View source for Boric acid

{{Short description|Weak acid with formula B(OH)3}}
{{About|boric acid as a chemical and in general|boric acid as a vaginal treatment|Boric acid (vaginal)}}
{{Use dmy dates|date=February 2023}}
{{Chembox
|Watchedfields = changed
|verifiedrevid = 476995365
|ImageFileL1_Ref = {{chemboximage|correct|??}}
|Name = Orthoboric acid
|ImageFileL1 = Boric-acid-2D.png
|ImageNameL1 = Structural formula
|ImageFileR1 = Boric-acid-3D-vdW.png
|ImageNameR1 = Space-filling model
|ImageFile2 = Boric acid.jpg
|ImageName2 = Boric acid crystals
|IUPACName = Boric acid<ref name=ncbi/>
|OtherNames = Orthoboric acid, Boracic acid, Sassolite, Borofax, Trihydroxyborane, Boranetriol, Hydrogen borate, ''Acidum boricum''
|Section1 = {{Chembox Identifiers
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 7346
|PubChem = 7628
|SMILES1 = [OH+]=[B-](O)O
|ChEMBL_Ref = {{ebicite|correct|EBI}}
|ChEMBL = 42403
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/BH3O3/c2-1(3)4/h2-4H
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = KGBXLFKZBHKPEV-UHFFFAOYSA-N
|CASNo = 10043-35-3
|CASNo_Ref = {{cascite|correct|CAS}}
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 33118
|SMILES = OB(O)O
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = R57ZHV85D4
|KEGG_Ref = {{keggcite|correct|kegg}}
|KEGG = D01089
|InChI = 1/BH3O3/c2-1(3)4/h2-4H
|InChIKey = KGBXLFKZBHKPEV-UHFFFAOYAI
|EINECS = 233-139-2
}}
|Section2 = {{Chembox Properties
|H=3 | B=1 | O=3
|Appearance = White crystalline solid
|Density = 1.435 g/cm<sup>3</sup>
|Solubility = 2.52 g/100 mL (0&nbsp;°C) <br /> 4.72 g/100 mL (20&nbsp;°C) <br /> 5.7 g/100 mL (25&nbsp;°C) <br /> 19.10 g/100 mL (80&nbsp;°C) <br /> 27.53 g/100 mL (100&nbsp;°C)
|SolubleOther = Soluble in lower [[Alcohol (chemistry)|alcohol]]s <br /> moderately soluble in [[pyridine]] <br /> very slightly soluble in [[acetone]]
|Solvent = other solvents
|MeltingPtC = 170.9
|BoilingPtC = 300
|ConjugateBase = [[Borate]]
|pKa = {{awrap|9.24 ([[Acid dissociation constant#Polyprotic acids|first proton]]),}} {{awrap|12.4 (second),}} {{awrap|13.3 (complete)}}
|MagSus = −34.1·10<sup>−6</sup> cm<sup>3</sup>/mol
|LogP = −0.29<ref name=chemsrc/>
}}
|Section3 = {{Chembox Structure
|MolShape = Trigonal planar
|Dipole = 0 D
}}
|Section4 = {{Chembox Pharmacology
|ATCCode_prefix = S02
|ATCCode_suffix = AA03
|ATC_Supplemental = {{ATC|D08|AD}}
}}
|Section5 = {{Chembox Hazards
|GHSPictograms = {{GHS08}}
|NFPA-H = 1
|NFPA-F = 0
|NFPA-R = 0
|FlashPt = Nonflammable
|LD50 = 2660 mg/kg, oral (rat)
}}
|Section6 = {{Chembox Related
|OtherCompounds = [[Boron trioxide]]<br />[[Borax]]}}
}}

'''Boric acid''', more specifically '''orthoboric acid''', is a compound of [[boron]], [[oxygen]], and [[hydrogen]] with formula {{chem2|B(OH)3}}. It may also be called '''hydrogen [[orthoborate]]''', '''trihydroxidoboron''' or '''boracic acid'''.<ref name=webster/> It is usually encountered as colorless crystals or a white powder, that dissolves in [[water]], and occurs in nature as the mineral [[sassolite]].   It is a weak [[acid]] that yields various [[borate]] [[anion]]s and [[salt (chemistry)|salts]], and can react with [[Alcohol (chemistry)|alcohol]]s to form [[borate ester]]s.

Boric acid is often used as an [[antiseptic]], [[insecticide]], [[flame retardant]], [[neutron absorber]], or precursor to other boron compounds.

