Calcium Chloride, CaCl₂

Calcium chloride, CaCl₂, occurs in nature as the mineral tachydrite, a double chloride of magnesium and calcium, CaCl₂,2MgCl₂,12H₂O. It is also present in sea-water. The part played by tachydrite in the formation of oceanic salt deposits has been studied by van't Hoff and his colleagues.

Calcium chloride may be obtained by direct combination of its elements. The reaction does not take place in the cold, but is very vigorous on the application of heat. The heat of formation is 190.44 Cal.

It can be prepared by neutralisation of calcium carbonate or oxide with hydrochloric acid and evaporation to dryness. To obtain the anhydrous salt the residue must be fused, but as the action of water vapour tends to decompose it and make it alkaline, hydrochloric acid gas followed by nitrogen must be passed over it.

Anhydrous calcium chloride is a white solid of density 2.15 at 20° C. The density in the molten state has also been determined. The following values at different temperatures have been found: -

The more recent determinations of the melting-point give values lying between 770° and 780° C.

Arndt and Willner found the decomposition potential at 800° C. to be 3.24 volts. According to Neumann and Bergve it is 2.85 volts at 585° C., and the temperature coefficient is 0.685×10^-3. The actual working voltage in the separation of the metal is very much higher.

The fused salt becomes phosphorescent on exposure to sunlight, and really constitutes Homberg's phosphorus.

Calcium chloride is very deliquescent, which makes it a valuable drying agent in the laboratory and in technology. According to McPherson, the porous granular calcium chloride, obtained by drying a little above 260° C., is a more rapid and more efficient desiccating agent than the fused salt, because the action is due to absorption rather than to hydration, and the porous form naturally exposes more surface. At 25° C. the vapour pressure of the fused salt is 0.35 mm. and of the granular 0.14 - 0.25 mm.

The solution of anhydrous calcium chloride in water is accompanied by considerable heat evolution. The molecular heat of solution is 17.48 Cal.

Equilibria in the system calcium cloride - water

Berthelot and Ilosvay stated that the heat of solution of recently fused salt is about 0.300 Cal. greater than that of the same salt two months later, apparently indicating the presence of another modification. An observation made by Richards and Honigschmid supports this conclusion. In the last stages of the preparation of pure calcium chloride for their atomic weight determinations, they heated it first in a current of gaseous hydrochloric acid and then replaced this by nitrogen. When the product was cooled, a clear glassy solid was at first formed, but this soon became flecked with white spots which increased until they covered the whole surface. They were caused by the shattering of the glassy chloride as it crystallised, sometimes almost explosively, at a lower temperature . The transition could be prevented by heating for a long time in nitrogen. The authors concluded that either traces of hydrochloric acid dissolved in the fused salt hastened crystallisation, or that traces of alkali formed by long ignition in nitrogen retarded it. The change was accompanied by considerable increase in volume, but the density was not determined.

Calcium chloride is obtained in large quantities as a waste product from certain industrial processes, notably the ammonia-soda process, the Weldon recovery process, and the manufacture of potassium chlorate. Many proposals have been made for its utilisation - for example, in the manufacture of pearl-hardening, as a metallurgical flux, or as a source of chlorine. For the latter purpose it may be heated with sand or clay under various conditions. So far, however, not much practical success has been attained. It may also be used for the production of chlorate electrolytically.

A few minor uses have been found for calcium chloride, for instance as a refrigerating agent, and, owing to its hygroscopic nature, for the laying of dust.

Calcium chloride forms four hydrates containing 6, 4, 2, and 1 molecules of water of crystallisation respectively. The behaviour of the system, calcium chloride: water, under varying conditions of temperature and concentration, was first explained by Roozeboom from the point of view of the phase rule. In fig. the temperatures of saturation with respect to the different solid phases are plotted against the concentrations of the saturated solutions. A represents the freezing-point of pure water. The addition of calcium chloride lowers the freezing-point until the point B is reached. The line AJB represents the temperatures of equilibrium between ice and solution for concentrations less than that of point B. On further addition of calcium chloride the freezing-point begins to rise again, but now the solid phase in contact with the solution is calcium chloride hexahydrate, CaCl₂.6H₂O, and the conditions of equilibrium are represented by the curve BD. It is customary to describe this as the solubility curve rather than the freezing-point curve of the hexahydrate. B is a point on both curves, and therefore represents the temperature at which both ice and the hexahydrate are in equilibrium with the solution. It is, in fact, a eutectic point, or, as the particular case of ice in contact with a salt, the cryohydric point of the system.

