Boron dioxide, B₂O₂

This oxide may be prepared as follows. Crude magnesium boride, made as described below, is decomposed by cold water; the filtered solution is then treated by either of the following processes: (i.) the solution is evaporated to dryness in vacuo and the residue heated; (ii.) the solution is treated with ammonia, filtered from magnesium hydroxide, and evaporated to dryness in vacuo and the residue heated. In method (i.) part of the boron dioxide obtained is present in combination with magnesia (as a magnesium borite); in method (ii.) the dioxide is obtained practically free from magnesia but contaminated with about five per cent, of the lower oxide B₄O₃.2H₂O.

Boron dioxide is soluble in water. Its aqueous solution can be oxidised to boric acid by evaporation in air or by heating with nitric acid, but is not affected by iodine. Freezing-point measurements indicate that the molecular formula of the oxide is B₂O₂. The oxide apparently combines with water to form an acid, and the aqueous solution dissolves an amount of freshly precipitated magnesium hydroxide corresponding to the production of a magnesium borite, MgO.2B₂O₂.