不同于一般的Bronsted酸,硼酸中的氢不属于活泼氢,因为其在溶液中不电离,其弱酸性是其与水复合形成的共轭离子的酸性。
详细可参考如下,引自Wikipedia:
Properties
Boric acid is soluble in boiling water. When heated above 170 °C, it dehydrates, forming
metaboric acid (HBO
2):
H
3BO
3 → HBO
2 + H
2O
Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. Boric acid melts at about 236 °C, and when heated above about 300 °C further dehydrates, forming
tetraboric acid or
pyroboric acid (H
2B
4O
7):
4 HBO
2 → H
2B
4O
7 + H
2O
The term
boric acid may sometimes refer to any of these compounds. Further heating leads to
boron trioxide.
H
2B
4O
7 → 2 B
2O
3 + H
2O
Boric acid does not dissociate in
aqueous solution as a
Brønsted acid, but is a
Lewis acid which interacts with
water molecules to form the
tetrahydroxyborate ion, as observed by
Raman spectroscopy[2]:
B(OH)
3 + H
2O
B(OH)−4 + H
+ (
Ka = 5.8x10
−10 mol/l; p
Ka = 9.24)
Polyborate anions are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the
tetraborate ion, found in the mineral borax:
4 B(OH)−4 + 2 H
+ B
4O2−7 + 9 H
2O
Boric acid makes an important contribution to the absorption of low frequency sound in seawater.