Complex Boric Acids

Complex acids containing boron are formed by the action of boric acid upon numerous substances. The best defined inorganic compounds of this nature are the two borotungstic acids, [6H₂O.B₂O₃.28WO₃].56H₂O and [5H₂O.B₂O₃.24WO₃].61H₂O, prepared by Copaux.

The interaction of boric acid with numerous organic compounds has been studied by Magnanini, who found that in aqueous or alcoholic solution, boric acid interacts with all hydroxycarboxylic acids in which at least one hydroxyl group is present in the a-position to a carbonyl group, the complex acids formed (which he did not isolate) being stronger than the organic acids from which they are produced. The other organic acids do not interact with boric acid in solution. A crystalline potassium hydrogen boro-oxalate 2K(BO)C₂O₄.3H₂O, can nevertheless be readily prepared by heating potassium metaborate with oxalic acid solution, or by heating boric acid with potassium hydrogen oxalate.

The interaction of boric acid with alcohols has been studied by Magnanini, P. Muller and Abegg, and others. It has been shown by Miiller and Abegg that complex acids are formed (by condensation of the alcohols with boric acid) to an appreciable extent when boric acid is dissolved in various liquid alcohols, but that the extent to which complex acid formation occurs, steadily diminishes as the alcoholic solvent is replaced by water. Complex acid formation between boric acid and both mannitol and glycerol, two polyhydric alcohols, is of considerable interest, both theoretically and practically. In each case the complex acid formed is much stronger than boric acid; and whereas mannitol is optically inactive, mannitoboric acid is decidedly dextro-rotatory. The extent to which these complex acids are formed in solution increases with the concentration of either the boric acid or the polyhydric alcohol, and diminishes with the dilution; in consequence of which the molecular conductivity of mannitoboric acid decreases with the dilution. Magnanini concluded from his experiments that mannitoboric acid was produced by the condensation of three molecules of boric acid with one of mannitol, but the subsequent isolation of the complex acid has shown that the condensation proceeds as expressed by the equation: -

H₃BO₃ + C₆H₁₄O₆ ⇔ C₆H₁₅O₈B + H₂O.

The preparation of mannitoboric acid is carried out by dissolving 12 grams of mannitol and 8 2 grams of boric acid in 95 cubic centimetres of absolute alcohol, and setting the solution aside for a few days in a cold place. The first few crystals formed are discarded; those subsequently deposited are pure mannitoboric acid, C₆H₁₅O₈B, and melt at 89.5°. The molecular weight of the acid in acetone solution is rather greater than that required for the preceding formula. The acid is largely decomposed by water, but its ammonium, silver, calcium, and barium salts are more stable.