Cadmium Chloride, CdCl₂

Anhydrous Cadmium Chloride, CdCl₂, is a very stable salt, and can be prepared by dehydrating one of its hydrates or by heating the oxide in a current of chlorine. Baxter and Hines obtained a pure salt by igniting the double chloride NH₄Cl.CdCl₂. It has also been prepared pure by dissolving redistilled cadmium in hydrochloric acid, evaporating to dryness with excess of the acid, and distilling in a current of hydrogen chloride, and by fusing cadmium bromide in a current of chlorine. The pure chloride can always be rendered anhydrous by heating in dry hydrogen chloride. The transparent pearly mass melts at about 568° C., boils at about 964° C., and condenses, when sublimed, in transparent micaceous laminae.

The fused salt has a density of 4.049 at 25° C. (referred to water at 4° C.), and when strongly heated reacts with oxygen or hydrogen; it does not react with nitrogen peroxide.

[Cd] + (Cl₂) = [CdCl₂]+93.2 Cal. (+93.0 Cal.).

Molten cadmium chloride conducts electricity.

It dissolves somewhat in methyl and ethyl alcohols, slightly in acetone, and the depression of the freezing-points of its solutions in urethane indicates the formula CdCl₂.

The boiling-points of its solutions in quinoline also indicate a normal formula.

Hydrates of cadmium chloride with 5, 4, 2½, 2, and 1 molecules of water have been described, but those with 5 and 2 are doubtful.

Sulphuric acid added to a solution of cadmium chloride precipitates, according to Viard, the monohydrate or the dihydrate according to the amount of acid added. Needle-shaped crystals of the monohydrate separate from a 58 per cent, solution evaporated at 40° C., or by crystallising either a neutral solution of the carbonate in hydrochloric acid or a saturated solution of cadmium chloride saturated with hydrogen chloride. It is dehydrated completely at 120°-130° C. According to Viard, the anhydrous salt becomes the monohydrate in ordinary air, and in dry air the change is reversed.

Hauer described a dihydrate. It has been said to separate when cold solutions of cadmium chloride are evaporated, the monohydrate separating at higher temperatures, and the temperature of transition from the dihydrate to the monohydrate has been given as 34.1° C. Viard says that the dihydrate becomes the monohydrate when exposed to air. According to Dietz, monoclinic crystals of CdCl₂.2½H₂O separate from concentrated solutions at ordinary temperatures. This hydrate seems to exist in two modifications, and its transition temperature into the monohydrate appears to be about 34° C. The crystals may be thick tables or short pyramids. Sudhaus claims to have confirmed the existence of both CdCl₂.2½H₂O and CdCl₂.2H₂O.

The tetraliydrate separates in prismatic crystals from a 56.58 per cent, solution of CdCl₂ at -10° C. Above -5° C., though it may exist in a labile form, it passes into CdCl₂.2½H₂O.

The pentahydrate, said to be obtained by crystallisation below -14° C., has not been confirmed.

[CdCl₂] + 400Aq. = CdCl₂.Aq. +3.382 Cal. (prepared at 200° C. The fused salt gave +3.211 Cal.).

[CdCl₂.H₂O]+400Aq. = CdCl₂.Aq. +0.625 Cal.

[CdCl₂.2H₂O]+400Aq. = CdCl₂.Aq. -2.284 Cal.

The solubility of cadmium chloride in gram CdCl₂ per 100 grm. solution has been determined as follows: -

CdCl2.4H2O.
Temperature, ° C.	-9	0	+10	+15
Solubility	43.58	49.39	55.58	59.12

CdCl2.2½H2O.
Temperature, ° C.	-10	0	+18	+30	+36
Solubility	44.35	47.37	52.53	56.27	57.91

CdCl2.H2O.
Temperature, ° C.	10	20	40	60	80	100
Solubility.	57.47	57.35	57.51	57.77	58.41	59.32

Cadmium chloride is strongly ionised in aqueous solution. Hydrolysis is slight, though it becomes noticeable at higher temperatures.

By heating anhydrous cadmium chloride and metallic cadmium in vacuo or in an atmosphere of nitrogen to the fusing-point of the metal, a garnet-red-coloured liquid is produced that solidifies to a greyish- white mass. Its composition corresponds to the formula Cd₄Cl₇, and it may be a loose compound of CdCl and CdCl₂ (CdCl.3CdCl₂), or a solution of the former in the latter. It is a powerful reducing agent, and water decomposes it into cadmium chloride and cadmous hydroxide, CdOH. It decomposes into cadmium metal and cadmium chloride when strongly heated.

Anhydrous cadmium chloride absorbs ammonia gas with increasing rapidity. The hexammoniate, CdCl₂.6NH₄, is produced if the temperature is kept well below 0° C. Ammoniacal compounds are obtained by dissolving cadmium chloride in ammonia solution and passing in ammonia gas. If the temperature is kept at 0° C. small transparent crystals of CdCl₂.6NH₄ separate out. This substance, which is only slightly soluble in water, cannot exist above 62° C., and decomposes at 100° C. into CdCl₂.2NH₄, which is quite stable under ordinary conditions and only begins to decompose at 210° C. CdCl₂.2NH₄ crystallises out under ordinary conditions from solutions of cadmium chloride in ammonia, and is also obtained as a white crystalline powder by passing ammonia gas into an alcoholic solution of cadmium chloride. When heated in a glass retort a transparent, yellowish, glassy mass of CdCl₂.NH₄ distils over.

The unstable acid salt, 2HCl.CdCl₂.7H₂O, is precipitated by saturating a cold saturated solution of cadmium chloride with hydrogen chloride.

Some oxychlorides of cadmium have been described. Hexagonal crystals of Cd(OH)Cl, of density 4.56 at 15° C., were obtained by heating cadmium chloride with powdered marble at 200° C. in sealed tubes. Cadmium hydroxide can be substituted for the marble.

The following double chlorides of cadmium are known: LiCl.CdCl₂.3iH₂O; 2NaCl.CdCl₂.3H₂O and 2NaCl.CdCl₂; KCl.CdCl₂, KCl.CdCl₂.H₂O, 2KCl.CdCl₂, 4KCl.CdCl₂; RbCl.CdCl₂, 4RbCl.CdCl₂; CsCl.CdCl₂, 2CsCl.CdCl₂; NH₄Cl.CdCl₂, 4NH₄Cl.CdCl₂, NH₄Cl.2CdCl₂; CuCl₂.CdCl₂.4H₂O, 4CuCl₂.CdCl₂, 2Cu₂Cl₂.CdCl₂; BaCl₂.CdCl₂.4H₂O. BaCl₂.CdCl₂.5H₂O; SrCl₂.2CdCl₂.7H₂O, CaCl₂.2CdCl₂.7H₂O; HgCl₂. 2CdCl₂.12H₂O; 2CdCl₂.MnCl₂.12H₂O, 2CdCl₂.FeCl₂.12H₂O, 2CdCl₂. CoCl₂.12H₂O, CdCl₂.2NiCl₂.12H₂O, 2CdCl₂.NiCl₂.12H₂O.