Boric Acid: Structure, Bonding, Preparation, Properties and Uses

Boric Acid (H₃BO₃)

Boric Acid is also known as Hydrogen Borate, Orthoboric acid, Boracic acid, Sassolite, Borofax, Trihydroxyborane, Boranetriol, and Acidum boricum

1. Structure

• Boric acid has the molecular formula H₃BO₃ (also written as B(OH)₃).
• It is a monobasic Lewis acid, not a protonic (Brønsted) acid.
• In solid state, it exists as planar triangular B(OH)₃ units.
• These units are linked together by symmetrical hydrogen bonds (O–H···O) forming infinite two-dimensional layers (sheet-like structure).
• The layers are held together by weak van der Waals forces.
• B–O bond length: ~136 pm; O-H: bond length ~97 pm; O–H···O hydrogen bond length: ~270 pm.
• Boron is sp² hybridized with trigonal planar geometry (bond angle ≈ 120°), and C_3h molecular symmetry.

Structure of Boric Acid representing O-B-O Bond Angle, B-O and O-H Bond Length

Structure of Boric Acid representing O-B-O Bond Angle, B-O and O-H Bond Length, and Hydrogen Bonding

2. Bonding

• Covalent bonding in boric acid involves σ-bonding between boron's sp² hybrid orbitals and oxygen p orbitals, with partial π-character resulting from oxygen lone pair donation into boron's empty p orbital.
• Boron has only 6 electrons in valence shell → acts as electron-deficient Lewis acid.
• It accepts a lone pair from OH⁻ to form [B(OH)₄]⁻ (tetrahedral).
• No B–H bonds (unlike boranes); only B–O and O–H bonds.
• The B-O bond energy ~536 kJ/mol, significantly higher than typical B-O single bonds due to the partial double bond character.
• Extensive intermolecular hydrogen bonding gives it a layered, flaky crystalline appearance.

3. Preparation

Method	Reaction	Conditions
From Borax (Commercial)	Na2B4O7·10H2O + 2HCl → 4H3BO3 + 2NaCl + 5H2O or Na2B4O7 + H2SO4 + 5H2O → 4H3BO3 + Na2SO4	Concentrated acid, heating, then cooling to crystallize
From Colemanite	Ca2B6O11 + 2H2SO4 + 11H2O → 6H3BO3 + 2CaSO4·2H2O	SO2 is passed to precipitate CaSO4
Hydrolysis of Boron Halides	BCl3 + 3H2O → H3BO3 + 3HCl	Lab method

4. Physical Properties

• Appearance: White, shiny flaky crystals (pearly luster).
• Solubility: Sparingly soluble in cold water (~5 g/100 mL at 20°C), highly soluble in hot water (~40 g/100 mL at 100°C).
• Taste: Slightly bitter.
• Melting point: 170°C (decomposes on strong heating >100°C).
• Volatility: Volatile in steam (steam distillation property).

5. Chemical Properties (Reactions)

1. Acidic nature (Lewis acid)
   B(OH)₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺
2. With ethyl alcohol (Borate ester formation – flame test)
   H₃BO₃ + 3C₂H₅OH → B(OC₂H₅)₃ + 3H₂O
   Triethyl borate burns with green-edged flame.
3. Dehydration on heating
   H₃BO₃ → HBO₂ (metaboric) → H₂B₄O₇ (tetraboric) → B₂O₃ (boron trioxide)
   Heating boric acid at 1700°C gives metaboric acid.
   H₃BO₃ → HBO₂ + H₂O
   Heating boric acid at 3000°C gives tetraboric acid.
   4HBO₂ → H₂B₄O₇ + H₂O
   Heating boric acid at 3300°C gives boron trioxide.
   H₂B₄O₇ → 2B₂O₃ + H₂O
4. With NaOH
   Forms sodium tetraborate (borax):
   4H₃BO₃ + 2NaOH → Na₂B₄O₇ + 7H₂O
5. With HF + H₂SO₄
   Forms volatile BF₃ (used for purification).
6. With Sulphuric Acid
   Easily dissolved in sulphuric acid
   B(OH)₃ + 6H₂SO₄ → B(HSO₄)₄^– + 2HSO₄^– + 3H₃O⁺

6. Uses of Boric Acid

• Antiseptic: Mild antiseptic in eye washes (boric acid solution), talc powders, ointments (e.g., Boroline).
• Insecticide: Especially against ants, cockroaches, and termites (disrupts their digestive system).
• Glass manufacture: As a flux in borosilicate glass (Pyrex).
• Enamels and glazes: In ceramic industry.
• Flame retardant: In textiles and wood.
• Neutron absorber: In nuclear reactors (due to high neutron capture by ¹⁰B).
• Analytical reagent: In qualitative analysis (borax bead test, green flame test).
• pH buffer: In swimming pools and cosmetics.
• Preservative: In some food items (limited use).

Note: Use as antiseptic has declined due to toxicity concerns (especially in infants); now restricted in many countries.

Summary: Boric acid is a weak, monobasic Lewis acid with extensive hydrogen-bonded layer structure, commercially prepared from borax, and widely used as mild antiseptic, insecticide, and in glass/ceramics industry.

Frequently Asked Questions (FAQ) - Boric Acid

1. What is the molecular structure of Boric Acid?

View Answer

Boric acid (H₃BO₃) consists of planar triangular units. In its solid crystalline state, these units are held together by a network of symmetrical hydrogen bonds, forming infinite, two-dimensional, sheet-like layers.

2. Is Boric Acid a strong or weak acid?

View Answer

Boric acid is a very weak, monobasic Lewis acid. It does not donate protons directly; instead, it acts as an electron-deficient molecule that accepts a lone pair of electrons from a hydroxyl ion (OH^-) to form the tetrahedral [B(OH)₄]^- ion.

3. How is Boric Acid prepared commercially?

View Answer

It is primarily produced by reacting borax (Na₂B₄O₇ · 10H₂O) with a strong mineral acid, such as hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), followed by crystallization.

4. What are the common industrial uses of Boric Acid?

View Answer

Due to its unique chemical properties, it is widely used as an antiseptic, an insecticide, a flux in the manufacturing of borosilicate glass (like Pyrex), and as a flame retardant in textiles and wood products. It also serves as a neutron absorber in nuclear reactors.