Boron occurs in two isotopic forms. The abundance of boron in the earth's crust is less than 0.001% by mass. Boron occurs mainly as borates and orthoboric acid. Boron is extremely hard, low density solid with a melting point greater than 2540 K and low electrical conductivity. Elemental boron exists in several different modifications. The phenomenon of existence of different forms of an element is called allotropy. Under different conditions at least four allotropes of boron may be obtained.
Boric acid, B(OH)3 or H3BO3 is obtained by the action of hydrochloric acid or sulphuric acid on borax. On cooling the reaction mixture, white flakes of the acid are obtained.
Na2[B4O5(OH)4] . 8H2O + 2HCl 2NaCl + 4H3BO3 + 5H2O
Na2[B4O5(OH)4]. 8H2O + H2SO4 Na2SO4 + 4H3BO3 + 5H2O
It may also be obtained by the hydrolysis of most boron compounds like nitrides, sulphides, etc.
2BN + 6H2O 2H3BO3 + 2NH3
B2S3 + 6H2O 2H3BO3 + 3H2S
Boric acid is a white crystalline substance, soft and soapy to touch. It is moderately soluble in cold water. On heating it decomposes to form metaboric acid at 375 K, tetra boric acid at 435 K and boron trioxide at red heat.
Boric acid is a very weak monobasic acid. It does not liberate hydrogen ion but accepts a hydroxyl ion, i.e. it behaves as Lewis acid.
|B(OH)3 + 2H2O H3O+ + [B(OH)4]- pKa = 9.25
(or H3BO3) Metaborate ion
The structures of B(OH)3 and [B(OH)4]- are as follows.
The boric acid cannot be titrated satisfactorily with NaOH as sharp end point is not obtained. However, in the presence of a cis-diol (glycerol, mannitol or sugars), boric acid acts as a strong acid and can be titrated with NaOH in the presence of phenolphthalein indicator. With a cis-diol, the product [B(OH)4]- in the above reaction forms a strong complex, causing the reaction to move in the forward direction.
Boric acid contains triangular planar BO3 units. In the solid the B(OH)3 units are hydrogen bonded together into two-dimensional sheets with almost hexagonal symmetry.