The molecular lowering of the freezing-point of water by calcium chloride decreases until a concentration of about 0.1 normal is reached, after which it increases with concentration. The first change is probably due to diminishing ionisation of the salt, and the second to hydrate formation in solution.

Ignoring for the present the curve GR, it may be seen that BD passes through a maximum at point M, where the compositions of the solid and liquid phases are identical. M therefore represents the melting-point of calcium chloride hexahydrate. Such a point is called a " congruent melting-point."calcium and its compounds.

Between M and D the liquid is richer in calcium chloride than the solid. D is a second eutectic at which a tetrahydrate begins to separate out, and the curve DF is the solubility or freezing-point curve for this compound. At D the composition of the solution is 112.8 parts of calcium chloride to 100 of water. More or less parallel to DF is a second curve CR, which also represents the solubility curve of a tetrahydrate, so that there must be two modifications of the tetrahydrate, an α- and a β-form. These were first distinguished by Roozeboom. The β-form, separating along DF, is more soluble than the α-compound, and is, therefore, a metastable hydrate. By inoculating a solution, of which the temperature and pressure are given by any point inside the region CRFD, with a minute crystal of α-CaCl₂.4H₂O, the latter will begin to separate out until the temperature and composition of the remaining solution correspond to a point on the curve CR. The melting-point of the hexahydrate is, therefore, in a metastable region, and represents a case of suspended transformation, illustrating the general tendency of a system to pass through an unstable phase before reaching a state of stable equilibrium. Under stable conditions the α-tetrahydrate would begin to separate when the composition had reached 100.6 parts of calcium chloride per 100 parts of water, and when the temperature was still a little under the melting-point of the hexahydrate.

Supersaturation of both the α- and β-tetrahydrate solutions can be obtained. The solubility curve of the α-compound may be extended to point G, where the concentration of the liquid phase is 91 grm. of calcium chloride per 100 grm. of water at 20° C., and the β-curve to E, concentration 104.5 grm. at 20° C. In either case inoculation of the solution with the hexahydrate causes rapid separation of the latter.

At R and F, that is at temperatures 45.3° and 38.4° C., and solubilities 130.2 and 127.5 respectively, the α- and β-tetrahydrates are in equilibrium with the dihydrate of which the solubility curve is represented by FL. At a temperature of 175.5° C., and solubility 297 grm. of calcium chloride per 100 grm. of water, the monohydrate appears, but the solubility curve of the anhydrous salt does not begin until a temperature of over 260° C. is reached.

The solubility of calcium chloride is considerably diminished by the presence of hydrochloric acid, and the transition-points are lowered. Migration experiments in hydrochloric acid solutions indicate the existence of complex anions of the type (CaCl₂)_yCl_x.

The following values have been found for the boiling-points of solutions of different concentrations:

Grams CaCl2 in grams water.	6	16.5	25	41.5	69.0	101	137.5	178	220	268	292	305
Boiling-point, °C.	101	103	105	110	120	130	140	150	160	170	175	178

The properties of calcium chloride solutions have also been investigated from the point of view of density, freezing-point, vapour pressure, specific heat, viscosity, surface tension, electrical conductivity, compressibility, and refractive index.

6H₂O.Calcium Chloride Hexahydrate, CaCl₂.6H₂O, forms hexagonal prisms by crystallisation from the saturated solution at ordinary temperatures. Its density at 22° C. is 1.7182. The heat of formation of the hydrate from the anhydrous salt and water is 21.750 Cal. The heat of solution is -4.596 Cal. At 17.9° C., or -4.562 Cal. at 22° C. The heat of fusion is 11.417 Cal.

Tammann studied the effect of pressure on the melting-point of the hexahydrate. His results are contained in the table on following page.

4H₂O.Lefebvre and Hammerl each discovered a Calcium Chloride Tetrahydrate, CaCl₂.4H₂O. These were at first considered to be identical, since the solubilities were not determined. Roozeboom distinguished between them. Hammerl's was the α-form, which belongs to the rhombic system. It can be obtained by repeated fusion of the hexahydrate. The unstable β-form separates in large transparent plates on cooling a solution containing more than one molecule of calcium chloride to six molecules of water.

