Tetraboron pentoxide, B₄O₅

Tetraboron pentoxide, B₄O₅, is prepared by treating the compound Mg₃B₂(OH)₆ with strong ammonia for several days in an atmosphere of hydrogen, filtering, evaporating the filtrate to dryness in vacuo, and heating the residue. Thus prepared, it forms a pale brown solid, infusible at the softening point of Jena glass. When allowed to remain in vacuo in contact with water it dissolves, forming a golden-yellow solution that oxidises rapidly in the air; a small flocculent residue, apparently consisting of B₄O₃.2H₂O, is also obtained. It is possible that the residue is produced by the following reaction: -

3B₄O₅ = B₄O₃ + 4B₂O₃.