Barium Orthophosphates

Tribarium Orthophosphate, Ba₃(PO₄)₂, may be formed as large transparent plates belonging to the cubic system, and of density 4.1 at 16° C., by fusing barium oxide with sodium metaphosphate at red heat, with or without sodium or barium chlorides. It is obtained as the monohydrate by precipitation of excess of barium chloride by trisodium phosphate, or by disodium phosphate in alkaline solution. In the latter case excess of barium hydroxide solution should be added to prevent the formation of a double salt. When trisodium phosphate is used a colloidal precipitate is first formed without perceptible heat change. Afterwards, crystallisation takes place with the evolution of heat. This has resulted in considerable variation in the values obtained for the heat of formation. It is soluble in most acids, giving the mono- or di-barium phosphates. Its use as a pigment has been suggested.

A naturally occurring barium apatite, 3Ba₃(PO₄)₂.BaCl₂, corresponding to calcium apatite, has been found, and can be prepared artificially by fusing together barium phosphate and barium chloride. There are also a bromapatite, an iodapatite, and a fluorapatite.

Dibarium Orthophosphate, BaHPO₄, is obtained by precipitating barium chloride with disodium phosphate. It is also formed by the action of phosphoric acid on tribarium phosphate, or of water on monobarium phosphate. It forms a white crystalline powder, probably containing 1 molecule of water of crystallisation, or is obtained as barium monetite in rhombic prisms, of density 4.165 and containing no water of crystallisation.

It is soluble in acids, including sulphurous acid, and also in solutions of ammonia salts.

Monobarium Orthophosphate, BaH₄(PO₄)₂, forms anhydrous triclinic crystals of density 2.8 – 2.9, when an acid solution of barium phosphate is evaporated. With hydrochloric acid the double salt, 4BaH₄(PO₄)₂.BaCl₂, is formed.

For reasons already mentioned under calcium and strontium phosphates, very little confidence can be placed in the solubility determinations for the different barium phosphates.