Bismuth Orthophosphate, BiPO₄

The normal salt only, BiPO₄, is obtained by the addition of disodium phosphate to a nitric acid solution of bismuth nitrate. Acid salts do not appear to have been prepared. The microscopic, monoclinic prisms of the normal salt have a density of 6.323 at 15° C. The substance is slightly soluble in bismuth trichloride, and from molecular weight determinations in this solvent the formula appears to be BiPO₄. It is reduced by heating in a current of hydrogen, but not by carbon monoxide. It is not hydrolysed by water, even after prolonged boiling. A trihydrate, BiPO₄.3H₂O, has been prepared, and also a white, insoluble basic salt, 2BiPO₄.3Bi₂O₃. Investigation of the solubility of bismuth orthophosphate in hydrochloric acid at 25° C. indicated that it was conditioned by the concentration of the chloride ion and of the hydrogen ion, but was independent of the concentration of phosphoric acid. It is suggested that the main reaction which occurs on solution may be represented by

BiPO₄ + 3HCl = H[BiCl₃.H₂PO₄]

It has been stated that pyrophosphate and metaphosphate of bismuth cannot be prepared in the wet way owing to their rapid conversion into orthophosphate by the action of water.