Cobaltous Chloride, CoCl₂

Cobaltous Chloride, CoCl₂, results in the anhydrous condition when metallic cobalt or its sulphide is heated in chlorine; by heating the hydrated salt to 140° C., or by calcination of the chloropentammine chloride, [CoCl.5NH₃]Cl₂, in either case in a current of hydrogen chloride; by distilling a solution of the hexahydrated chloride in anhydrous ethylene glycol under reduced pressure; and finally by treatment of a solution of the hydrated chloride with gaseous hydrogen chloride. By the first of these methods blue crystalline scales are obtained which admit of purification by sublimation in a current of chlorine or carbon dioxide. Density, 2.937.

At red heat moist hydrogen reduces it to metallic cobalt. Dry hydrogen acts less readily, and a portion of the chloride sublimes. Magnesium likewise reduces it at high temperatures. The salt dissolves in alcohol to a blue solution, which becomes violet and later rose-coloured upon addition of water. Upon exposure to moist air, the anhydrous salt takes up water, forming first the di-hydrate and then the tetrahydrate.

Aqueous solutions of cobalt chloride may be obtained by dissolving the anhydrous salt in water, or the oxides or carbonate in hydrochloric acid. Upon concentration in the warm the hexahydrate, CoCl₂.6H₂O, is obtained as dark red monoclinic prisms of density 1.84. These melt at 60° C. in their own water of crystallisation. They lose four molecules of water either when warmed to 50° C. over sulphuric acid, or when kept for a prolonged period in vacuo over the same, the resulting dihydrate, CoCl₂.2H₂O, being rose-coloured. The dihydrate is also obtained by precipitation from solution on addition of concentrated hydrochloric acid.

By raising the temperature to 100° C. one further molecule of water is expelled, a violet monohydrate, CoCl₂.H₂O, remaining. The mono-hydrate may also be prepared by concentrating a solution of the hexahydrate in absolute alcohol at 95° C. The salt crystallises out in pale violet needles. At 110° to 120° C. the anhydrous salt is obtained as a blue mass.

The tetrahydrate, CoCl₂.4H₂O, is obtained by allowing either the anhydrous salt or the dihydrate to remain exposed to moist air. Further exposure yields the hexahydrate.

The solubility of cobalt chloride in water is as follows

Temperature ° C.	- 4	+ 7	11	12	25	34	41	45	49	56	78	94	96	112
Grams of CoCl2 per 100 grams solution	28.0	31.2	31.3	32.5	34.4	37.5	39.8	41.7	46.7	48.4	48.8	50.5	51.2	52.3

..

. .

. The solubility curves of cobalt chloride.

In the cold the saturated solution is rose-coloured, like the crystalline hexahydrated salt. On warming it becomes violet between 25° and 50° C., above which latter temperature it is blue. This is explained by some as due to a change in hydration of the cobalt salt in solution from the red hexahydrate, through the violet monohydrate, to the blue anhydrous salt. Certainly the changes in colour correspond to breaks in the solubility curve as shown in Fig. A similar change in colour from red to blue likewise occurs with increase of concentration of the solution.

This simple hydration theory cannot explain all the known phenomena, as, for example, the opposite effects of calcium chloride and zinc chloride on the colours. Engel therefore assumed that the observed colours were due to certain double salts present in the solutions. In the case of pure cobalt chloride, hydrolysis was supposed to occur on heating the solution, the hydrochloric acid liberated uniting with unchanged cobalt chloride; and as an explanation of the colour change this is almost certainly incorrect. Ostwald suggested a simple ionic explanation, namely, that the red colour is that of the cobalt cation, and the blue that of the undissociated salt. This is certainly not a complete explanation, and seems to necessitate a very marked decrease in ionisation with rise of temperature, which experiment, so far, does not support.

