Boric acid

Boric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a mild acid often used as an antiseptic, insecticide, flame retardant, in nuclear power plants to control the fission rate of uranium, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a white powder and dissolves in water. It has the chemical formula H₃BO₃, sometimes written B(OH)₃. When occurring as a mineral, it is called sassolite.

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Preparation

Boric acid is produced mainly from borate minerals by the reaction with sulfuric acid. The largest source of borates in the world is an open-pit mine in Boron, California, USA.

Properties

Boric acid was first prepared by Wilhelm Homberg (1652-1715) from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi ("sedative salt of Homberg"). The presence of boric acid or its salts has been noted in sea-water. It is also said to exist in plants and especially in almost all fruits (A. H. Allen, Analyst, 1904, 301). The free acid is found native in certain volcanic districts such as Tuscany, the Lipari Islands and Nevada, issuing mixed with steam from fissures in the ground; it is also found as a constituent of many minerals (borax, boracite, boronatrocaicite and colemanite). Boric acid is soluble in boiling water. When heated above 170°C it dehydrates, forming metaboric acid HBO₂. Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. It melts at about 236°C, and when heated above about 300°C further dehydrates, forming tetraboric acid or pyroboric acid, H₂B₄O₇. Boric acid can refer to any of these compounds. Further heating leads to boron trioxide.

Boric acid does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules:

B(OH)₃ + H₂O ⇌ B(OH)₄⁻ + H⁺

K_a = 5.8x10⁻¹⁰ mol/l; pK_a = 9.24.

Polyborate anions are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the tetraborate ion, found in the mineral borax:

4B(OH)₄⁻ + 2H⁺ ⇌ B₄O₇²⁻ + 9H₂O

Crystal structure

Crystalline boric acid consists of layers of B(OH)₃ molecules held together by hydrogen bonds. The distance between two adjacent layers is 318 pm.

Toxicology

Boric acid is poisonous if taken internally or inhaled although it is generally not considered to be much more toxic than table salt (based on its mammal LD₅₀ rating of 2660mg/kg body mass).[1] The Thirteenth Edition of the Merck Index indicates that the LD₅₀ of boric acid is 5.14 g/kg for oral dosages given to rats, and that 5 to 20 g/kg has produced death in adult humans. The LD₅₀ of sodium chloride is reported to be 3.75 g/kg in rats according to the Merk Index. According to the Dutch Health Council(1998/19) Boric Acid should be regarded as if it impairs fertility in humans(R60).

However, boric acid is toxic to unborn infants. Also, it has been associated with low birth weight, eye malformations and problems with the nervous system.^{[citation needed]}

Uses

Medicinal uses

It can be used as an antiseptic for minor burns or cuts and is sometimes used in dressings or salves or is applied in a very dilute solution as an eye wash. (1.5% solution or 1 tbsp per quart of boiled water has been suggested for the latter.) As an anti-bacterial compound, boric acid can also be used as an acne treatment. Boric acid can be used to treat yeast and fungal infections such as candidiasis (vaginal yeast infections) by inserting a vaginal suppository containing 600 mg of boric acid daily for 14 days (PMID 10865926). It is also used as prevention of athlete's foot, by inserting powder in the socks or stockings, and in solution can be used to treat some kinds of otitis externa (ear infection) in both humans and animals. The preservative in urine sample bottles (red cap) in the UK is boric acid.

Boric acid has the distinction of being the only known acid that is actually beneficial (rather than harmful) to the eyes, and as such is used by ophthalmologists and in some commercial eye drops.

Insecticidal use

Boric acid was first registered as an insecticide in 1948 by the EPA for control of cockroaches, termites, fire ants, fleas, silverfish, and many other insects. [2] It acts as a stomach poison affecting the insects' metabolism, and the dry powder is abrasive to the insects' exoskeleton.

Boric acid may be used either in an insect bait formulation containing a feed attractant or as a dry powder. The powder may be injected into cracks and crevices, where it forms a fine layer of dust. Insects travel through the boric acid dust, which adheres to their legs. When the insects groom themselves, they then ingest the poison, which causes death three to ten days later of starvation and dehydration.

Preservative Use

In combination with its use as an insecticide it also prevents and destroys existing wet and dry rot in timbers. It can be used in combination with an ethylene glycol carrier to treat external wood against fungal and insect attack. It is possible to buy Borate impregnated rods for insertion into wood via drill holes where damp and moisture is known to collect and sit. It is available in a gel form and injectable paste form for treating rot affected wood without the need to replace the timber. You can buy concentrates of Borate based timber treatments which can be sprayed or dipped. Surface treatments prevent slime, mycelium and algae growth even in marine environments. There is a wide range of manufacturers of wood preservers based on boric acid/ borate mineral salts.

Industrial uses

Boric acid is used in nuclear power plants to slow down the rate at which fission is occurring. Fission chain reactions are generally driven by the amount of neutrons present (as products from previous fissions). Natural Boron is 20% Boron-10 and about 80% Boron-11. Boron-10 has a high cross-section for absorption of low energy (thermal) neutrons. By adding more boric acid to the reactor coolant which circulates through the reactor, the probability that a neutron can survive to cause fission is reduced. Therefore, boric acid concentration changes effectively regulate the rate of fissions taking place in the reactor. This is only done in Pressurized Water Reactors (PWR's). Boron is also dissolved into the spent fuel pools containing used uranium rods. The concentration is high enough to keep fissions at a minimum.

In the jewelry industry, boric acid is often used in combination with denatured alcohol to reduce surface oxidation and firescale from forming on metals during annealing and soldering operations.

It is also used in the manufacturing of remming mass, a fine silica-containing powder used for producing induction furnace linings and ceramics.

Miscellaneous uses

Borates including boric acid have been used since the time of the Greeks for cleaning, preserving food, and other activities.^{[citation needed]}

Silly Putty was originally made by adding boric acid to silicone oil. Now name-brand Silly Putty also contains significant amounts of elemental silicon (silicon binds to the silicone and allows the material to bounce 20% higher).

Lithium borate is the lithium salt of boric acid and is used in the laboratory as buffer for gel. TBE buffer is widely used for the electrophoresis of nucleic acids and has a higher buffer capacity than a TAE Buffer. It can be used for DNA and RNA polyacrylamide and agarose gel electrophoresis.

It is used in pyrotechnics to prevent the amide-forming reaction between aluminum and nitrates. A small amount of boric acid is added to the composition to neutralize alkaline amides that can react with the aluminum.

Boric acid is popularly used among fire jugglers and fire spinners dissolved in methanol to give a deep green flame.

It is also used in India and across the world to dust down Carrom boards to decrease friction and increase speed of play.^{[citation needed]}

References

• Jolly, W. L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1. 

Further reading

• Louis Goodman, Alfred Gilman, Laurence Brunton, John Lazo and Keith Parker (2006). Goodman & Gilman's The Pharmacological Basis of Therapeutics. New York: McGraw Hill.