Orthoboric Acid, H3BO3

Orthoboric Acid, H3BO3

Orthoboric acid, H3BO3 is a white crystalline solid with soapy touch. It is sparingly soluble in water but highly soluble in hot water. It can be prepared by acidifying an aqueous solution of borax.

Na2B4O7   +   2HCl   +   5H2O    →    2NaCl   +   4B(OH)3

It is also formed by the hydrolysis (reaction with water or dilute acid) of most boron compounds (halides, hydrides, etc.). It has a layer structure in which planar BO3 units are joined by hydrogen bonds as shown in below figure.
Orthoboric Acid, H3BO3

Boric acid is a weak monobasic acid. It is not a protonic acid but acts as a Lewis acid by accepting electrons from a hydroxyl ion:

B(OH)3   +   2HOH    →   [B(OH)4 ]-    +    H2O+

On heating, orthoboric acid above 370 K forms metaboric acid, HBO2 which on further heating yields boric oxide, B2O3.

H3BO3 (Heat)   →   HBO2 (Heat)    →   B2O3