Boric acid, more specificallyorthoboric acid, is a compound ofboron,oxygen, andhydrogen with formulaB(OH)3. It may also be calledhydrogenorthoborate,trihydroxidoboron orboracic acid.[3] It is usually encountered as colorless crystals or a white powder, that dissolves inwater, and occurs in nature as the mineralsassolite. It is a weakacid that yields variousborateanions andsalts, and can react withalcohols to formborate esters.
Orthoboric acid was first prepared byWilhelm Homberg (1652–1715) fromborax, by the action of mineral acids, and was given the namesal sedativum Hombergi ("sedative salt of Homberg"). However, boric acid andborates have been used since the time of theancient Greeks for cleaning, preserving food, and other uses.[4]
Two crystalline forms of orthoboric acid are known:triclinic withspace group P1, andtrigonal with space group P32. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method.[6]
The triclinic form of boric acid consists of layers ofB(OH)3 molecules held together by hydrogen bonds with an O...O separation of 272 pm. The distance between two adjacent layers is 318 pm.[7] While the layers of the triclinic phase are nearly trigonal withγ = 119.76°,a = 701.87 pm, andb = 703.5 pm (compared toa = 704.53(4) pm for the trigonal form), the stacking of the layers is somewhat offset in the triclinic phase, withα = 92.49° andβ = 101.46°. The triclinic phase hasc = 634.72 pm and the trigonal one hasa = 956.08(7) pm.[8][9]
When heated, orthoboric acid undergoes a three-step dehydration. The reported transition temperatures vary substantially from source to source.[citation needed]
When heated above 140 °C, orthoboric acid yieldsmetaboric acid (HBO2) with loss of one water molecule:[11][12]
B(OH)3 →HBO2 +H2O
Heating metaboric acid above about 180 °C eliminates another water molecule formingtetraboric acid, also called pyroboric acid (H2B4O7):[11][12]
When orthoboric acid is dissolved in water, it partially dissociates to givemetaboric acid:
B(OH)3 ⇌HBO2 +H2O
The solution is mildly acidic due to the ionization of the acids:
B(OH)3 +H2O ⇌[BO(OH)2]− +H3O+
HBO2 +H2O ⇌[BO2]− +H3O+
However,Raman spectroscopy of strongly alkaline solutions has shown the presence of[B(OH)4]− ions,[14] leading some to conclude that the acidity is exclusively due to the abstraction ofOH− from water:[14]
B(OH)3 +HO− ⇌B(OH)−4
Equivalently,
B(OH)3 +H2O ⇌B(OH)−4 +H+ (Ka = 7.3×10−10; pKa = 9.14)
Or, more properly,
B(OH)3 + 2H2O ⇌B(OH)−4 +H3O+
This reaction occurs in two steps, with the neutral complex aquatrihydroxyboronB(OH)3(OH2) as an intermediate:[15]
B(OH)3 +H2O →B(OH)3(OH2)
B(OH)3(OH2) +H2O →[B(OH)4]− +H3O+
This reaction may be characterized asLewis acidity of boron towardHO−, rather than asBrønsted acidity.[16][17][18] However, some of its behaviour towards some chemical reactions suggest it to be a tribasic acid in the Brønsted-Lowry sense as well.
Boric acid, mixed withboraxNa2B4O7·10H2O (more properlyNa2B4O5(OH)4·8H2O) in the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately.[19]
Boric acid reacts with alcohols to formborate esters,B(OR)3 where R isalkyl oraryl. The reaction is typically driven by a dehydrating agent, such as concentratedsulfuric acid:[20]
The tetrahydroxyborate anion formed in the dissolution spontaneously reacts with these diols to form relatively stable anion esters containing one or two five-member−B−O−C−C−O− rings. For example, the reaction with mannitolH(HCOH)6H, whose two middle hydroxyls are incis orientation, can be written as:
The stability of these mannitoborate ester anions shifts the equilibrium to the right, thereby increasing the solution's acidity by five orders of magnitude compared to that of pure boric oxide. This lowers the pKa from 9 to below 4 for a sufficient concentration of mannitol.[21][7][22][23] The resulting solution is referred to as mannitoboric acid.