The term "boric acid" is also used generically for any [[oxyacid]] of boron, such as [[metaboric acid]] {{chem2|HBO2}} and [[tetraboric acid]] {{chem2|H2B4O7}}.

==History==
Orthoboric acid was first prepared by [[Wilhelm Homberg]] (1652–1715) from [[borax]], by the action of mineral acids, and was given the name {{lang|la|sal sedativum Hombergi}} ("sedative salt of Homberg"). However, boric acid and [[borate]]s have been used since the time of the [[ancient Greece|ancient Greek]]s for cleaning, preserving food, and other uses.<ref name=eisler2007/>

==Molecular and crystal structure==
The three oxygen atoms form a [[Trigonal planar molecular geometry|trigonal planar geometry]] around the boron. The B-O bond length is 136 pm, and the O-H is 97 pm. The [[molecular symmetry|molecular point group]] is ''C''<sub>3h</sub>.<ref>{{Greenwood&Earnshaw2nd|page=1291}}</ref>

Two crystalline forms of orthoboric acid are known: [[triclinic crystal system|triclinic]] with [[space group]] P{{overline|1}}, and [[hexagonal crystal system|trigonal]] with space group P3<sub>2</sub>. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method.<ref name=rota2017/>

The triclinic form of boric acid consists of layers of {{chem2|B(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.<ref name=GandE/> While the layers of the triclinic phase are nearly trigonal with {{nowrap|1=γ = 119.76°}}, {{nowrap|1=''a'' = 701.87 pm}}, and {{nowrap|1=''b'' = 703.5 pm}} (compared to {{nowrap|1=''a'' = 704.53(4) pm}} for the trigonal form), the stacking of the layers is somewhat offset in the triclinic phase, with {{nowrap|1=α = 92.49°}} and {{nowrap|1=β = 101.46°}}. The triclinic phase has {{nowrap|1=''c'' = 634.72 pm}} and the trigonal one has {{nowrap|1=''a'' = 956.08(7) pm}}.<ref>{{cite journal |last1=Gajhede |first1=M. |last2=Larsen |first2=S. |last3=Rettrup |first3=S. |title=Electron density of orthoboric acid determined by X-ray diffraction at 105 K and ab initio calculations |journal=Acta Crystallographica Section B Structural Science |date=1 December 1986 |volume=42 |issue=6 |pages=545–552 |doi=10.1107/S0108768186097720 |bibcode=1986AcCrB..42..545G |url=https://www.crystallography.net/cod/9007675.html}}</ref><ref>{{cite journal |last1=Shuvalov |first1=Robert R. |last2=Burns |first2=Peter C. |title=A new polytype of orthoboric acid, H 3 BO 3 -3 T |journal=Acta Crystallographica Section C Crystal Structure Communications |date=15 June 2003 |volume=59 |issue=6 |pages=i47–i49 |doi=10.1107/S0108270103009685|pmid=12794317 |bibcode=2003AcCrC..59I..47S }}</ref>

{| style="margin:auto;" class="wikitable"
|[[File:Boric-acid-unit-cell-3D-balls.png|center|200px]]||[[File:Boric-acid-layer-3D-balls.png|center|250px]]
|-
|{{center|The [[unit cell]] of boric acid}}||{{center|[[hydrogen bond]]ing (dashed lines)<br />allows boric acid molecules to form<br />parallel layers in the solid state}}
|}

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

: {{chem2|Na2B4O7}}·10{{chem2|H2O}} + 2 HCl → 4 {{chem2|B(OH)3}} + 2 NaCl + 5 {{chem2|H2O}}

It is also formed as a byproduct of hydrolysis of boron trihalides and [[diborane]]:<ref name=hous2008/>

: {{chem2|B2H6}} + 6 {{chem2|H2O}} → 2 {{chem2|B(OH)3}} + 6 {{chem2|H2}}

: {{chem2|BX3}} + 3 {{chem2|H2O}} → {{chem2|B(OH)3}} + 3 HX (X = Cl, Br, I)

==Reactions==
===Pyrolysis===
When heated, orthoboric acid undergoes a three-step dehydration. The reported transition temperatures vary substantially from source to source.{{citation needed|date=September 2023}}

When heated above 140&nbsp;°C, orthoboric acid yields [[metaboric acid]] ({{chem2|HBO2}}) with loss of one water molecule:<ref name=kaur2021/><ref name=aghi2018/>

