Perboric Acid and Perborates

Perboric acid is unknown in the pure state, but it is possible that ethereal solutions have been prepared. A number of metallic perborates, however, are known. They were discovered by Etard, and first prepared in a pure state by Melikoff and Pissarjewsky.

The best known perborates are derivatives of a perboric acid of the composition HBO₃. They are readily hydrolysed in aqueous solution, boric acid, or rather its salts, and hydrogen peroxide being produced. Accordingly, the perboric acid is considered to have the constitution O:BO.OH: -

O:BO.OH + 2H₂O = B(OH)₃ + HO.OH.

This constitution, however, has been disputed on the ground that hydrated podium or potassium perborate does not give off hydrogen peroxide when heated to 50°-60° under diminished pressure or in a current of air free from carbon dioxide; and the alternative constitution has been proposed by Bosshard and Zwicky. The following argument, based upon the preparation and properties of KBO₄, is also given by the same chemists. The compound KBO₄, or potassium hyperborate, crystallises with 1H₂O and is fairly stable. When dried over phosphoric anhydride in vacuo, 4KBO₄.H₂O is produced, so that the original compound cannot be formulated as KBO₃.H₂O₂. It is rather to be considered as derived from potassium hyperoxide KO.OH, and perboric acid. Hence, two constitutions are possible, viz. KO.O.OB:O and , and it is considered that the latter is much more probable than the former owing to the stability of the salt.