Boric acid has a layered structure composed of asymmetric hydrogen-bonded B(OH)3 units, with an interlayer spacing of 318 picometers. It is soluble in water, alcohol, glycerol, ethers, and essential oils; its aqueous solution is weakly acidic. The solubility of boric acid in water increases with increasing temperature and it can volatilize with water vapor. In aqueous solution, boric acid exists primarily as unionized B(OH)3 monomers, but a small portion of B(OH)3 molecules complex with OH- ions in water, making the solution slightly acidic.
Boric acid is a monoprotic, very weak acid. Its acidity does not originate from the donation of protons. Because boron is an electron-deficient atom, it can add hydroxide ions from water molecules, releasing a proton. This electron-deficient property is utilized by adding polyhydroxy compounds (such as glycerol and glycerol) to form stable complexes, thereby enhancing its acidity.
Reason for the acidity of boric acid (pKa = 9.25):
However, the substances in the solution are not actually that simple. It has been reported that boric acid does exist in dilute solutions (<0.025 mol/L) in the forms of B(OH)3 and [B(OH)4]-. However, at higher concentrations, the acidity of the solution increases significantly. Through pH and conductivity studies, it is believed that a stronger triboronic acid (pKa = 6.84) is formed in more concentrated boric acid solutions. Its anion structure is a cyclic triboronic acid with a hydroxyl group added.
Even tetraboronic acid H2B4O7 (pKa1≈4, pKa2≈9) has been formed. Furthermore, the presence of triborate ions has been confirmed in mixed solutions of boric acid and borate. Of course, there are many theories regarding the existence and forms of polyboronic acids and polyborates. However, the most popular polyborate ions are HB2O4-, H4B3O7-, B4O72-, HB4O7-, and B5O8-, among which the triborate ion is the most plausible.
Adding glycerol to boric acid triggers esterification, reducing the borate ion concentration and promoting boric acid ionization, thus increasing the acidity of the solution. Besides glycerol, boric acid and its borates can rapidly form stable chelates with other polyols and α-hydroxycarboxylic acids, predominantly 1:1, but also sometimes 1:2. The acidity of boric acid can be significantly enhanced by the integrative effect of polyols; for example, in the presence of mannitol, the ionization constant of boric acid can increase by 104 times (from pKa = 9.25 to 5.15), which is the basis for the titration of boric acid with NaOH solution in the presence of mannitol in analytical chemistry.
Boric acid reacts with hydrofluoric acid in aqueous solution to immediately form trifluoroboric acid, which then slowly converts to tetrafluoroboric acid:
A series of intermediate products are present, forming a complete reaction sequence: H[B(OH)4], H[BF(OH)3], H[BF2(OH)2], H[BF3(OH)], and H[BF4]. Of these, only monofluoroboric acid H[BF(OH)3] is a temporary intermediate; the other three fluoroacids are stable and can be prepared individually.
