【讨论】硼酸中的H

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 (HBO₂):

H₃BO₃ → HBO₂ + H₂O

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 (H₂B₄O₇):

4 HBO₂ → H₂B₄O₇ + H₂O

The term boric acid may sometimes refer to any of these compounds. Further heating leads to boron trioxide.

H₂B₄O₇ → 2 B₂O₃ + H₂O

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 + H₂O B(OH)−4 + H⁺ (K_a = 5.8x10⁻¹⁰ mol/l; pK_a = 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⁺ B₄O2−7 + 9 H₂O

Boric acid makes an important contribution to the absorption of low frequency sound in seawater.