H₃BO₃ — Boric Acid

Boric acid (H₃BO₃), also known as hydrogen borate or orthoboric acid, is a weak inorganic acid of boron. It naturally occurs in volcanic regions, hot springs, and certain minerals such as borax and colemanite. Boric acid is one of the most versatile boron compounds, serving multiple roles in medicine, industry, and chemistry.

In its pure form, boric acid appears as colorless, pearly crystals that are soluble in water, alcohol, and glycerol. It acts as a mild antiseptic, insecticide, flame retardant, and lubricant additive. In laboratory settings, it is used as a pH buffer and as a precursor to other boron compounds such as borates and borax.

Boric acid has the chemical formula \(H_3BO_3\) and a trigonal planar structure. Each boron atom is bonded to three hydroxyl (–OH) groups. In the solid state, molecules are linked together through extensive hydrogen bonding, forming a layered crystal structure.

The central boron atom is sp² hybridized, and each B–O bond lies in a single plane. Although boric acid contains hydroxyl groups, it does not act as a proton donor like typical Brønsted acids. Instead, it behaves as a Lewis acid by accepting hydroxide ions from water:

This reaction explains its weak acidity in aqueous solutions.

Boric acid occurs naturally in volcanic craters, hot springs, and seawater. It is also found in certain minerals like:

• Borax (Na₂B₄O₇·10H₂O)
• Colemanite (Ca₂B₆O₁₁·5H₂O)
• Ulexite (NaCaB₅O₉·8H₂O)

Commercially, boric acid is extracted from borax by reaction with mineral acids such as hydrochloric acid:

1. From Borax (Sodium Tetraborate)

The most common method for producing boric acid involves treating borax with dilute hydrochloric or sulfuric acid:

This reaction yields boric acid crystals upon cooling and evaporation.

2. From Colemanite

Calcium borate (colemanite) reacts with sulfuric acid to form boric acid:

3. By Hydrolysis of Boron Trifluoride

Boron trifluoride hydrolyzes in the presence of water to form boric acid:

• Physical State: White, crystalline solid that dissolves in hot water.
• Acidity: Weak monobasic acid that acts as a Lewis acid, accepting hydroxide ions instead of donating protons.
• Solubility: Soluble in water and alcohol; solubility increases with temperature.
• Thermal Decomposition: On heating, boric acid loses water to form metaboric and boron trioxide:

\(H_3BO_3 \xrightarrow{100°C} HBO_2 + H_2O\)

\(2HBO_2 \xrightarrow{150°C} B_2O_3 + H_2O\)

• Reaction with Bases: Reacts with bases to form borates:

\(H_3BO_3 + NaOH \rightarrow NaBO_2 + 2H_2O\)

• Reaction with Polyhydric Alcohols: With mannitol or glycerol, boric acid forms complex acids that exhibit stronger acidity.

• Medical Applications: Used as an antiseptic for treating minor burns, wounds, and eye infections in dilute solutions (around 2–3%).
• Insecticide: Effective against cockroaches, termites, ants, and other insects when mixed with sugar or flour as bait.
• Glass and Ceramics Industry: Improves thermal and chemical resistance of borosilicate glass and ceramics.
• Flame Retardant: Added to materials like wood, paper, and textiles to enhance fire resistance.
• Laboratory Reagent: Acts as a weak acid and buffering agent in chemical and biological experiments.
• Nuclear Reactors: Used as a neutron absorber in pressurized water reactors due to the boron isotope (¹⁰B).
• Cosmetics and Skincare: Serves as a pH stabilizer and preservative in lotions, creams, and powders.
• Adhesives and Lubricants: Added to adhesives and coolants to prevent bacterial growth and improve performance.

Boric acid is generally safe in low concentrations but can be toxic if ingested in large amounts. Prolonged exposure or contact with concentrated boric acid may cause skin irritation, nausea, or respiratory discomfort. It should not be used internally or on deep wounds.

When handling boric acid, gloves, masks, and safety glasses are recommended. It is relatively stable under normal conditions but decomposes at high temperatures. Despite these precautions, its mild antiseptic nature makes it a widely used compound in medicine and household applications.