The addition of mannitol to an initially neutral solution containing boric acid or simple borates lowers itspH enough for it to be titrated by a strong base such as NaOH, including with an automated potentiometrictitrator. This property is used in analytical chemistry to determine the borate content of aqueous solutions, for example to monitor the depletion of boric acid byneutrons in the water of the primary circuit oflight-water reactor when the compound is added as aneutron poison during refueling operations.[7]
Based on mammalianmedian lethal dose (LD50) rating of 2,660 mg/kg body mass, boric acid is only poisonous if taken internally or inhaled in large quantities. The Fourteenth Edition of theMerck 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. For a 70 kg adult, at the lower 5 g/kg limit, 350 g could produce death in humans. For comparison's sake, the LD50 ofsalt is reported to be 3.75 g/kg in rats according to theMerck Index. According to theAgency for Toxic Substances and Disease Registry, "The minimal lethal dose of ingested boron (as boric acid) was reported to be 2–3 g in infants, 5–6 g in children, and 15–20 g in adults. [...] However, a review of 784 human poisonings with boric acid (10–88 g) reported no fatalities, with 88% of cases being asymptomatic."[24] Human studies in three borate exposure-rich comparison groups (U.S. Borax mine and production facility workers, Chinese boron workers, Turkish residents living near boron-rich regions) produced no indicators of developmental toxicity in blood and semen tests. The highest estimated exposure was 5 mg B/kg/day, likely due to eating in contaminated workplaces, more than 100 times the average daily exposure.[25]
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 becarcinogenic, studies in dogs have reportedtesticular atrophy after exposure to 32 mg/(kg⋅day) for 90 days. This level, were it applicable to humans at like dose, would equate to a cumulative dose of 202 g over 90 days for a 70 kg adult, not far lower than the above LD50.[26]
According to the CLH report for boric acid published by the Bureau for Chemical Substances Lodz, Poland, boric acid in high doses shows significant developmental toxicity andteratogenicity in rabbit, rat, and mouse fetuses, as well as cardiovascular defects, skeletal variations, and mild kidney lesions.[25] As a consequence in the 30th ATP to EU directive 67/548/EEC of August 2008, the European Commission decided to amend its classification asreprotoxic category 2 and to apply therisk phrases R60 (may impair fertility) and R61 (may cause harm to the unborn child).[27][28][29][30][31]
At a 2010 European Diagnostics Manufacturing Association (EDMA) Meeting, several new additions to thesubstance of very high concern (SVHC) candidate list in relation to theRegistration, Evaluation, Authorisation and Restriction of Chemicals Regulations 2007 (REACH) were discussed. Following the registration and review completed as part of REACH, the classification of boric acid CAS 10043-35-3 / 11113-50-1 is listed from 1 December 2010 isH360FD (May damage fertility. May damage the unborn child).[32][33]
The primary industrial use of boric acid is in the manufacture of monofilamentfiberglass, which is 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.[39]
Inelectroplating, boric acid is used as part of some proprietary formulas. One known formula uses about a 1 to 10 ratio ofH 3BO 3 toNiSO 4, a very small portion ofsodium lauryl sulfate and a small portion ofH 2SO 4.
