Strontium Chloride, SrCl₂

Strontium Chloride, SrCl₂, was first prepared by Crawford in 1790. It may be obtained by direct combination of its elements, by the action of chlorine or hydrochloric acid on strontium oxide at high temperatures, or by neutralising strontium hydroxide, carbonate, or sulphide with hydrochloric acid solution, evaporating and fusing the residue in a current of hydrochloric acid gas. On a large scale it is prepared by the action of calcium chloride on strontium carbonate. It may be obtained in a gelatinous form by precipitation under suitable conditions.

The heat of formation of strontium chloride from its elements is 195.66 Cal. The melting-point is 874° C. The density is 2.95 – 3.05 at ordinary temperatures, and at the melting-point 2.77. The specific heat is 0.1199. The following values have been found for the specific electrical conductivity of the fused salt: -

Temperature, ° C.	873	900	950	1000	1050	1100
Ohms-1 per c.c.	1.92	1.98	2.14	2.29	2.43	2.56

The decomposition potential of strontium chloride at 880° C. is 3.4 volts, and of the salt dissolved in potassium chloride and calcium fluoride to bring down the temperature of fusion, 3.0 volts at 615° C., whilst the temperature coefficient is 0.715×10^-3.

Strontium chloride is decomposed by superheated steam, giving hydrochloric acid, and when heated in contact with air it loses chlorine, forming the oxide. Bromine partially replaces the chlorine at high temperatures.

Anhydrous strontium chloride is very hygroscopic, but not quite to the same extent as calcium chloride. It dissolves in water with the evolution of 11.14 Cal. of heat per gram-molecule.

The following values have been found for the solubilities at different temperatures: -

Temperature, ° C.	-17	-11	-5	-1	2	7	18	35	55	64	75	98	105	118	132	153	175	215	250
Grams SrCl2 per grams solution	26.5	28.6	29.3	30.8	31.3	31.7	33.7	37.8	43.8	46.4	46.5	49.6	50.7	52.0	52.5	55.7	60.5	64.1	67.3

There seems to have been some doubt as to the nature of the solid phase in the neighbourhood of 75° C., so that probably equilibrium was not reached.

Harkins found that hydrochloric acid diminished the solubility of strontium chloride from 5160 milli-equivalents per litre at 0° to 425 milli-equivalents in a solution containing 5275 milli-equivalents of hydrochloric acid per litre.

The solubility in alcohol is not very great.

The boiling-points of concentrated aqueous solutions have been studied by Gerlach. A solution containing 487.5 grm. of the anhydrous chloride in 100 grm. of water has a boiling-point of 117° C. Freezing-point determinations of both dilute and concentrated solutions have been made. Aqueous solutions have also been studied from the point of view of surface tension, compressibility, viscosity, electrical conductivity, density, and vapour pressure. The equilibria between strontium and potassium amalgams of different concentrations and solutions of strontium and potassium chlorides have been investigated.

Vapour pressure measurements indicate only a hexa- and a di-hydrate, although Etard suggested the possibility of a mono-hydrate.

Strontium Chloride Hexahydrate, SrCl₂.6H₂O, crystallises from solutions at ordinary temperatures. It forms colourless hexagonal needles isomorphous with calcium chloride hexahydrate. The density is 1.954. The heat of formation of the hexahydrate from the anhydrous salt and water is 18.64 Cal., and the heat of solution is -7.50 Cal. The vapour pressure at 25° C. is 8.37 mm.

The hexahydrate is very soluble in methyl alcohol, much more so than in ethyl alcohol.

The temperature of transition to the dihydrate has been carefully determined by Richards and Yngve as 61.34° C. It is lowered by contact of the salt with a solution containing hydrochloric acid; at 25° C. the two hydrates are in equilibrium with a solution containing 2.11 per cent, of strontium chloride and 27.14 per cent, of acid.

Strontium Chloride Dihydrate, SrCl₂.2H₂O, crystallises from solutions above the transition point in slightly deliquescent prisms. It may be obtained at ordinary temperatures from a solution of strontium chloride in concentrated hydrochloric acid, or by passing hydrochloric acid gas into a saturated solution of strontium chloride. The heat of hydration of anhydrous strontium chloride to the dihydrate is 9.06 Cal.

There appears to be no record of the temperature of transition from the dihydrate to the anhydrous form, but it is not below 130° C.

Anhydrous strontium chloride absorbs dry ammonia, forming a voluminous white powder of approximate composition, SrCl₂.8NH₃. The dissociation pressure is 100 mm. At 3.5° C. There is also a monammoniate, SrCl₂.NH₃, which has a vapour pressure of 100 mm. At 45.5° C. The heat of formation of the octammoniate is 9.98 Cal., and of the monammoniate 10.5 Cal. at 0° C., 12.5 at 78° C., and 13.4 at 108° C. Addition compounds are also formed with hydroxylamine, 2SrCl₂.5NH₂OH.2H₂O and 2SrCl₂.9NH₂OH.3HCl.H₂O, and one with glycine, SrCl₂.2NH₂CH₂COOH.3H₂O.

On the following page is a list of the double salts formed by strontium chloride.

It has been suggested that double salts in solution may contain the ion SrCl₄^2-.

Strontium chloride forms mixed crystals or solid solutions, but not compounds with calcium chloride, barium chloride, and lead chloride.