Hypoborates

Stock and Kuss have shown that when either of the hydrides B₂H₆ or B₄H₁₀ dissolves in aqueous alkali hydroxide, the initial product is an alkali compound of the type MO.BH₃, which they term an alkali hypoborate; e.g.: -

B₂H₆ + 2KOH = 2KOBH₃ + H₂ B₄H₁₀ + 4KOH = 4KOBH₃ + H₂.

Solutions of the alkali hypoborates are fairly stable at 0°, particularly when excess of alkali is present, and may be exposed to air. When boiled, however, decomposition takes place rapidly: -

2KOBH₃ + 2H₂O = 2KBO₂ + 5H₂.

Potassium hypoborate, KOBH₃, may be isolated by dissolving potassium hydroxide in one and a half times its weight of water and treating it with an excess of B₄H₁₀ at 0°; the solid hypoborate is then obtained in colourless, glistening, octahedral crystals which may be dried in a high vacuum.

The compound is stable when dry. It is deliquescent, and its aqueous solution slowly decomposes at the ordinary temperature. Acids, even acetic acid, immediately decompose it. The aqueous solution is a powerful reducing agent, and gives precipitates with solutions of most salts, but insoluble hypoborates are never obtained. With copper sulphate, a precipitate of copper hydride, CuH₂, is obtained; with nickel sulphate a very remarkable change takes place, nickel boride, Ni₂B, being precipitated.

When potassium hypoborate is heated to 500°, potassium, hydrogen, and water are expelled. The water is evolved first, and is completely expelled at 200°; potassium begins to distil from the residue at 400°. The reaction may be expressed thus: -

5KOBH₃ = K₃B₅O₃ + 2K + 2H₂O + 11H.

The residue, K₃B₅O₃, is soluble in water, giving an alkaline solution the properties of which resemble those of a solution obtained by heating boric oxide with magnesium and extracting the product with water.

Sodium hypoborate, NaOBH₃, resembles the potassium compound. The barium and magnesium hypoborates are only known in aqueous solution.