The solution of orthoboric acid and borax in 4:5 ratio is used as afire retarding agent of wood by impregnation.[44] Also, it is used in combination with other chemicals for the fire retardancy of wood-based materials.[45]
Boric acid is also present in the list of chemical additives used forhydraulic fracturing (fracking) in theMarcellus Shale in Pennsylvania.[48] It is often used in conjunction withguar gum ascross-linking and gelling agent for controlling theviscosity and the rheology of the fracking fluid injected at high pressure in the well. It is important to control the fluid viscosity for keeping in suspension on long transport distances the grains of thepropping agents aimed at maintaining the cracks in theshales sufficiently open to facilitate the gas extraction after the hydraulic pressure is relieved.[49][50][51] The rheological properties of borate cross-linked guar gumhydrogel mainly depend on thepH value.[52]
Boric acid is used in someexpulsion-type electrical fuses as a de-ionization/extinguishing agent.[53] During anelectrical fault in an expulsion-type fuse, aplasma arc is generated by the disintegration and rapidspring-loaded separation of the fusible element, which is typically a specialized metal rod that passes through a compressed mass of boric acid within the fuse assembly. The high-temperature plasma causes the boric acid to rapidly decompose intowater vapor andboric anhydride, and in turn, the vaporization products de-ionize the plasma, helping to interrupt the electrical fault.[54]
Boric acid can be used as anantiseptic for minor burns or cuts and is sometimes used in salves anddressings, such asboracic lint. Boric acid is applied in a very dilute solution as an eye wash. Boric acidvaginal suppositories can be used for recurrentcandidiasis due to non-albicans candida as a second line treatment when conventional treatment has failed.[55][56] It is less effective than conventional treatment overall.[55] Boric acid largely spareslactobacilli within the vagina.[57] AsTOL-463, it is under development as anintravaginal medication for the treatment forvulvovaginal candidiasis.[58][59][60]
As anantibacterial compound, boric acid can also be used as anacne treatment. It is also used to preventathlete's foot, by inserting powder in the socks or stockings. Various preparations can be used to treat some kinds ofotitis externa (ear infection) in both humans and animals.[61] The preservative inurine sample bottles in the UK is boric acid.[62]
Boric acid solutions used as an eye wash or on abraded skin are known to be toxic, particularly to infants, especially after repeated use; this is because of its slow elimination rate.[63]
Boric acid was first registered in the US as an insecticide in 1948 for control ofcockroaches,termites,fire ants,fleas,silverfish, and many otherinsects. The product is generally considered safe in household kitchens to control cockroaches and ants. It acts as a stomach poison affecting the insects'metabolism. The dry powder isabrasive to the insects'exoskeletons.[64][65][66] It is in non-specificIRAC group 8D. Boric acid is also known as "the gift that keeps on killing" because cockroaches cross over lightly dusted areas and do not die immediately. Still, the effect is like shards of glass cutting them apart. This often allows a cockroach to return to the nest, where it soon dies. Cockroaches, beingcannibalistic, eat others killed by contact or consumption of boric acid, consuming the powder trapped in the dead roach and killing them, too.[citation needed]
Boric acid is also widely used in wood treatment to protect against termites. The full complexity of its mechanism is not fully understood. Still, aside from causing dose-dependent mortality, boric acid causesdysbiosis in theEastern Subterranean termite, leading to the opportunistic rise of insect pathogens that could be contributing to mortality.[67] In Japan the practice of laying newspapers treated with o-boric acid and borax under buildings has been effective in controllingCoptotermes formosanus andReticulitermes speratus populations. Decaying wood treated with 0.25 to 0.5 percentdisodium octaborate (Na2B8O13·4H2O, commonly abbreviated DOT) is also effective for baitingHeterotermes aureus populations. A 1997 paper concluded: "Borate baits would undoubtedly be helpful in the long-term, but do not appear sufficient as a sole method of structural protection."[68]
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 anethylene 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 are known to collect and sit. It is available in a gel form and injectable paste form for treating rot-affected wood without replacing the timber. Concentrates of borate-based treatments can be used to prevent slime, mycelium, and algae growth, even in marine environments.[citation needed]
Boric acid is added to salt in the curing of cattle hides,calfskins, andsheepskins. This helps to control bacterial development and insects.[citation needed]
Boric acid predominates in solution below pH 9Boric acid buffers against rising pH in swimming pools
