Boric Acid

Preparation of Boric Acid

1) From Borax

Boric acid is obtained by treating a hot concentrated solution of borax with hydrochloric acid or sulphuric acid. The resulting solution on concentration and cooling gives crystals of boric acid.

Na₂B₄O₇ + 2 HCl + 5 H₂O → 4 H₃BO₃ + 2 NaCl

Na₂B₄O₇ + H₂SO₄ + 5 H₂O → 4 H₃BO₃ + 2 Na₂SO₄

2) By hydrolysis of boron compounds

Boric acid can also be prepared by the hydrolysis of boron compounds such as halides, hydrides and nitrides.

BCl₃ + 3 H₂O → H₃BO₃ + 3 HCl

B₂H₆ + 6 H2O → 2H₃BO₃ + 6 H₂

BN + 3 H₂O → H₃BO₃ + NH₃

 

3) From Colemanite

Boric acid is obtained by passing sulphur dioxide through the solution of the mineral colemanite in boiling water. The resulting solution on concentration and cooling gives crystals of boric acid while calcium bisulphite being highly soluble in water remains in the mother liquid.

 

Ca₂B₆O₁₁ + 11H₂O → 2 Ca(OH)₂ + 6 H₃BO₃

2 Ca(OH)₂ + 4 SO₂ → 2 Ca(HSO₃)₂

–———————————————————————

Ca₂B₆O₁₁ + 11H₂O + 4 SO₂ → 2 Ca(HSO₃)₂

 

Properties of Boric Acid

1) It is a white crystalline solid with a soft soapy touch having a low density of 1.48 gcm^–3.

2) It is sparingly soluble in cold water but fairly soluble in hot water.

3) Acidic Nature

Boric acid behaves as a very weak monobasic acid. It does not act as a proton-donor i.e. protonic acid. Due to the small size of Boron and presence of only 6 electrons in its valence shell , B(OH)₃ behaves as a Lewis acid, by accepting a pair of electrons from OH‾ ion of water thereby releasing a proton.

4) Action of Heat

Boric acid ,on heating, loses water in three different stages at different temperature ultimately giving boron trioxide. 

H₃BO₃ → HBO₂ + H₂O

4 HBO₂ → H₂B₄O₇ → 2 B₂O₃ + H₂O

 

5) Reaction with Ethyl Alcohol

Orthoboric acid reacts with ethyl alcohol in presence of to form conc H₂SO₄ to form triethylborate.

B(OH)₃ + 3 C₂H₅OH → B(OC₂H₅)₃ + 3 H₂O

The vapours of triethyl borate when ignited burns with a green edged flame. This forms the basis for detecting borates and boric acid in qualitative analysis.

Uses of Boric Acid

Boric acid is used

1) in the manufacture of heat resistant borosilicate glass.

2) as a preservative for milk and food stuffs.

3) in the manufacture of enamel and glazes in pottery.

4) the aqueous solution of boric acid is used as a mild antiseptic especially as eye wash under the name boric lotion.

Structure of Boric Acid

The ground state outer electronic configuration of Boron is 2s² 2p_x¹ 

In excited state, one of the 2s electron gets  promoted to the vacant orbital 2p_y orbital. The three half filled atomic orbitals thus obtained undergo hybridization to give sp² hybridised orbitals. Each one of these three sp² orbital overlap with 2p orbitals of O‾ forming 3 B-O‾ bonds.

In boric acid, planar BO₃^3- units are joined by unsymmetrical hydrogen bonds to give a layered structure. The adjacent layers in the crystal of boric acid are held together by weak forces of attraction. The distance between any two successive layer is 318 pm.the Because of weak forces of attraction, one layer can slide over the other. This makes boric acid soft and Soapy to touch.