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.


Boric acid is produced mainly from borate minerals by the reaction with sulfuric acid. The largest source of borates in the world is Eti Mine Works of Turkey.


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:

B(OH)3 + H2O ⇌ B(OH)4 + 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:
4B(OH)4 + 2H+ ⇌ B4O72− + 9H2O.

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

Crystaline 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


Based on mammal median lethal dose (LD50) 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. 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.



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 in a 1.5% solution or 1 tbsp per quart 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). 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 can be poisonous. The lethal dose is approximately 15 grams in adults and 5 gram in small children. Symptoms include:anorexia, diarrhea, headache, nausea, emesis, restlessness and weakness. Higher dosages can produce hyperthermia, huypothermia, hypotension and shock. Dialysis is used to treat the poison victim. Dopamine or norepinephrine drip can reverse symptoms of hypotension. Protocols for treating shock are similar to standard protocols.


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 (5 mL) powdered boric acid and 10 teaspoons (50 mL) 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, decimating or even completely wiping out the colony.


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.


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.

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

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


Borates including boric acid have been used since the time of the Greeks 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 India 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, calf skins and sheep skins. Used in that way it helps to control bacteria development and also aids in the control of insects.


Further reading

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

External links

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