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


  1. I love how my chem teacher was like "Here are super basic things about lewis structures. Now go figure out H3BO3." Like, he couldn't be bothered to tell us that oxygen attaches before the hydrogen for these special cases at all but I can guarantee he will grade us on it. I an also guarantee the book he's provided doesn't cover this either.


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