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. Metaboric 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 confirmed by Raman spectroscopy:
- B(OH)3 + H2O B(OH)−
4 + H+ (Ka = 5.8x10−10 mol/l; pKa = 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.
Read more about this topic: Boric Acid
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