கொஞ்சம் கெமிஸ்ட்ரி ! - EP.009

 

Chemical Properties

• Formula: H₃BO₃

• Molar Mass: 61.83 g/mol

• Appearance: White crystalline solid or powder

• Odor: Odorless

• Density: 1.435 g/cm³

• Melting Point: 170.9°C (339.6°F)

• Boiling Point: 300°C (572°F)

• Solubility: Soluble in water, lower alcohols, moderately soluble in pyridine, very slightly soluble in acetone

• Acidity (pKa): 9.24 (first proton), 12.4 (second), 13.3 (complete)

• Conjugate Base: Borate

• Magnetic Susceptibility: -34.1·10⁻⁶ cm³/mol

Structure

Boric acid has a trigonal planar molecular geometry with three oxygen atoms forming a planar structure around the boron atom. The bond lengths are approximately 136 pm for B-O and 97 pm for O-H. The dipole moment is 0 D, indicating a non-polar molecule.

Preparation

• From Borax: Boric acid can be prepared by reacting borax (sodium tetraborate) with a mineral acid such as hydrochloric acid:

$${\text{<span style="font-family: Diphylleia;">Na</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}{\text{<span style="font-family: Diphylleia;">B</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}{\text{<span style="font-family: Diphylleia;">O</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">7</span>}}<span\; style="font-family:\; Diphylleia;">\cdot </span>\mathrm{<span\; style="font-family:\; Diphylleia;">10</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}\text{<span style="font-family: Diphylleia;">HCl</span>}<span\; style="font-family:\; Diphylleia;">\to </span>\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}{\text{<span style="font-family: Diphylleia;">BO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">5</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}\text{<span style="font-family: Diphylleia;">NaCl</span>}\backslash text\{Na\}\_2\backslash text\{B\}\_4\backslash text\{O\}\_7\; \backslash cdot\; 10\backslash text\{H\}\_2\backslash text\{O\}\; +\; 2\backslash text\{HCl\}\; \backslash rightarrow\; 4\backslash text\{H\}\_3\backslash text\{BO\}\_3\; +\; 5\backslash text\{H\}\_2\backslash text\{O\}\; +\; 2\backslash text\{NaCl\}$$

• From Hydrolysis of Diborane: Boric acid can also be prepared from the hydrolysis of diborane (B₂H₆):

$${\text{<span style="font-family: Diphylleia;">B</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">6</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">6</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}<span\; style="font-family:\; Diphylleia;">\to </span>\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}{\text{<span style="font-family: Diphylleia;">BO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">6</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\backslash text\{B\}\_2\backslash text\{H\}\_6\; +\; 6\backslash text\{H\}\_2\backslash text\{O\}\; \backslash rightarrow\; 2\backslash text\{H\}\_3\backslash text\{BO\}\_3\; +\; 6\backslash text\{H\}\_2$$

• From Hydrolysis of Boron Trihalides: Boric acid can be formed as a by-product of the hydrolysis of boron trihalides (BX₃):

$${\text{<span style="font-family: Diphylleia;">BX</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}<span\; style="font-family:\; Diphylleia;">\to </span>{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}{\text{<span style="font-family: Diphylleia;">BO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}\text{<span style="font-family: Diphylleia;">HX</span>}\backslash text\{BX\}\_3\; +\; 3\backslash text\{H\}\_2\backslash text\{O\}\; \backslash rightarrow\; \backslash text\{H\}\_3\backslash text\{BO\}\_3\; +\; 3\backslash text\{HX\}$$

Uses

• Antiseptic: Boric acid is widely used as an antiseptic for minor cuts and burns. It is also used in medical dressings and salves.

• Insecticide: It is used as an insecticide to control pests such as ants, cockroaches, and termites.

• Flame Retardant: Boric acid is used as a flame retardant in various materials, including textiles and plastics.

• Neutron Absorber: It is used in nuclear power plants as a neutron absorber to control the rate of fission.

• Eyewash: Very dilute solutions of boric acid can be used as an eyewash to treat minor eye infections.

• Acne Treatment: Due to its antibacterial properties, boric acid can be used to treat acne.

• Athlete's Foot: In its powdered form, boric acid can be sprinkled into socks and shoes to prevent athlete's foot (tinea pedis).

Safety and Hazards

• Hazard Statements: H360FD (May damage fertility or the unborn child), H319 (Causes serious eye irritation)

• Precautionary Statements: P201 (Obtain special instructions before use), P280 (Wear protective gloves/protective clothing/eye protection/face protection), P305+P351+P338 (IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing), P308+P313 (IF exposed or concerned: Get medical advice/attention)

• NFPA 704 (Fire Diamond): 1 (Health), 0 (Flammability), 0 (Reactivity)

• Lethal Dose (LD50): 2660 mg/kg (oral, rat)

Environmental Impact

Boric acid is not biodegradable but is generally considered safe for the environment when used appropriately. However, it can be toxic to plants and aquatic life in high concentrations.

