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Boric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a weak 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.

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 seawater. It is also said to exist in plants and especially in almost all fruits. 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 to form the tetrahydroxyborate ion:

B(OH)3 + H2O B(OH) + H+ (Ka = 5.8x10−10 mol/l; pKa = 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:

4 B(OH) + 2 H+ ⇌ B4O + 9 H2O


Boric acid makes an important contribution to the absorption of low frequency sound in seawater.

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.

The unit cell of boric acid
Hydrogen bonding (dashed lines)
allows boric acid molecules to form
parallel layers in the solid state



Toxicology

Based on mammal median lethal dose rating of 2,660 mg/kg body mass, boric acid is poisonous if taken internally or inhaled. However, it is generally considered to be not much more toxic than table salt. 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 Merck Index.

Long term exposure to boric acid may be of more concern, causing kidney damage and eventually kidney failure (see links below). Although it does not appear to be carcinogenic, studies in dogs have attributed testicular atrophy after exposure to 32 mg/kg bw/day for 90 days. This level is far lower than the LD50.

According to boric acid IUCLID Dataset published by the European Commission, boric acid in high doses shows significant developmental toxicity and teratogenicity in rabbit, rat, and mouse fetus such as cardiovascular defects, skeletal variations, mild kidney lesions. As a consequence, in August 2008, in the 30th ATP to EU directive 67/548/EEC, the EC decided to amend its classification as reprotoxic cat. 2 and R60 (may impair fertility) R61 (may cause harm to the unborn child).

Uses

Medicinal

Boric acid 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 in a 1.5% solution or 1 tbsp (15 ml) per quart (very close to a liter) of sterilised water. 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) or for yeast infection of the male pubic region (jock-itch or strong genital odor) by applying the powder to the skin all over the pubic region for several days to a week. 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.

Insecticidal

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

Boric acid is generally considered to be safe to use in household kitchens to control cockroaches and ants. Homemade ant bait can be made by dissolving 1 teaspoon powdered boric acid and 10 teaspoons sugar into 2 cups (500 mL) of water; this mixture can then be absorbed into cotton balls which are left near ant trails. This reportedly will be carried back into the ants' nest, killing any ants that eat it, potentially destroying the entire colony.

Boric acid is also made into a paste or gel form as a powerful and effective insecticide much safer to humans than many other insecticides. The paste or gel has attractants in it to attract insects, especially cockroaches, that take the bait back to the nest. Little is necessary for killing insects for up to a year; a small dab under sinks, counters, refrigerators, and stoves is wholly adequate. The boric acid slowly and imperceptibly causes dehydration for which insects can never develop immunity.

Preservation

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 dampness 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.

Lubrication

Colloidal suspensions of nano-particles of boric acid dissolved in petroleum or vegetable oil can form a remarkable lubricant on ceramic or metal surfaces with a coefficient of sliding friction that decreases with increasing pressure to a value ranging from 0.1 to 0.02.

Self-lubricating H3BO3 films result from a spontaneous chemical reaction between water molecules and B2O3 coatings in humid environment. In bulk-scale, an inverse relationship exists between friction coefficient and Hertzian contact pressure induced by applied load.

Industrial

The primary industrial use of boric acid is in the manufacture of monofilament fiberglass usually referred to as textile fiberglass. Textile fiberglass is used to reinforce plastics in applications that range from boats, to industrial piping to computer circuit boards.

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 neutron multiplication 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.

Boric acid is used in producing the glasses of LCD flat panel displays.

In electroplating, boric acid is used as part of some proprietary formulas. One such known formula calls for about a 1 to 10 ratio of to , a very small portion of sodium lauryl sulfate and a small portion of .

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

Miscellaneous

Borates including boric acid have been used since the time of the Greek for cleaning, preserving food, and other activities.

Silly Putty was originally made by adding boric acid to silicone oil.

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 aluminium and nitrates. A small amount of boric acid is added to the composition to neutralize alkaline amides that can react with the aluminium.

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

The white powder is also used in Indiamarker and across the world to dust down Carrom boards to decrease friction and increase speed of play.

Boric acid is added to salt in the curing of cattle hides, calfskins and sheepskins. Used in that way it helps to control bacteria development and also aids in the control of insects.

Boric acid is added to borax for use as welding flux by blacksmiths and farriers.

References

Further reading





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