Strontium Arsenates

Strontium Orthoarsenate, Sr₃(AsO₄)₂, may be prepared by neutralising aqueous arsenic acid with strontia-water, or by adding ammonia to a solution of strontium monohydrogen arsenate in dilute hydrochloric acid; in either case the precipitate is separated and well washed with water. The salt may also be obtained by fusing together equal quantities of sodium chloride and sodium orthoarsenate and adding the required proportion of strontium oxide; after cooling, the sodium salts are removed by washing with water. Colourless rhombic crystals of density 4.601 are obtained, readily soluble in dilute acids. The salt melts at 1635° C. When heated at 400° to 450° C. in hydrogen, reduction occurs to give strontium oxide, arsenic and water.

Strontium Monohydrogen Arsenate, SrHAsO₄, may be prepared as follows: Strontium hydroxide is added slowly and with stirring to a solution of arsenic acid (containing 75 g. As₂O₅ in 1.5 litres H₂O) until a slight turbidity appears. Sodium hydroxide, free from carbonate, is then added to bring the pH to about 5.4, the faintly turbid solution heated in portions of about 150 c.c. and stirred until the monohydrogen orthoarsenate separates in fine crystals. The supernatant liquid is removed, again treated with sodium hydroxide and heated to give a further yield. The crystals are washed with alcohol and dried in air. They usually contain a slight excess of arsenic acid.

The salt may also be obtained by the action of hot water on strontium dihydrogen orthoarsenate or on the pyroarsenate; or by the action of ammonia on a hot solution in hydrochloric acid of the precipitate obtained by mixing solutions of strontium chloride and sodium monohydrogen orthoarsenate. In the last case the anhydrous salt crystallises out, but if the mother liquor is kept in an atmosphere of ammonia, the monohydrate, SrHAsO₄.H₂O, crystallises out. The density of the latter is 3.606 at 15° C. and of the anhydrous salt 4.035. The monohydrate becomes anhydrous at 130° C. and at a higher temperature yields the pyroarsenate.

Strontium Dihydrogen Orthoarsenate, Sr(H₂AsO₄)₂, may be prepared by adding 24 g. of strontium hydroxide to an aqueous solution of arsenic acid (containing 60 g. As₂O₅ in 90 c.c.) and heating until a clear solution is obtained. On prolonged boiling, much of the dihydrogen salt crystallises out and, after filtration, a further crop may be obtained from the mother liquor. The crystals should be washed with alcohol and dried. Like the monohydrogen salt they usually contain a slight excess of arsenic acid.

The dihydrate, Sr(H₂AsO₄)₂.2H₂O, may be obtained by treating strontium carbonate with an excess of arsenic acid. The crystals lose their water of crystallisation at 240° to 250° C. and on further heating yield strontium metarsenate, Sr(NaO₃)₂.

Strontium Pyroarsenate, Sr₂As₂O₇, is formed when the monohydrogen orthoarsenate is heated above 360° C. or when potassium monohydrogen orthoarsenate is fused with strontium oxide or carbonate. It is slightly soluble in mineral acids.

A basic arsenate of composition 3Sr₃(AsO₄)₂.Sr(OH)₂ has been obtained as a fine flaky precipitate by the action of sodium hydroxide and strontia-water on an acidified solution of sodium monohydrogen orthoarsenate, care being taken to ensure the absence of carbonate.

Salts of the type MSrAsO₄ (where M = NH₄, K or Na) have been obtained, as also have the following pyroarsenates, SrCuAs₂O₇ and SrHgAs₂O₇.