Boric acid in equilibrium with its conjugate base the borate ion is widely used (in the concentration range 50–100 ppm boron equivalents) as a primary or adjunct pH buffer system inswimming pools. Boric acid is a weak acid, with pKa (the pH at which buffering is strongest because the free acid and borate ion are in equal concentrations) of 9.24 in pure water at 25 °C. But apparent pKa is substantially lower in swimming pool or ocean waters because of interactions with various other molecules in solution. It will be around 9.0 in a saltwater pool. No matter which form of soluble boron is added, within the acceptable range of pH and boron concentration for swimming pools, boric acid is the predominant form in aqueous solution, as shown in the accompanying figure. The boric acid – borate system can be useful as a primary buffer system (substituting for thebicarbonate system with pKa1 = 6.0 and pKa2 = 9.4 under typical salt-water pool conditions) in pools with salt-water chlorine generators that tend to show upward drift in pH from a working range of pH 7.5–8.2.Buffer capacity is greater against rising pH (towards the pKa around 9.0), as illustrated in the accompanying graph. The use of boric acid in this concentration range does not allow any reduction in freeHOCl concentration needed for pool sanitation. Still, it may add marginally to the photo-protective effects ofcyanuric acid and confer other benefits through anti-corrosive activity or perceived water softness, depending on overall pool solute composition.[69]
Colloidal suspensions of nanoparticles of boric acid dissolved in petroleum or vegetable oil can form a remarkable lubricant on ceramic or metal surfaces[70] with a coefficient of sliding friction that decreases with increasing pressure to a value ranging from 0.10 to 0.02. Self-lubricatingB(OH)3 films result from a spontaneous chemical reaction between water molecules andB2O3 coatings in a humid environment. On a bulk scale, an inverse relationship exists between the friction coefficient and the Hertzian contact pressure induced by the applied load.[citation needed]
Boric acid is used to lubricatecarrom andnovuss boards, allowing for faster play.[71]
Boric acid is used in somenuclear power plants as aneutron poison. The boron in boric acid reduces the probability ofthermal fission by absorbing somethermal neutrons.Fission chain reactions are generally driven by the probability that free neutrons will result in fission and is determined by the material and geometric properties of the reactor. Natural boron consists of approximately 20% boron-10 and 80% boron-11 isotopes. Boron-10 has a highcross-section for absorption of low-energy (thermal) neutrons. By increasing boric acid concentration in the reactor coolant, the probability that a neutron will cause fission is reduced. Changes in boric acid concentration can effectively regulate the rate of fission taking place in the reactor. During normal at power operation, boric acid is used only inpressurized water reactors (PWRs), whereasboiling water reactors (BWRs) employcontrol rod pattern and coolant flow for power control. However, BWRs can use an aqueous solution of boric acid andborax orsodium pentaborate for an emergency shutdown system if the control rods fail to insert. Boric acid may be dissolved inspent fuel pools used to storespent fuel elements. The concentration is high enough to keep neutron multiplication at a minimum. Boric acid was dumped over Reactor 4 of theChernobyl nuclear power plant after itsmeltdown to prevent another reaction from occurring.[citation needed]
Boron is used inpyrotechnics to prevent theamide-forming reaction betweenaluminium andnitrates. 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 inmethanol, it is popularly used byfire jugglers and fire spinners to create a deep green flame much stronger than copper sulfate.[72]
^Andrei Rotaru (2017): "Thermal and kinetic study of hexagonal boric acid versus triclinic boric acid in air flow."Journal of Thermal Analysis and Calorimetry, volume 127, pages 755–763.doi:10.1007/s10973-016-5583-7
^Housecroft, C. E.; Sharpe, A. G. (2008). "Chapter 13: The Group 13 Elements".Inorganic Chemistry (3rd ed.). Pearson. p. 340.ISBN978-0-13-175553-6.
^abcGurwinder Kaur, Shagun Kainth, Rohit Kumar, Piyush Sharma and O. P. Pandey (2021): "Reaction kinetics during non-isothermal solid-state synthesis of boron trioxide via boric acid dehydration."Reaction Kinetics, Mechanisms and Catalysis, volume 134, pages 347–359.doi:10.1007/s11144-021-02084-8
^abcSiavash Aghili, Masoud Panjepour, and Mahmood Meratian (2018): "Kinetic analysis of formation of boron trioxide from thermal decomposition of boric acid under non-isothermal conditions."Journal of Thermal Analysis and Calorimetry, volume 131, pages 2443–2455.doi:10.1007/s10973-017-6740-3
^Balci, Suna; Sezgi, Naime; Eren, Esin (2012). "Boron Oxide Production Kinetics Using Boric Acid as Raw Material".Industrial & Engineering Chemistry Research.51 (34):11091–11096.doi:10.1021/ie300685x.hdl:11511/40051.
^abJolly, W. L. (1984).Modern Inorganic Chemistry. McGraw-Hill. p. 198.
^Masanori Tachikawa (2004): "A density functional study on hydrated clusters of orthoboric acid, B(OH)3(H2O)n (n=1–5)".Journal of Molecular Structure: THEOCHEM, volume 710, issues 1–3, pages 139-150.doi:10.1016/j.theochem.2004.09.008
^Tsuyumoto, I.; Oshio, T.; Katayama, K. (2007). "Preparation of highly concentrated aqueous solution of sodium borate".Inorganic Chemistry Communications.10 (1):20–22.doi:10.1016/j.inoche.2006.08.019.
