Boric Acid
Boric Acid
Boric acid, also known as hydrogen borate, boracic acid, orthoboric acid, and acidum boricum, is a weak, monobasic Lewis acid of boron. It has the chemical formula H₃BO₃, sometimes written B(OH)₃. Boric acid is often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other chemical compounds.
Properties of Boric Acid
Boric acid is a colorless crystal or white powder that dissolves in water. When occurring as a mineral, it is called sassolite.
- Molecular Formula: H₃BO₃ or B(OH)₃
- Molar Mass: 61.83 g/mol
- Density: 1.435 g/cm³
- Melting Point: 170.9 °C
- Boiling Point: 300 °C (decomposes)
- Solubility in Water: 5.7 g/100 mL (20 °C)
- pKa: ~9.24 (at 25 °C)
Structure and Bonding
Boric acid consists of boron atoms bonded to three hydroxyl groups. In its crystalline form, the molecules are held together by hydrogen bonds, forming a layered structure.
Chemical Structure
The chemical structure of boric acid can be represented as:
H
|
H–O–B–O–H
|
H
Preparation of Boric Acid
Boric acid can be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:
[ Na_2B_4O_7 \cdot 10H_2O + 2HCl \rightarrow 4H_3BO_3 + 2NaCl + 5H_2O ]
Uses of Boric Acid
Boric acid has a variety of uses in different fields:
- Antiseptic: Used in the treatment of minor cuts and burns.
- Insecticide: Effective against cockroaches, termites, ants, and other insects.
- Flame Retardant: Used in the manufacture of textiles and plastics.
- Neutron Absorber: Used in nuclear power plants to control the fission rate of uranium.
- Precursor to Other Chemicals: Used to produce borate esters and other boron-containing compounds.
Toxicity and Safety
Boric acid is relatively non-toxic to humans and animals, but it can be harmful if ingested in large amounts or over a long period. It is also an eye irritant.
Differences between Boric Acid and Other Acids
Here is a table comparing boric acid with sulfuric acid (a strong acid) and acetic acid (a weak acid):
| Property | Boric Acid (H₃BO₃) | Sulfuric Acid (H₂SO₄) | Acetic Acid (CH₃COOH) |
|---|---|---|---|
| Strength | Weak acid | Strong acid | Weak acid |
| pKa | ~9.24 | -3 (first dissociation) | ~4.75 |
| Uses | Antiseptic, insecticide | Battery acid, industrial processes | Vinegar, solvent |
| Toxicity | Low toxicity | High toxicity | Low toxicity |
Examples of Boric Acid Use
Antiseptic
Boric acid can be used as an eyewash to treat bacterial conjunctivitis. A dilute solution of boric acid is used to cleanse and soothe the irritated eyes.
Insecticide
A common use of boric acid is as a bait for cockroaches. The powder is mixed with food attractants and placed in areas where cockroaches are found. When ingested, it disrupts their digestive system and results in death.
Flame Retardant
Boric acid is used in the manufacture of cellulose insulation as a flame retardant. It reduces the flammability of the material and increases its resistance to heat.
Conclusion
Boric acid is a versatile compound with a wide range of applications. Its weak acidity and low toxicity make it suitable for various uses in medical, industrial, and household contexts. Understanding its properties, preparation, and uses is essential for students and professionals in chemistry and related fields.