Zinc Borates

Anhydrous 3ZnO.B₂O₃, or zinc orthoborate, Zn₃(BO₃)₂, has been prepared by fusing zinc oxide with molecular proportions of potassium hydrogen fluoride and boron trioxide and treating with cold water. The flattened prismatic crystals belong to the rhombic system and act markedly on polarised light. They dissolve in acids, and hot water acts upon them to leave an amorphous residue that consists largely of zinc oxide.

The compound 3ZnO.2B₂O₃ has been obtained from the fusion of zinc oxide and boron trioxide. The crystals are triclinic.

Precipitates of the general formula xZnO.yB₂O₃.zH₂O, varying in composition, have been obtained by the action of boric acid on zinc carbonate, or by precipitating solutions of zinc salts with borax.

The largest yield of zinc borate is obtained by adding a concentrated solution of a zinc salt to a slight excess of saturated borax solution, or by using a supersaturated solution of borax or solid borax as precipitant.

Precipitation is more complete if a little sodium hydroxide is added, though sodium zincoxide gives no precipitate with borax.