BORIC ACID: CHEMICAL STRUCTURE, PROPERTIES, PREPARATION, USES - CHEMISTRY - 2025

The boric acid is an inorganic compound with the chemical formula H ₃ BO ₃. It consists of a white or colorless solid. It is a weak acid that in aqueous solution produces, depending on its concentration, a pH of between 3.8 and 4.8. It is poorly soluble in cold water and moderately soluble in hot water.

Boric acid was discovered in 1702 by Wilhelm Homberg (1652-1713), who treated borax with sulfuric acid, obtaining a solution with medicinal action that received the name of Homberg's sedative water.

Solid sample of boric acid on watch glass. Source: Walkerma via Wikipedia.

However, it is also noted that Homberg prepared the boric acid by adding water to the borax and heating the solution until evaporation, thus leaving the boric acid crystals in the sediment.

This acid is a highly useful compound that has numerous applications in industry and medicine, also being used as an insecticide, wood preservation, fire retardant and is a constituent of a pH buffer solution.

Chemical structure

Boric acid molecule represented by a model of spheres and bars. Source: Benjah-bmm27

In the upper image we have the actual molecule of H ₃ BO ₃. Note that the hydrogen atoms, represented by white spheres, are not bound to the central boron atom, as the chemical formula might suggest; but rather, to the oxygen atoms, represented by red spheres.

Thus, a more convenient formula, although less used, for boric acid is B (OH) ₃, indicating that its acidity is due to the H ⁺ ions released from its OH groups. The B (OH) ₃ molecule has a trigonal plane geometry, with its boron atom having an sp ² chemical hybridization.

B (OH) ₃ is a highly covalent molecule, since the electronegativity difference between the boron atom and oxygen is not very large; so BO bonds are essentially covalent. Also note that the structure of this molecule resembles that of a spinner. Will it be able to rotate on its own axis in the same way?

Crystals

Unit cell for the H3BO3 crystal. Source: Benjah-bmm27

The upper image shows the unit cell corresponding to the triclinic crystalline structure for boric acid which, under certain synthetic conditions, can adopt the compact hexagonal structure. Note that there are four molecules per unit cell, and that they are arranged in two layers, A and B, alternating (they are not superimposed one on top of the other).

Due to the symmetry and orientation of the B-OH bonds, it can be assumed that B (OH) ₃ is apolar; however, the existence of intermolecular hydrogen bonds changes the story. Each B (OH) ₃ molecule will give or receive three of these bridges, with a total of six dipole-dipole interactions, as seen in the image below:

Crystalline layers of H3BO3 observed from a higher axis. Source: Benjah-bmm27

Note that these hydrogen bonds are the directional interactions that govern boric acid crystals, and establish patterns of ornamental aspects; inner rings perhaps with enough space to occlude some impurities that are not in the crystal defects.

These hydrogen bonds, despite the low molecular mass of B (OH) ₃, keep its crystal sufficiently cohesive that it needs a temperature of 171 ºC to melt. It is unknown what effects high pressures (in the order of GPa) would have on the molecular layers of B (OH) ₃.

Properties

Names

IUPAC: boric acid and boron trihydrooxide. Other names: orthoboric acid, boracic acid, sassolite, borofax, and trihydroxyborane.

Molar mass

61.83 g / mol

Physical appearance

Clear, colorless, crystalline white solid. It is also available as granules or as a white powder. Slightly creamy to the touch.

Odor

Toilet

Taste

Slightly bitter

Melting point

170.9 ºC

Boiling point

300 ºC

Water solubility

Moderately soluble in cold water and soluble in hot water:

2.52 g / 100 mL (0 ºC)

27.50 g / 100 mL (100 ºC)

Boric acid is partially soluble in water and tends to sink in it. Said solubility tends to increase in the presence of acids, such as hydrochloric, citric and tartaric.

Solubility in other solvents

-Glycerol: 17.5% at 25 ºC

-Ethylene glycol: 18.5% at 25 ºC

-Acetone: 0.6% at 25 ºC

-Ethyl acetate: 1.5% at 25 ºC

-Meethanol: 172 g / L at 25 ºC

-Ethanol: 94.4 g / L at 25 ºC

Octanol / water partition coefficient

Log P = -0.29

Acidity (pKa)

9.24. 12.4. 13.3. These are the three constants of their respective dissociations to release H ⁺ in water.

pH

3.8 - 4.8 (3.3% in aqueous solution)

5.1 (0.1 molar)

Vapor pressure

1.6 10 ^-6 mmHg

Stability

Stable in water

Decomposition

It decomposes when heated above 100ºC, forming boric anhydride and water.

