Boric Acid

Boric acid is an inorganic compound that is a weak monobasic Lewis acid of boron. Although in some chemical reactions it acts as a tribasic acid. It is also known as hydrogen borate, boracic acid and orthoboric acid. Its IUPAC name is trihydrooxidoboron.  

Molecular Formula of Boric Acid – H₃BO₃

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When boric acid occurs as a mineral in nature, it is called sassolite. It is a crystalline solid at room temperature. It is found as a constituent of many naturally occurring minerals such as borax, boracite, ulexite and colemanite. Its salts are found in seawater. It is also found in all fruits and many plants. 

Wilhelm Homberg first prepared boric acid crystals in 1702. He gave it the name sal sedativum Hombergi (sedative salt of Homberg). Although boric acid compounds have been used since the time of the ancient Greeks for cleaning, preserving food and other activities. 

Crystal Structure of Boric Acid 

It has trigonal planar geometry due to three oxygen atoms around the boron. In this bond length of B-O is 136pm and O-H is 97pm. The structure of the unit cell of boric acid is given below:

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Properties of Boric Acid

Physical Properties of Boric Acid are as follows.

• It is a colourless or white crystalline solid at room temperature. 

• Its molar mass is 61.83 g/mol.

• Its melting point is 170.9℃.

• Its boiling point is 300℃.

• It is soluble in water.

Chemical Properties of Boric Acid are as follows.

• On heating boric acid gives metaboric acid. The reaction at 170℃ is given below.

              H₃BO₃ → HBO₂ + H₂O

At 300℃, it gives tetraboric acid. The reaction is given below.

             4HBO₂ → H₂B₄O₇ + H₂O

On heating above 330℃, it gives boron trioxide. The reaction is given below.

H₂B₄O₇→ 2B₂O₃ + H₂O

• Boric acid reacts with alcohol and gives borate esters. The reaction is given below.

B(OH)₃ + 3ROH → B(OR)₃ + 3H₂O

• It also dissolves in anhydrous sulfuric acid. The reaction is given below.

B(OH)₃ + 6H₂SO₄→ B(HSO₄)₄^- + 2HSO₄^- + 3H₃O⁺

Preparation of Boric Acid 

• By Borax - Boric acid is prepared by reaction of borax with mineral acid (or hydrochloric acid). The reaction involved is given below.

Na₂B₄O₇.10H₂O + 2HCl →4B(OH)₃ + 2NaCl + 5H₂O

• By Hydrolysis of Diborane - Boric acid is also formed as a by-product of hydrolysis of diborane. The reaction involved is given below.

B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂

• By Hydrolysis of Trihalide - Boric acid is also formed as a by-product of hydrolysis of boron trihalides. The reaction involved is given below.

BX₃ + 3H₂O → B(OH)₃ + 3HX (X = Cl, Br, I)

Uses of Boric Acid 

We have been using boric acid since ancient times. Presently it is mainly used in industries. A few of its uses are listed below.

• It is used in the manufacturing of monofilament fibreglass. 

• It is used in the jewellery industry in combination with denatured alcohol. 

• It is used in the production of glass in LCD flat panel displays.

• It is used in electroplating.

• The boric acid mixture is used as a fire retarding agent. 

• It is used in the manufacturing of ramming mass.

• It has medicinal uses as well such as it reduces the harmful effect of HF.

• Boric acid mixed with borax is used for welding flux by blacksmiths.

• It is often used as an antiseptic, insecticide, flame retardant, neutron absorber etc. 

• It is used to prevent or destroy existing wet and dry rot in timbers. 

• It is used as a primary buffer system in swimming pools. 

• It is used in some nuclear power plants as a neutron poison. 

• Its mixture with petroleum or vegetable oil works as an excellent lubricant. 

Conclusion

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