Handling and Storage

• Handling: Use in well-ventilated areas; avoid inhalation and contact with skin and eyes; use appropriate personal protective equipment (PPE).

• Storage: Store in tightly sealed containers made of corrosion-resistant materials; keep away from incompatible substances such as bases and oxidizers.

Historical Background

Boric acid was first prepared by Wilhelm Homberg in the early 18th century from borax by the action of mineral acids. It was initially named "sal sedativum Hombergi" (sedative salt of Homberg). Boric acid and borates have been used since ancient times for cleaning, preserving food, and other activities.

Molecular and Crystal Structure

The three oxygen atoms form a trigonal planar geometry around the boron atom. The bond lengths are approximately 136 pm for B-O and 97 pm for O-H. The structure of the unit cell of boric acid is trigonal planar.

Chemical Reactions

• Dehydration: On heating, boric acid undergoes dehydration to form metaboric acid (HBO₂) and tetraboric acid (H₂B₄O₇):

$${\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}{\text{<span style="font-family: Diphylleia;">BO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">\to </span>{\text{<span style="font-family: Diphylleia;">HBO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}<span\; style="font-family:\; Diphylleia;">+</span>{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}\backslash text\{H\}\_3\backslash text\{BO\}\_3\; \backslash rightarrow\; \backslash text\{HBO\}\_2\; +\; \backslash text\{H\}\_2\backslash text\{O\}$$

$$\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}{\text{<span style="font-family: Diphylleia;">HBO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}<span\; style="font-family:\; Diphylleia;">\to </span>{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}{\text{<span style="font-family: Diphylleia;">B</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}{\text{<span style="font-family: Diphylleia;">O</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">7</span>}}<span\; style="font-family:\; Diphylleia;">+</span>{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}4\backslash text\{HBO\}\_2\; \backslash rightarrow\; \backslash text\{H\}\_2\backslash text\{B\}\_4\backslash text\{O\}\_7\; +\; \backslash text\{H\}\_2\backslash text\{O\}$$

• Reaction with Alcohols: Boric acid reacts with alcohols to form borate esters:

$${\text{<span style="font-family: Diphylleia;">B(OH)</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}\text{<span style="font-family: Diphylleia;">ROH</span>}<span\; style="font-family:\; Diphylleia;">\to </span>{\text{<span style="font-family: Diphylleia;">B(OR)</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}\text{<span style="font-family: Diphylleia;">O</span>}\backslash text\{B(OH)\}\_3\; +\; 3\backslash text\{ROH\}\; \backslash rightarrow\; \backslash text\{B(OR)\}\_3\; +\; 3\backslash text\{H\}\_2\backslash text\{O\}$$

• Reaction with Anhydrous Sulfuric Acid: Boric acid dissolves in anhydrous sulfuric acid:

$${\text{<span style="font-family: Diphylleia;">B(OH)</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">6</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}}{\text{<span style="font-family: Diphylleia;">SO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}<span\; style="font-family:\; Diphylleia;">\to </span>{\text{<span style="font-family: Diphylleia;">B(HSO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}{\text{<span style="font-family: Diphylleia;">)</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}^{<span\; style="font-family:\; Diphylleia;">-</span>}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">2</span>}{\text{<span style="font-family: Diphylleia;">HSO</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">4</span>}}^{<span\; style="font-family:\; Diphylleia;">-</span>}<span\; style="font-family:\; Diphylleia;">+</span>\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}{\text{<span style="font-family: Diphylleia;">H</span>}}_{\mathrm{<span\; style="font-family:\; Diphylleia;">3</span>}}{\text{<span style="font-family: Diphylleia;">O</span>}}^{<span\; style="font-family:\; Diphylleia;">+</span>}\backslash text\{B(OH)\}\_3\; +\; 6\backslash text\{H\}\_2\backslash text\{SO\}\_4\; \backslash rightarrow\; \backslash text\{B(HSO\}\_4\backslash text\{)\}\_4^-\; +\; 2\backslash text\{HSO\}\_4^-\; +\; 3\backslash text\{H\}\_3\backslash text\{O\}^+$$

Industrial Applications

• Glass and Ceramics: Used in the production of borosilicate glass and ceramics.

• Fiberglass: Used in the manufacturing of monofilament fiberglass.

• Adhesives and Sealants: Used in the formulation of adhesives and sealants.

• Electronics: Used in the production of electronic components and semiconductors