Boric acid

Boric acid
IUPAC name Boric acid Trihydroxidoboron
Other names Orthoboric acid, Boracic acid, Sassolite, Optibor, Borofax
Identifiers
CAS number	10043-35-3 Y
PubChem	7628
ChemSpider	7346
SMILES OB(O)O
InChI InChI=1/BH3O3/c2-1(3)4/h2-4H Key: KGBXLFKZBHKPEV-UHFFFAOYAI
Properties
Molecular formula	H3BO3
Molar mass	61.83 g mol−1
Appearance	White crystalline solid
Density	1.435 g/cm3
Melting point	170.9 °C, 444 K, 340 °F
Boiling point	300 °C, 573 K, 572 °F
Solubility in water	2.52 g/100 mL (0 °C) 4.72 g/100 mL (20 °C) 5.7 g/100 mL (25°C) 19.10 g/100 mL (80 °C) 27.53 g/100 mL (100 °C)
Solubility in other solvents	Soluble in lower alcohols moderately soluble in pyridine very slightly soluble in acetone
Acidity (pKa)	5.2 (see text)
Structure
Molecular shape	Trigonal planar
Dipole moment	Zero
Hazards
EU classification	Harmful (Xn) Repr. Cat. 2
R-phrases	R60 R61
S-phrases	S53 S45
NFPA 704	0 1 0
Flash point	Non-flammable.
LD50	2660 mg/kg, oral (rat)
Related compounds
Related compounds	Boron trioxide Borax
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.

Occurrence

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). The presence of boric acid and its salts has been noted in seawater. It also exists in plants and especially in almost all fruits.^[1]

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

Preparation

Boric acid may be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:

Na₂B₄O₇·10H₂O + 2 HCl → 4 B(OH)₃ [or H₃BO₃] + 2 NaCl + 5 H₂O

Properties

Boric acid is soluble in boiling water. When heated above 170 °C, it dehydrates, forming metaboric acid (HBO₂):

H₃BO₃ → HBO₂ + H₂O

Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. Boric acid melts at about 236 °C, and when heated above about 300 °C further dehydrates, forming tetraboric acid or pyroboric acid (H₂B₄O₇):

4 HBO₂ → H₂B₄O₇ + H₂O

The term boric acid may sometimes refer to any of these compounds. Further heating leads to boron trioxide.

H₂B₄O₇ → 2 B₂O₃ + H₂O

Boric acid does not dissociate in aqueous solution as a Brønsted acid, but is a Lewis acid which interacts with water molecules to form the tetrahydroxyborate ion, as observed by Raman spectroscopy^[2]:

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:

4 B(OH)−4 + 2 H⁺ ⇌ B₄O2−7 + 9 H₂O

Boric acid makes an important contribution to the absorption of low frequency sound in seawater.^[3]

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 mammalian median lethal dose (LD₅₀) rating of 2,660 mg/kg body mass, boric acid is poisonous if taken internally or inhaled in large quantities. However, it is generally considered to be not much more toxic than table salt.^[4] 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 reported testicular atrophy after exposure to 32 mg/kg bw/day for 90 days. This level is far lower than the LD₅₀.^[5]

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 fetuses as well as cardiovascular defects, skeletal variations, mild kidney lesions.^[6] 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 category 2 and to apply the risk phrases R60 (may impair fertility) and R61 (may cause harm to the unborn child).^{[7][8][9][10][11]}

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 (1 tbsp per quart or 15 cm³ per L) of sterilized 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^[12] 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.

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.

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.^[13] 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^[14].

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. The boric acid slowly causes dehydration.

Preservation

In combination with its use as an insecticide, boric acid 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. Concentrates of borate-based treatments can be used to prevent slime, mycelium and algae growth, even in marine environments.

Boric acid is added to salt in the curing of cattle hides, calfskins and sheepskins. This helps to control bacteria development and helps to control insects.

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^[15] with a coefficient of sliding friction that decreases with increasing pressure to a value ranging from 0.10 to 0.02. Self-lubricating H₃BO₃ films result from a spontaneous chemical reaction between water molecules and B₂O₃ coatings in a 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.^[16]

Boric acid is used in nuclear power plants as a neutron poison 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, changes in boric acid concentration effectively regulate the rate of fission taking place in the reactor. This method is only used 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 the production of the glass in 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 H₃BO₃ to NiSO₄, a very small portion of sodium lauryl sulfate and a small portion of H₂SO₄.

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

Pyrotechnics

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 can be used as a colorant to make fire green. For example, when dissolved in methanol it is popularly used among fire jugglers and fire spinners to create a deep green flame.

Other

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

The white powder is also used to dust down Carrom boards to decrease friction and increase speed of play.^[17]

Boric acid is added to borax for use as welding flux by blacksmiths and farriers.^[18]

References

Further reading

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

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

External links

• Boric Acid Technical Fact Sheet - National Pesticide Information Center
• Boric Acid General Fact Sheet - National Pesticide Information Center
• International Chemical Safety Card 0991
• US EPA Pesticide Reregistration Eligibility Decision
• National Pollutant Inventory - Boron and compounds
• Boric acid at ChemicalLand21
• European Chemicals Agency (ECHA)"New Public Consultation on Eight Potential Substances of Very High Concern" - includes Boric Acid. Closes 22nd April 2010
• ChemSub Online: Boric acid