^Brown, Herbert C.; Mead, Edward J.; Shoaf, Charles J. (1956). "Convenient procedures for the preparation of alkyl borate esters".J. Am. Chem. Soc.78 (15):3613–3614.Bibcode:1956JAChS..78.3613B.doi:10.1021/ja01596a015.
^abMendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000),Vogel's Quantitative Chemical Analysis (6th ed.), New York: Prentice Hall, p. 357,ISBN0-582-22628-7
^abNIST Special Publication. U.S. Government Printing Office. 1969.
^Ishii, Y.; Fujizuka, N.; Takahashi, T.; et al. (1993). "A fatal case of acute boric acid poisoning".Clinical Toxicology.31 (2):345–352.doi:10.3109/15563659309000402.PMID8492348.
^Restuccio, A.; Mortensen, M. E.; Kelley, M. T. (1992). "Fatal ingestion of boric acid in an adult".American Journal of Emergency Medicine.10 (6):545–547.doi:10.1016/0735-6757(92)90180-6.PMID1388380.
^Duldner, J. E. (30 January 2009)."Boric acid poisoning".A.D.A.M. Medical Encyclopedia. MedLine Plus.
^NSW Food Authority."Borax and boric acid". Australia: New South Wales Government. Archived fromthe original on 15 October 2009. Retrieved24 September 2009.
^Kistler, R. B.; Helvaci, C. (1994). "Boron and Borates". In Carr, D. D. (ed.).Industrial Minerals and Rocks (6th ed.). Littleton, CO: SME. pp. 171–186.
^Tsuyumoto, I.; Oshio, T. (2009). "Development of fire resistant laminated wood using concentrated sodium polyborate aqueous solution".Journal of Wood Chemistry and Technology.29 (4):277–285.doi:10.1080/02773810903033721.S2CID98730912.
^Bishop, Maximilienne; Shahid, Naureen; Yang, Jianzhong; Barron, Andrew R. (2004). "Determination of the mode and efficacy of the cross-linking of guar by borate using MAS11B NMR of borate cross-linked guar in combination with solution11B NMR of model systems".Dalton Trans. (17):2621–2634.doi:10.1039/B406952H.ISSN1477-9226.PMID15514744.
^Nyirjesy P, Brookhart C, Lazenby G, Schwebke J, Sobel JD (April 2022). "Vulvovaginal Candidiasis: A Review of the Evidence for the 2021 Centers for Disease Control and Prevention of Sexually Transmitted Infections Treatment Guidelines".Clin Infect Dis.74 (Suppl_2):S162 –S168.doi:10.1093/cid/ciab1057.PMID35416967.
^"R.E.D. Facts – Boric Acid"(PDF). United States Environmental Protection Agency.Archived(PDF) from the original on 23 December 2021. Retrieved2 April 2022.
^Düzcükoğlu, H.; Acaroğlu, M. (2009). "Lubrication Properties of Vegetable Oils Combined with Boric Acid and Determination of Their Effects on Wear".Energy Sources, Part A: Recovery, Utilization, and Environmental Effects.32 (3):275–285.Bibcode:2009EneSA..32..275D.doi:10.1080/15567030802606053.S2CID97537085.
^Singh, Harpreet."Standard equipments". Punjab State Carrom Association. Archived fromthe original on 14 March 2007. Retrieved24 September 2009.
Jolly, W. L. (1991).Modern Inorganic Chemistry (2nd ed.). New York: McGraw-Hill.ISBN978-0-07-112651-9.
Goodman, L.; Gilman, A.; Brunton, L.; Lazo, J.; Parker, K. (2006).Goodman & Gilman's The Pharmacological Basis of Therapeutics. New York: McGraw Hill.
Cordia, J.A., Bal, E.A., Mak, W.A. and Wils, E.R.J. (2003).Determination of some physico-chemical properties of Optibor EP. Rijswijk, The Netherlands: TNO Prins Maurits Laboratory, report PML 2002-C42rr, GLP, Unpublished, confidential data provided by Borax Europe Limited.