: {{chem2|B(OH)3}} → {{chem2|HBO2}} + {{chem2|H2O}}

Heating metaboric acid above about 180&nbsp;°C eliminates another water molecule forming [[tetraboric acid]], also called pyroboric acid ({{chem2|H2B4O7}}):<ref name=kaur2021/><ref name=aghi2018/>

: 4 {{chem2|HBO2}} → {{chem2|H2B4O7}} + {{chem2|H2O}}

Further heating (to about 530&nbsp;°C) leads to [[boron trioxide]]:<ref name=balci2012/><ref name=kaur2021/><ref name=aghi2018/>

: {{chem2|H2B4O7}} → 2 {{chem2|B2O3}} + {{chem2|H2O}}

===Aqueous solution===
When orthoboric acid is dissolved in water, it partially dissociates to give [[metaboric acid]]:
: {{chem2|B(OH)3}} {{Eqm}}  {{chem2|HBO2}} + {{chem2|H2O}}
The solution is mildly acidic due to the ionization of the acids:
: {{chem2|B(OH)3}} + {{chem2|H2O}} {{Eqm}} {{chem2|[BO(OH)2]-}} + {{H3O+}}
: {{chem2|HBO2}} + {{chem2|H2O}} {{Eqm}} {{chem2|[BO2]-}} + {{H3O+}}

However, [[Raman spectroscopy]] of strongly alkaline solutions has shown the presence of [[tetrahydroxyborate|{{chem2|[B(OH)4]−}} ions]],<ref name=jolly1984/> leading some to conclude that the acidity is exclusively due to the abstraction of {{chem2|OH-}} from water:<ref name=jolly1984/>
: {{chem2|B(OH)3}} + {{chem2|HO-}} {{Eqm}} {{chem2|B(OH)4-}}
Equivalently,
: {{chem2|B(OH)3}} + {{chem2|H2O}} {{Eqm}} {{chem2|B(OH)4-}} + {{H+}} (''K''<sub>a</sub> = 7.3×10<sup>−10</sup>; p''K''<sub>a</sub> = 9.14)
Or, more properly,
:{{chem2|B(OH)3}} + 2 {{chem2|H2O}} {{Eqm}} {{chem2|B(OH)4-}} + {{H3O+}}
This reaction occurs in two steps, with the neutral complex aquatrihydroxyboron {{chem2|B(OH)3(OH2)}} as an intermediate:<ref name=tachi2004/>
: {{chem2|B(OH)3}} + {{chem2|H2O}} → {{chem2|B(OH)3(OH2)}}
: {{chem2|B(OH)3(OH2)}} + {{chem2|H2O}} → {{chem2|[B(OH)4]-}} + {{H3O+}}

This reaction may be characterized as [[Lewis acid]]ity of boron toward {{chem2|HO-}}, rather than as [[Brønsted acid]]ity.<ref name=hous2005/><ref name=JEE2014/><ref name=darp2000/> 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 with [[borax]] {{chem2|Na2B4O7*10H2O}} (more properly {{chem2|Na2B4O5(OH)4*8H2O}}) in the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately.<ref name=tsuy2007/>

===Sulfuric acid solution===
Boric acid also dissolves in anhydrous sulfuric acid according to the equation:<ref name=GandE/>
:{{chem2|B(OH)3}} + 6 {{chem2|H2SO4}} → {{chem2|[B(SO4H)4]-}} + 2 {{chem2|[HSO4]-}} + 3 {{H3O+}}
The product is an extremely strong acid, even stronger than the original sulfuric acid.<ref name=GandE/>

===Esterification===
Boric acid reacts with alcohols to form [[borate ester]]s, {{chem2|B(OR)3}} where R is [[alkyl]] or [[aryl]]. The reaction is typically driven by a dehydrating agent, such as concentrated [[sulfuric acid]]:<ref name=brown1956/>

:{{chem2|B(OH)3}} + 3 ROH → {{chem2|B(OR)3}} + 3 {{chem2|H2O}}

===With vicinal diols===
The acidity of boric acid solutions is considerably increased in the presence of [[cis–trans isomerism|cis]]-[[diol#Vicinal diols|vicinal diols]] ([[organic compound]]s containing similarly oriented [[hydroxyl]] groups in adjacent [[carbon]] atoms, {{chem2|(R1,R2)\dC(OH)\sC(OH)\d(R3,R4)}}) such as [[glycerol]] and [[mannitol]].<ref name=vogelP357/><ref name=GandE/><ref name=NIST1969/><ref name="Belcher1960"/>

