Strontium Carbonate, SrCO₃

Strontium Carbonate, SrCO₃, occurs in nature as the mineral strontianite, of density 3.4 – 3.7, and isomorphous with aragonite. It may be formed by precipitation from strontium chloride solution by ammonium carbonate, or by the action of carbon dioxide on heated strontium hydroxide, or strontium oxide at red heat.

For the technical production several methods may be employed: -

1. Fusion of celestine with sodium carbonate.
2. Boiling celestine with a concentrated solution of ammonium carbonate.
3. Action of a mixture of calcium chloride, carbon, and metallic iron on celestine at red heat. The sulphate is thus converted into the carbonate by ammonium carbonate, or by milk of lime and carbon dioxide.
4. Boiling with magnesium chloride a solution of strontium hydrosulphide through which carbon dioxide is passing.

The precipitated carbonate can be transformed into strontianite by crystallisation from solution in fused alkali chlorides.

The specific heat of strontianite is 0.1445, and of the precipitated carbonate 0.14483. The density of the precipitated carbonate is 3.55 – 3.62.

Strontium carbonate is less readily decomposed by heat than calcium carbonate. Its dissociation pressure reaches 1 atmosphere of carbon dioxide at about 1155° C., but the rate of decomposition only becomes rapid when a temperature of 1255° C. is reached.

The heat of formation of crystallised strontium carbonate from strontium oxide and carbon dioxide is 57.3 Cal.

Strontium carbonate is dimorphic, the transition point to the α-form being 920°-929° C. α-Strontium carbonate belongs to the hexagonal system. The melting-point in an atmosphere of carbon dioxide under pressure is 1497° C.

Strontium carbonate is only very slightly soluble in water, the solubility at 18° C. being 11 mgm. per litre, and the solubility product [Sr²⁺]×[CO₃^2-] = 1.567×10^-9. This value was deduced from experiments carried out under a pressure of carbon dioxide varying from 0.05 – 1.1 atmosphere. A saturated solution of strontium carbonate under ordinary atmospheric conditions reacts alkaline, owing to hydrolysis. The solubility is increased by the presence of carbon dioxide in the water, through the formation of the acid carbonate in solution. It is also increased by the presence of ammonium chloride or nitrate.

A colloidal solution of strontium carbonate may be formed by passing carbon dioxide through a methyl alcoholic solution of strontium oxide. No separation of the gel takes place.

By the addition of a solution of strontium chloride to a saturated solution of potassium carbonate a transparent jelly-like mass, consisting of the double salt, SrCO₃.K₂CO₃, is obtained, and this, on stirring, becomes opaque and finally granular.

The crystalline double salts, SrCO₃.K₂CO₃ and SrCO₃.Na₂CO₃, may be prepared by fusion.