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Boric Acid - Formula, Structure, Synthesis, Physical properties, Reactions and Uses

Boric acid is a monobasic lewis acid consisting of four oxygen atoms, hydrogen atoms, and phosphorus. The molecular formula of boric acid is H3BO3. The other common names of boric acid include acidum boricum, orthoboric acid, hydrogen borate, and boracic acid. The effective use of boric acid is in treating burns and minor cuts because of its antiseptic properties. Dilute solutions of boric acid are used for a wide variety of purposes. Let us study various details about boric acid in detail.

Structure of boric acid

The chemical formula of boric acid is H3BO3. Boron is the central atom to which three hydroxyl groups are attached. Boron contains 3 electrons in its valence shell and hence undergoes sp2 hybridization. On the other hand, every oxygen atom in boric acid is sp3 hybridized. As a result, the three oxygen atoms form a sigma bond with the boron, which is the central atom. The shape of the molecule so formed is trigonal planar. The bond lengths of O-H and B-O are 97pm and 136pm, respectively.

Synthesis of boric acid

Boric acid is mainly synthesized from borax, trihalides, and diborane. First, let us have a look at the essential synthesis methods of boric acid.

1. Synthesis of boric acid from borax

Boric acid can be synthesized from borax with the help of any mineral acid like sulphuric acid or hydrochloric acid. The solution of mineral acid and borax produces boric acid crystals upon concentration and cooling. Hence, borax is said to be the principal source for the synthesis of boric acid.
The equation representing the synthesis of boric acid from borax is given below.

Na2B4O7.10H2O + 2HCl → 4H3BO3 + 5H2O + 2NaCl

Na2B4O7 + H2SO4 + 5H20 → 4H3BO3 + 5H2O + 2Na2SO4

2. Synthesis of boric acid from trihalides

Boron trihalides, in the presence of water, produce boric acid. Boric acid can be synthesized from three boron trihalides: boron tribromide, boron trichloride, and boron trifluoride. This is a hydrolysis reaction.
The equation representing the synthesis of boric acid from boron trihalides is given below.

BX3 + 3H2O 🡪 B(OH)3 + 3HX

Where X is any halogen atom (Fluorine, chlorine, or bromine)

3. Synthesis of boric acid from diborane

Diborane compounds, in the presence of water, produce boric acid. As a result, diborane undergoes a hydrolysis reaction to produce boric acid and hydrogen gas.
The equation representing the synthesis of boric acid from diborane is given below.

B2H6 + 6H2O 🡪 2B(OH)3 + 6H2

Physical properties of boric acid

At room temperature, boric acid is a white or colourless crystalline solid. Boric acid is soft and soapy to touch with a density of 1.435 g/cm3. Boric acid is freely soluble in hot water and sparingly soluble in cold water. Its solubility in water depends on the temperature of the water. It has a solubility of 57 grams per litre at 25 degrees celsius and increases to 275 grams per litre at 100 degrees celsius. Boric acid is slightly soluble in the solvent acetone and sparingly soluble in pyridine.

 

Molecular formula 

H3BO3

Molecular weight 

61.83 g·mol−

Physical state 

Solid 

Color

Colorless or white 

Density 

1.435 g/cm3

Melting point 

170.9 °C

Boiling point 

300 °C

Solubility 

Fairly soluble in hot water and sparingly soluble in cold water. 


Acidity of boric acid

Boric acid acts as a weak monobasic acid. Boric acid has a small size with 6 electrons in its valence shell. Boron accepts a pair of electrons from the hydroxyl ion and acts as a lewis acid.
In the presence of polyols like glycerol and mannitol, the acidity of boric acid increases significantly. The acidic solution of boric acid in mannitol is known as mannitoboric acid. Borate anion is the conjugate base of boric acid.

Chemical reactions of boric acid

Boric acid is one of the essential chemicals in the field of medicine and the pharmaceutical industry. The various chemical properties of boric acid are given below.

1. Action of heat in boric acid

At a temperature of 170°C, boric acid produces metaboric acid. Metaboric acid is a cubic crystalline solid soluble in water. At 236 °C, metaboric acid further undergoes dehydration and produces tetraboric acid. The further heating of tetraboric acid at 236 °C produces boron trioxide.
The reactions of boric acid in the presence of heat are given below.

  • Formation of metaboric acid: H3BO3 → HBO2 + H2O
  • Formation of tetraboric acid: 4 HBO2 → H2B4O7 + H2O
  • Formation of boron trioxide: H2B4O7 → 2 B2O3 + H2O

All the above reactions are dehydration reactions.

1. Reaction of boric acid with ethyl alcohol

In the presence of concentrated sulphuric acid, Boric acid reacts with ethyl alcohol to produce triethyl borate.
When the vapours of triethyl borate are ignited, they produce a green-edged flame. This property is used as a qualitative test to identify boric and borates in the given compound.

Uses of boric acid

The uses of boric acid are as follows:

  • Production of heat-resistant borosilicate glass.
  • Synthesis of glazes and enamel in pottery.
  • The boric acid aqueous solution is used as an antiseptic in eyewashes or other medicines.
  • Used as an insecticide and flame retardant.
  • Used to preserve foodstuffs and milk.
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