The tetrahydroxyborate anion formed in the dissolution spontaneously reacts with these diols to form relatively stable anion esters containing one or two five-member {{chem2|\sB\sO\sC\sC\sO\s}} rings. For example, the reaction with mannitol {{chem2|H(HCOH)6H}}, whose two middle hydroxyls are in ''cis'' orientation, can be written as:

: {{chem2|B(OH)3}} + {{chem2|H2O}} {{Eqm}} {{chem2|[B(OH)4]-}} + {{chem2|H+}}

: {{chem2|[B(OH)4]-}} + {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(OH)2(H(HCOH)2(HCO\s)2(HCOH)2H)]-}} + 2 {{chem2|H2O}}

: {{chem2|[B(OH)2(H(HCOH)2(HCO\s)2(HCOH)2H)]-}} + {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(H(HCOH)2(HCO\s)2(HCOH)2H)2]-}} + 2 {{chem2|H2O}}

Giving the overall reaction:

: {{chem2|B(OH)3}} + 2 {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(H(HCOH)2(HCO\s)2(HCOH)2H)2]-}} + 3 {{chem2|H2O}} + {{chem2|H+}}

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 p''K''<sub>a</sub> from 9 to below 4 for a sufficient concentration of mannitol.<ref name=vogelP357/><ref name=GandE/><ref name=NIST1969/><ref name="Belcher1960"/> 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 its [[pH]] enough for it to be titrated by a strong base such as NaOH, including with an automated potentiometric [[titrator]]. This property is used in analytical chemistry to determine the borate content of aqueous solutions, for example to monitor the depletion of boric acid by [[neutron]]s in the water of the primary circuit of [[light-water reactor]] when the compound is added as a [[neutron poison]] during refueling operations.<ref name=GandE/>

==Toxicology==
Based on mammalian [[median lethal dose]] (LD<sub>50</sub>) rating of 2,660&nbsp;mg/kg body mass, boric acid is only poisonous if taken internally or inhaled in large quantities. The Fourteenth Edition of the ''[[Merck Index]]'' indicates that the LD<sub>50</sub> of boric acid is 5.14&nbsp;g/kg for oral dosages given to rats, and that 5 to 20&nbsp;g/kg has produced death in adult humans. For a 70&nbsp;kg adult, at the lower 5&nbsp;g/kg limit, 350&nbsp;g could produce death in humans. For comparison's sake, the LD<sub>50</sub> of [[sodium chloride|salt]] is reported to be 3.75&nbsp;g/kg in rats according to the ''Merck Index''. According to the [[Agency for Toxic Substances and Disease Registry]], "The minimal lethal dose of ingested boron (as boric acid) was reported to be 2–3&nbsp;g in infants, 5–6&nbsp;g in children, and 15–20&nbsp;g in adults. [...] However, a review of 784 human poisonings with boric acid (10–88&nbsp;g) reported no fatalities, with 88% of cases being asymptomatic."<ref name=HHS2010/> 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&nbsp;mg B/kg/day, likely due to eating in contaminated workplaces, more than 100 times the average daily exposure.<ref name=CLH2018/>

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&nbsp;mg/(kg⋅day) for 90&nbsp;days. This level, were it applicable to humans at like dose, would equate to a cumulative dose of 202&nbsp;g over 90&nbsp;days for a 70&nbsp;kg adult, not far lower than the above LD<sub>50</sub>.<ref name=FQPA2006/>

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 and [[teratogenicity]] in rabbit, rat, and mouse fetuses, as well as cardiovascular defects, skeletal variations, and mild kidney lesions.<ref name=CLH2018/> As a consequence in the 30th ATP to EU directive 67/548/EEC of August 2008, the European Commission<!--EC--> decided to amend its classification as [[reprotoxic]] category&nbsp;2 and to apply the [[risk phrase]]s R60 (may impair fertility) and R61 (may cause harm to the unborn child).<ref name=BYU2008/><ref name=ishii1993/><ref name=rest1992/><ref name=duld2009/><ref name=NSWFA2009/>

At a 2010 European Diagnostics Manufacturing Association (EDMA) Meeting, several new additions to the [[substance of very high concern]] (SVHC) candidate list in relation to the [[Registration, 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&nbsp;December 2010 is ''H360FD (May damage fertility. May damage the unborn child)''.<ref name=ECHA2010/><ref name=EUP2008/>