Donnan and Bassett suggest that cobalt chloride solution, in addition to simple ions Co^•• and Cl', contains complex anions CoCl₃' or CoCl₄'', there being two equilibrium reactions in solution, as follow:

1. CoCl₂ ⇔ Co^•• + 2Cl'
2. CoCl₂ + 2Cl' ⇔ CoCl₄''
   CoCl₂ + Cl' ⇔ CoCl₃'

Granting that the cobalt ion in solution is red, and that the complete anion is blue and increases in concentration with rise of temperature, the observed colour changes are readily explained qualitatively; for the complex ions will break down with dilution, and also if there be added the chloride of a metal with a greater tendency to form complex ions, e.g. zinc chloride, while the formation of the complex ions CoCl₄" (or CoCl₃') will be augmented by increasing the concentration of chlorine ions, i.e. by adding hydrochloric acid or the highly dissociated chloride of a metal like calcium, which has little or no tendency to form complex ions. Donnan and Bassett found by electrolytic experiments that the blue solutions contain a blue anion and the red solutions a red cation, and further supported their view by other physico-chemical data; Denham has furnished additional corroborative evidence. Donnan and Bassett conclude that when the cobalt atom is in close association with chlorine, e.g. in CoCl₂ and CoCl₃' or CeCl₄'', a blue colour is developed, but that when, by dissociation or the presence of water molecules this close association is broken, e.g. in Co^•• and CoCl₂.6H₂O, a red colour is observed.

The work of Vaillant and Lewis has shown that the colour changes cannot be quantitatively interpreted without considering that water plays a definite role in the reactions. It follows that if Donnan and Bassett's views on complex ion formation be correct, water is either produced or used up when cobalt chloride and chloride ion interact; thus, for example, where the ion CoCl₃' is assumed for simplicity:

CoCl₂.nH₂O + Cl'.mH₂O ⇔ CoCl₃'.pH₂O + (n + m - p)H₂O.

Kotschubei has determined the extent of the hydration of the cobalt ion in cobalt chloride solution by the electrolytic method briefly indicated in the first volume of this Series, and finds that the hydration increases with the dilution. He concludes from his own and other workers' experiments that the hydration diminishes with rise of temperature; also that the hydration of the cobalt chloride molecule varies in the same manner as that of the cobalt ion. He doubts the existence of complex ions in solutions of cobalt chloride, and considers that in the blue solutions formed by the addition of hydrochloric acid, calcium chloride, etc., the evidence for the existence of complex ions is inconclusive. On the other hand, he admits the presence of complex ion in the red solutions containing mercuric chloride, etc., and formulates the ions as probably being:

, , etc.

Cobalt chloride dissolves in alcohol to a blue solution, which becomes violet and then red on the addition of water. The alcoholic solution becomes red when very diluted or when cooled much below 0° C. The blue colour in these solutions has been attributed to the formation of double compound by Engel, to complex ion formation by Donnan and Bassett, and to both these causes by Kotschubei.

The change in colour undergone by cobalt chloride on varying the temperature is taken advantage of in the preparation of sympathetic inks.

The molecular weight of cobalt chloride as determined by the freezing-point method, with urethane as solvent, corresponds to the double formula, Co₂Cl₄ (compare ferrous chloride), but the results obtained by the boiling-point method indicate that under those conditions the molecule is single, namely, CoCl₂.

The hexammoniate, CoCl₂.6NH₃, is produced by passing ammonia into a concentrated aqueous solution of cobalt chloride in the entire absence of air, or by passing it into a saturated solution of cobalt chloride in methyl acetate. It yields dark rose-red octahedral crystals.

An unstable acid chloride, CoCl₂.HCl.3H₂O, is obtained as blue crystals by cooling to -23° C. a saturated solution of cobalt chloride in aqueous hydrogen chloride. CoCl₂.LiCl.3H₂O and CoCl₂.NH₄Cl.6H₂O have also been obtained. A blue alcoholate, CoCl₂.2CH₃OH, is known. With iodine trichloride the complex, CoCl₂.2ICl₃.8H₂O, is formed as orange-red crystals.