Boric Acid

1.0What Is Boric Acid?

Boric acid is a weak monobasic acid of boron with the chemical formula H₃BO₃. It is also known as orthoboric acid.

It occurs as white crystalline solid or colorless plates, and is slightly soluble in cold water but more soluble in hot water and alcohol. It shows weak acidic properties and is often used as an antiseptic, insecticide, flame retardant, and in laboratory experiments.

$\text{ Chemical Formula: }{\mathrm{H}}_3{\mathrm{BO}}_3\text{ or }\mathrm{B}(\mathrm{OH}{)}_3$

$\text{ Molar Mass: }61.83\mathrm{g}/\mathrm{mol}$

2.0Occurrence of Boric Acid in Nature

Boric acid occurs naturally in:

• Volcanic regions (as steam deposits and hot springs)
• Minerals such as borax (Na₂B₄O₇·10H₂O), colemanite (Ca₂B₆O₁₁·5H₂O), and boracite (Mg₃B₇O₁₃Cl)
• Sea water, plants, and even in certain fruits in trace amounts

It is mainly obtained industrially by treating borax with acids such as hydrochloric acid or sulfuric acid.

3.0Structure of Boric Acid

Boric acid has the molecular formula H₃BO₃, but structurally it is B(OH)₃.

Structural Characteristics:

• It contains one boron atom linked to three hydroxyl groups (-OH).
• The structure is planar and covalent, with sp² hybridization of boron.
• Each boron atom forms three B–O bonds, and molecules are held together by hydrogen bonds.
• In solid state, it forms layered sheets due to extensive hydrogen bonding.

4.0Physical Properties of Boric Acid

Boric acid has a soapy or greasy feel when touched, and sublimes at higher temperatures.

5.0Chemical Properties of Boric Acid

1. Weak Acidic Nature

Boric acid is a very weak monobasic acid.
Unlike typical proton donors, boric acid does not donate H⁺ ions directly; instead, it acts as a Lewis acid by accepting hydroxyl ions from water.

$\mathrm{B}(\mathrm{OH}{)}_3+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\left[\mathrm{B}(\mathrm{OH}{)}_4\right]}^{\ast }+{\mathrm{H}}^{\ast }$

Hence, it behaves as a Lewis acid (electron pair acceptor) rather than a Brønsted acid.

2. Reaction with Bases

When boric acid reacts with strong bases like NaOH, it forms borates.

$\mathrm{B}(\mathrm{OH}{)}_3+\mathrm{NaOH}\to {\mathrm{NaBO}}_2+2{\mathrm{H}}_2\mathrm{O}$

On heating, boric acid forms sodium metaborate (NaBO₂).

3. Reaction with Alcohols

When heated with alcohols (like ethanol or methanol) and concentrated sulfuric acid, boric acid forms trialkyl borates.

$\mathrm{B}(\mathrm{OH}{)}_3+3{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to \mathrm{B}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_3+3{\mathrm{H}}_2\mathrm{O}$

These ethyl or methyl borates burn with a green-edged flame, which is a test for boron.

4. Reaction on Heating (Dehydration)

When heated to about 100°C, boric acid loses one molecule of water and forms metaboric acid (HBO₂).

${\mathrm{H}}_3{\mathrm{BO}}_3\to {\mathrm{HBO}}_2+{\mathrm{H}}_2\mathrm{O}$

On further heating to 300°C, metaboric acid changes into tetraboric acid (H₂B₄O₇).
At very high temperature, boron trioxide (B₂O₃) is formed.

$2{\mathrm{H}}_3{\mathrm{BO}}_3\to {\mathrm{B}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$

5. Formation of Borax (from Boric Acid)

Boric acid reacts with sodium carbonate (Na₂CO₃) upon heating to produce borax (Na₂B₄O₇·10H₂O).

$4{\mathrm{H}}_3{\mathrm{BO}}_3+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7+6{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$

6.0Preparation of Boric Acid

Boric acid can be prepared both in laboratory and industrially using several methods.

1. From Borax (Laboratory Method)

Boric acid is commonly prepared by acidifying an aqueous solution of borax with hydrochloric acid (HCl) or sulfuric acid (H₂SO₄).

$N{a}_2{B}_4{O}_7\cdot 10H_{2}O+2HCl\to 4H_{3}B{O}_3+2NaCl+5H_{2}O$

Procedure:

1. Dissolve borax in hot water.
2. Add dilute HCl slowly while stirring.
3. On cooling, white crystals of boric acid are formed.
4. Filter, wash, and dry the crystals.

2. From Colemanite (Industrial Method)

Colemanite (Ca₂B₆O₁₁·5H₂O) is treated with sulfuric acid to produce boric acid.

$C{a}_2{B}_6{O}_{11}\cdot 5H_{2}O+2H_{2}S{O}_4\to 2CaS{O}_4+6H_{3}B{O}_3$

The insoluble calcium sulfate is filtered off, and boric acid is obtained from the filtrate upon cooling.

3. From Boron Halides (Hydrolysis Method)

Boron trichloride or boron tribromide reacts with water to yield boric acid.

${\mathrm{BCl}}_3+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{B}(\mathrm{OH}{)}_3+3\mathrm{HCl}$

7.0Structure and Bonding of Boric Acid Molecule

• The boron atom in boric acid is sp² hybridized.
• It forms three sigma bonds with oxygen atoms of hydroxyl groups.
• The structure is trigonal planar, with an angle of 120° between bonds.
• Hydrogen bonding between molecules leads to a layered crystalline structure.

These layers are held together by weak van der Waals forces and hydrogen bonds, making the solid soft and slippery.

8.0Tests for Boric Acid

1. Turmeric Paper Test

• When a drop of boric acid is placed on turmeric paper and dried, the paper turns red-brown.
• Upon moistening with alkali, it becomes greenish-black.
• This is a confirmatory test for boric acid.

2. Flame Test (Green Flame Test)

When ethyl borate, produced by reaction of boric acid with ethanol and sulfuric acid, is burned, it gives a green-edged flame.

$\mathrm{B}(\mathrm{OH}{)}_3+3{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\to \mathrm{B}{\left({\mathrm{OC}}_2{\mathrm{H}}_5\right)}_3+3{\mathrm{H}}_2\mathrm{O}$
This is a characteristic test for boron.

9.0Uses of Boric Acid

1. Medical and Antiseptic Uses

• Used as a mild antiseptic in eye washes, lotions, and ointments.
• Applied in boric acid powder form to treat fungal infections and cuts.

2. Industrial Uses

• Used in the manufacture of:
• • Borosilicate glass (Pyrex glass)
  • Enamels, glazes, and ceramics
  • Fiberglass and cement

3. Laboratory Uses

• Serves as a buffering agent in chemical solutions.
• Used in preparation of borax bead tests for detecting metallic ions.

4. Agricultural Uses

• Used as a micronutrient fertilizer for plants requiring boron.

5. Insecticidal Applications

• Acts as a slow-acting insecticide for ants, cockroaches, and termites.
• Works by damaging their digestive systems upon ingestion.

10.0Comparison Between Boric Acid and Borax