==Physics==
Sound absorption in [[ocean]]s not only depends on [[water molecule]]s but also on dissolved salts present in low [[concentration]] in [[seawater]].<ref name="NPL2018" /><ref name="Ainslie1998"/> Boric acid and [[borate]] {{Nowrap|([B] ≈ 4 × 10<sup>-4</sup> M}} in seawater)<ref name="Byrne2024" /> [[Relaxation (physics)|relaxation]] contributes to absorbing [[sound]]s in the low‐[[frequency]] region (0.2–10 [[Hertz|kHz]]).<ref name="Francois1982_Part_II" /> At higher frequencies, between 10 and 1000&nbsp;kHz [[magnesium sulfate]] (formed by the second most abundant [[cation]] and [[anion]] species in seawater) is the main contributor to the absorption of [[acoustic wave]]s in seawater.<ref name="Francois1982_Part_I" />

==Uses==
===Industrial===
The primary industrial use of boric acid is in the manufacture of monofilament [[fiberglass]], 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.<ref name=kist1994/>

In the jewelry industry, boric acid is often used in combination with [[denatured alcohol]] to reduce surface [[oxidation]] and formation of [[firescale]] on metals during [[annealing (metallurgy)|annealing]] and [[soldering]] operations.<ref>{{Cite book |last=Hettmansperger |first=Mary |url=https://books.google.com/books?id=tV4eh7W01y8C&dq=%2522boric+acid%2522+firescale&pg=PA33 |title=Wrap, Stitch, Fold & Rivet: Making Designer Metal Jewelry |date=2008 |publisher=Lark Books |isbn=978-1-60059-125-9 |language=en}}</ref><ref>{{cite web|url=https://www.ganoksin.com/article/dealing-fire-scale/|website=Ganoksin Jewelry Making Resources|first=Charles|last=Lewton-Brain|title=Dealing with Fire Scale|access-date=20 June 2024}}</ref>

Boric acid is used in the production of glass in [[liquid crystal display|LCD]] [[flat panel display]]s.<ref>{{cite web|url=https://www.borax.com/BoraxCorp/media/Borax-Main/Resources/Technical-Bulletin/borates-glasses.pdf?ext=.pdf|website=Borax|title=Technical Bulletin: Borates in glasses|year=2021}}</ref><ref>{{cite magazine|url=https://borates.today/boron-flat-panel-displays/|title=Boron Flat Panel Displays|date=9 February 2022|magazine=Borates Today}}</ref>

In [[electroplating]], boric acid is used as part of some proprietary formulas. One known formula uses about a 1 to 10 ratio of {{chem|H|3|BO|3}} to [[nickel(II) sulfate|{{chem|NiSO|4}}]], a very small portion of [[sodium lauryl sulfate]] and a small portion of [[sulfuric acid|{{chem|H|2|SO|4}}]].

The solution of orthoboric acid and borax in 4:5 ratio is used as a [[Fire retardant|fire retarding agent]] of wood by impregnation.<ref name=tsuy2009/> Also, it is used in combination with other chemicals for the fire retardancy of wood-based materials.<ref>{{cite journal | last1=Lin | first1=Chia-feng | last2=Karlsson | first2=Olov | last3=Martinka | first3=Jozef | last4=Rantuch | first4=Peter | last5=Garskaite | first5=Edita | last6=Mantanis | first6=George I. | last7=Jones | first7=Dennis | last8=Sandberg | first8=Dick | author6-link=George Mantanis |title=Approaching Highly Leaching-Resistant Fire-Retardant Wood by In Situ Polymerization with Melamine Formaldehyde Resin | journal=ACS Omega | volume=6 | issue=19 | date=2021-05-18 | issn=2470-1343 | pmid=34056425 | pmc=8154219 | doi=10.1021/acsomega.1c01044 | doi-access=free | pages=12733–12745}}</ref>

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

Boric acid is added to [[borax]] for use as [[welding]] [[flux (metallurgy)|flux]] by [[blacksmith]]s.<ref name=demp2010/>

Boric acid, in combination with [[polyvinyl alcohol]] (PVA) or [[silicone oil]], is used to manufacture [[Silly Putty]].<ref name=silly2013/>

Boric acid is also present in the list of chemical additives used for [[hydraulic fracturing]] (fracking) in the [[Marcellus Shale]] in Pennsylvania.<ref name=PDEP2010/> It is often used in conjunction with [[guar gum]] as [[cross-link]]ing and gelling agent for controlling the [[viscosity]] 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 the [[Hydraulic fracturing proppants|propping agents]] aimed at maintaining the cracks in the [[shale]]s sufficiently open to facilitate the gas extraction after the hydraulic pressure is relieved.<ref name=fink2015/><ref name=bish2004/><ref name=EUP3004279/> The rheological properties of borate cross-linked guar gum [[hydrogel]] mainly depend on the [[pH]] value.<ref name=wang2016/>

