Zinc Orthophosphate

The greyish-white mineral hopeite, Zn₃(PO₄)₂.4H₂O, of density 2.75-2.8 and hardness 2.5-3.9, crystallises in the orthorhombic system. It seems to occur in two modifications, and there is a parahopeite which is harder and denser than hopeite itself.

Crystals resembling the naturally occurring substance are obtained by prolonged heating of zinc oxide with phosphoric acid and much water at 180° C. This tetrahydrate has also been obtained in shining orthorhombic plates by heating a solution of zinc carbonate in phosphoric acid, or making a 50 per cent, solution of phosphorus pentoxide in water, treating with zinc oxide, cooling in ice, adding ten volumes of ice-cold water, stirring, filtering, and stirring the filtrate on the water-bath. Debray also obtained a crystalline product by prolonged digestion of a solution containing a zinc salt and an excess of sodium phosphate, and Skey by adding a soluble phosphate to a solution of a zinc salt so that the solution remained acid.

The tetrahydrate is easily soluble in acids and solutions of ammonia, ammonium carbonate, ammonium sulphate, and ammonium nitrate.

When it is heated to 190° C. it forms the monohydratewhich has also been prepared by heating a solution of zinc carbonate in phosphoric acid to about 250° C.

Rhombic prisms of anhydrous Zn₃(PO₄)₂, of density 3.998 at 15° C., and melting at a red heat, were obtained by dissolving the tetrahydrate in melted zinc chloride, or by heating it with a concentrated solution of zinc chloride in sealed tubes at 250° C.

It absorbs hydrogen sulphide perceptibly at 100° C., rapidly at 160° C., and the absorption at any temperature is proportional to the square of the pressure.

An amorphous hexahydrate has been obtained by the action of absolute alcohol on an acid phosphate of zinc.

The acid zinc orthophosphate, ZnHPO₄.H₂O, occurs in small transparent needles which are monoclinic. ZnHPO₄.3H₂O crystallises in sticky, thread-like, aggregative crystals, and Zn(H₂PO₄)₂.2H₂O in transparent triclinic rhombohedra. These three salts, along with Zn₃(PO₄)₂. 4H₂O, were identified during a study of the system zinc oxide - phosphoric oxide - water at 25° C. and 37° C.

Graham described ZnHPO₄.H₂O as very insoluble, losing its water on heating and becoming anhydrous when melted at a red heat. According to Heintz, Graham's method of precipitating zinc sulphate with ordinary sodium phosphate throws down Zn₃(PO₄)₂.4H₂O mixed with a smaller proportion of 2ZnHPO₄.3H₂O.

Zinc tetrahydrogen phosphate, Zn(H₂PO₄)₂.2H₂O, was prepared by dissolving zinc in an excess of aqueous phosphoric acid. It is easily soluble in hydrochloric acid, unchanged by alcohol or ether, and decomposed by excess of water into the insoluble 10ZnO.4P₂O₅.10H₂O.

The following basic salts occur naturally: tarbuttite, Zn₃(PO₄)₂. Zn(OH)₂; hibbenite, 2Zn₃(PO₄)₂.Zn(OH)₂.6½H₂O and Zn₃(PO₄)₂. Zn(OH)₂.3H₂O.

The basic salt 10ZnO.4P₂O₅.10H₂O, obtained by the action of water on some of the acid phosphates, is a white crystalline powder, insoluble in water and easily soluble in dilute acids.