Reactivity

Boric acid forms soluble salts with monovalent cations, for example: Na ₂ B ₄ O ₇.10H ₂ O, and insoluble salts with divalent cations, CaB ₄ O ₇.6H ₂ O.

In aqueous solution it produces an acidic solution, thinking that this property was due to the subtraction of OH ^- groups from water. Boric acid is classified as a weak acid of the Lewis type.

Boric acid reacts with glycerol and mannitol, increasing the acidity of the aqueous medium. The pKa is modified from 9.2 to 5, due to the formation of the boron-mannitol chelate ^-, which releases H ⁺.

Preparation

Boric acid is found in a free state in volcanic emanations in regions such as Tuscany in Italy, the Lipari Islands and in the state of Nevada, USA. It is also found in minerals such as borax, boracite, ulexite, and colemanite.

Boric acid is mainly prepared by the reaction of the mineral borax (sodium tetraborate decahydrate) with mineral acids, such as hydrochloric acid, sulfuric acid, etc.

Na ₂ B ₄ O ₉ 10H ₂ O + HCl => 4 H ₃ BO ₃ + 2 NaCl + 5 H ₂ O

It is also prepared by hydrolysis of boron trihalide and diborane.

Boric acid is prepared from colemanite (Ca ₂ B ₆ O ₁₁ · 6 H ₂ O). The procedure consists of treating the mineral with sulfuric acid to dissolve the boron compounds.

Then, the solution where boric acid is present is separated from the undissolved fragments. The solution is treated with hydrogen sulphide to precipitate the arsenic and iron impurities. The supernatant is cooled to produce a boric acid precipitate and separate it from the suspension.

Applications

In the industry

Boric acid is used in the manufacture of fiberglass. By helping to lower the melting point, it increases the strength and efficiency of textile fiberglass - a material used to reinforce plastic used in ships, industrial piping, and computer circuit boards.

Boric acid participates in the manufacture of borosilicate glass, which allows the obtaining of a glass resistant to temperature changes, usable in the kitchen of homes, in glassware in laboratories, fluorescent tubes, fiber optics, LCD screens, etc.

It is used in the metallurgical industry to harden and treat steel alloys, as well as to promote the metallic coating of materials.

It is used as a chemical component in hydraulic fracturing (fracking): a method used in oil and gas extraction. Boric acid acts as a fire retardant in cellulose materials, plastics and textiles that are impregnated with boric acid to increase their resistance to fire.

In medicine

Boric acid is used diluted in eye wash. Boric acid gelatin capsules are used to treat fungal infections, especially Candida albicans. It has also been used in the treatment of acne.

Boric acid is sprinkled on socks to prevent foot infections, such as athlete's foot. Likewise, solutions containing boric acid are used in the treatment of otitis externa in humans, as well as in animals.

Boric acid is added to the bottles used for urine collection, which prevents bacterial contamination before being examined in diagnostic laboratories.

Insecticide

Boric acid is used to control the proliferation of insects, such as cockroaches, termites, ants, etc. It does not instantly kill roaches, as it first affects their digestive and nervous systems, as well as destroying their exoskeleton.

Boric acid works slowly, allowing insects that have ingested it to come into contact with other insects, thus spreading poisoning.

Preservation

Boric acid is used to prevent the attack of wood by fungi and insects, being used for this purpose in conjunction with ethylene glycol. This treatment is also effective in controlling silt and algae.

PH buffer

Boric acid and its conjugate base form a buffer system of pKa = 9.24, which indicates that this buffer is most effective at alkaline pH, which is not usual in living beings. However, borate buffer is used in pH regulation in swimming pools.

Nuclear reactors

Boric acid has the ability to capture thermal neutrons, which reduces the possibility of uncontrolled nuclear fission that can lead to nuclear accidents.

farming

Boron is an essential element for plant growth, which led to the use of boric acid for the contribution of the element. However, too much boric acid can harm plants, especially citrus fruits.

Contraindications

The use of boric acid on denuded skin, wound or burn product that allows its absorption should be avoided. This is a toxic compound inside the body and in some cases, it can cause fainting, seizures, spasms, facial tics, and low blood pressure.

The doctor should be consulted for the use of boric acid vaginal capsules in pregnant women, since it has been indicated that it could cause fetal deformation and a decrease in the body weight of the child at birth.

The placement of boric acid, used as an insecticide, in places within the reach of children should also be avoided, since children have a greater susceptibility to the toxic action of boric acid, establishing the fatal dose in them at 2,000 to 3,000 mg.

References

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