2H₂O.Calcium Chloride Dihydrate, CaCl₂.2H₂O, may be obtained by the addition of hydrochloric acid to an aqueous solution of calcium chloride.

H₂O.Calcium Chloride Monohydrate, CaCl₂.H₂O, was first described by Roozeboom. As the vapour pressure of saturated calcium chloride solution, and, therefore, of the solid phase, is greater than one atmosphere at 175.5° C., the transition-point of the di- into the mono-hydrate, it is evident that, to obtain the latter, a higher hydrate must be heated under pressure, otherwise the anhydrous salt will be obtained.

Calcium chloride forms, with ammonia, four different compounds in which 1 molecule of the salt is combined with 8, 4, 2, and 1 molecules of ammonia respectively. Huttig has studied the dissociation pressure of ammonia at different temperatures and the heat of formation for all these compounds. Similar compounds, sometimes hydrated, are formed with a number of amino-compounds: - for example, with phenylhydrazine and aniline, with acetamide, with thiocarbamide, with carbamide and asparagine, and with α-amino-acids polypeptides. It has been suggested that the compound with carbamide, CaCl₂.4CO(NH₂)₂, which is stable in air and soluble in water, and has a melting-point of 158°-160° C., might be of value in subcutaneous injections for hay fever and asthma.

Calcium chloride forms addition compounds with the alcohols. On evaporating a solution in ethyl alcohol at a low temperature rectangular plates of 2CaCl₂.7C₂H₅OH are deposited. The compounds CaCl₂.3C₂H₅OH and CaCl₂.CH₃OH have also been separated, as well as a mono- and a di-acetone compound, and compounds with isobutyl and amyl alcohols.

Antonow obtained a number of addition compounds with hydroxylamine, 2CaCl₂.3NH₂OH.6H₂O, 2CaCl₂.5NH₂OH.4H₂O, CaCl₂.2NH₂OH. 2H₂O, CaCl₂.2NH₂OH.H₂O, CaCl₂.2NH₂OH, 4CaCl₂.NH₂OH.20H₂O, and CaCl₂.3NH₂OH.HCl.

Calcium chloride forms a number of double salts with other chlorides. A list is given below: - 4NaCl.CaCl₂ (Not separated. Existence supposed from heat of solution of mixed salts.); KCl.CaCl₂. (Melting-point, 754° C. - 740° C.); 2KCl.CaCl₂; 2KCl.3CaCl₂ (Melting-point, 725° C.); 2CsCl.CaCl₂ (Slender colourless prisms.); CaCl₂.BaCl₂ (Melting-point, 631° C.); CaCl₂. BaCl₂. SrCl₂. (Melting-point, 500° C.); CaCl₂.ZnCl₂.5.5H₂O. and 2CaCl₂. ZnCl₂.6H₂O. (Hygroscopic crystals.); 2TlCl₃.CaCl₂.6H₂O (Large colourless crystals.); PbCl₄.16CaCl₂. (Decomposed in dilute solution, forming PbO₂.); 5HgCl₂.CaCl₂.8H₂O (Decomposed by water.); 2HgCl₂. CaCl₂.6H₂O (Deliquescent.); 2CdCl₂.CaCl₂; 2CuCl₂.CaCl₂; CaCl₂. SnCl₄.6H₂O (Colourless deliquescent rhombohedra.); CaCl₂.3SeOCl₂.

The systems CaCl₂: KCl: H₂O and CaCl₂: MgCl₂: H₂O have been studied at 25° C.

A double compound of mercuric cyanide and calcium chloride, 2Hg(CN)₂.CaCl₂.6H₂O, is known. The appearance and gradual increase in quantity of the cyanide ion in the solution on warming leads to the conclusion that transformation to the compound Hg(CN)₂.Ca(CN)₂.HgCl₂, takes place.

At low temperatures chlorine is more soluble in calcium chloride solution than in pure water, which may indicate the formation of a perchloride. Determinations of the freezing-point lowering produced by chlorine in alkaline earth chloride solutions lead to the same conclusions. No solid perchlorides of the alkaline earths have been obtained.