Compounds of Boron

Boric Acid

(a) Orthoboric acid (H3BO3)

a1) (i) Preparation from borax

Na2B4O7 + H2SO4 + 5H2O —> Na2SO4 + 4H3BO3

(ii) From colemanite

Ca2B6O11 + 2SO2 + 11H2O —> 2Ca(HSO3)2 + 6H3BO3

Boric acid forms white needle like crystal.

It has a layer structure involving triangular BO3 groups joined by hydrogen bonds.

(a2) Action of heat

4H3BO3 4HBO2 H2B4O7 2B2O3

Metaboric acid Tetrabasic acid Boron trioxide

(a3) It behaves as a weak monobasic acid

B(OH)3 <—> H3BO3 <—> H+ + H2BO3- <—> H2O + BO2-

Thus on titration with NaOH, it gives sodium metaborate salt

H3BO3 + NaOH NaBO2 + 2H2O

(a4) B(OH)3 + MO M – borates

Metal oxide

Where M stands for a bivalent metal

(a5) B(OH)3 BF3

Borax (sodium tetraborate) Na2B4O7. 10H2O :

Borax occurs naturally and is also called Tinacal or suhaga. Tinacal contains 45% of borax

(a) Preparation from Boric Acid

4H3BO3 + Na2CO3 —> Na2B4O7 + 6H2O + CO2

(b) Basic Nature:- aqueous solution of borax is alkaline in nature due to its hydrolysis

Na2B4O7 + 3H2O —> NaBO2 + 3H3BO3

NaBO2 + 2H2O NaOH + H3BO3
Strong alkali

(c) Action of heat:
Na2B4O7.10H2O Na2B4O7 2NaBO2 + B2O3

(Anhydroussodium metaborate) anhydride basic Transparent glassy mass

When hot glassy mass is brought in contact with a coloured salt and heated again in the flame, B2O3 displaces the volatile oxides and reacts with basic oxides to form metaborates. Metaborates of basic radicals show characteristic colours. This test is known as borax bead test.

Colour of metaborates of    Cu    Fe      Co        Cr        Ni

Blue    Green   Blue    Green    Brown

Next Page→

←Back Page

Leave a Reply