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.
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
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