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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.

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