 2NH₄I.CdI₂.2H₂O; SrI₃.CdI₂.8H₂O; BaI₂.CdI₂.5H₂O; CdI₂.3HgI₂.

Cadmium iodide forms double salts with many iodides of substituted ammonium bases.