Boric acid is used in some [[Fuse (electrical)#Expulsion fuses|expulsion-type electrical fuses]] as a de-ionization/extinguishing agent.<ref>{{Cite book |url=https://mcemalta.com/pdfs/Eaton%203PS%20Vol%2014.pdf |title=Electrical Sector Solutions Product Overview |publisher=[[Eaton Corporation]] |year=2011 |volume=14: Fuses}}</ref> During an [[electrical fault]] in an expulsion-type fuse, a [[Plasma (physics)|plasma]] arc is generated by the disintegration and rapid [[Spring (device)|spring]]-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 into [[water vapor]] and [[Boron trioxide|boric anhydride]], and in turn, the vaporization products de-ionize the plasma, helping to interrupt the electrical fault.<ref>{{Cite journal |last1=Strom |first1=A. P. |last2=Rawlins |first2=H. L. |date=December 1932 |title=The Boric Acid Fuse |journal=Transactions of the American Institute of Electrical Engineers |volume=51 |issue=4 |pages=1020–1025 |doi=10.1109/T-AIEE.1932.5056215 |bibcode=1932TAIEE..51.1020S |s2cid=51650608 |issn=0096-3860}}</ref>

===Medical===
{{See also|Boric acid (vaginal)}}

Boric acid can be used as an [[antiseptic]] for minor burns or cuts and is sometimes used in salves and [[medical dressing|dressings]], such as [[boracic lint]]. Boric acid is applied in a very dilute solution as an eye wash. Boric acid [[Intravaginal administration|vaginal suppositories]] can be used for recurrent [[candidiasis]] due to non-albicans candida as a second line treatment when conventional treatment has failed.<ref name="iava2010" /><ref name=aber2010/> It is less effective than conventional treatment overall.<ref name="iava2010" /> Boric acid largely spares [[lactobacilli]] within the vagina.<ref name=sobel2021/> As [[TOL-463]], it is under development as an [[vaginal administration|intravaginal]] medication for the treatment for [[vulvovaginal candidiasis]].<ref name=adisTOL463/><ref name=nyir2022/><ref name=marra2019/>

As an [[antibacterial]] compound, boric acid can also be used as an [[acne]] treatment. It is also used to prevent [[athlete's foot]], by inserting powder in the socks or stockings. Various preparations can be used to treat some kinds of {{lang|la|[[otitis externa]]}} (ear infection) in both humans and animals.<ref name="adriz2018" /> The preservative in [[urine]] sample bottles in the UK is boric acid.<ref>{{Cite book |url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/636410/U_3i1.pdf |title=UK standards for microbiology investigations |publisher=[[Public Health England]] |date=2017-08-07}}</ref>

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.<ref name=harv1980/>

===Insecticidal===
Boric acid was first registered in the US as an insecticide in 1948 for control of [[cockroach]]es, [[termite]]s, [[fire ant]]s, [[flea]]s, [[silverfish]], and many other [[insect]]s. 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 is [[abrasive]] to the insects' [[exoskeleton]]s.<ref name=ABC/><ref name=boone/><ref name=REDF2022/> It is in non-specific [[Insecticide Resistance Action Committee#Table of modes of action and classes of insecticide|IRAC]] 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, being [[Cannibalism (zoology)|cannibalistic]], eat others killed by contact or consumption of boric acid, consuming the powder trapped in the dead roach and killing them, too.{{citation needed|date=July 2020}}

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 causes [[dysbiosis]] in the [[Reticulitermes flavipes|Eastern Subterranean termite]], leading to the opportunistic rise of insect pathogens that could be contributing to mortality.<ref>{{Cite journal |journal=Journal of Economic Entomology |doi=10.1093/jee/toae221 |title=Lethal disruption of the bacterial gut community in Eastern subterranean termite caused by boric acid |date=2024 |last1=Ashbrook |first1=Aaron R. |last2=Schwarz |first2=Melbert |last3=Schal |first3=Coby |last4=Mikaelyan |first4=Aram |volume=117 |issue=6 |pages=2599–2607 |pmid=39401329 |doi-access=free |pmc=11682946 }}</ref> In Japan the practice of laying newspapers treated with o-boric acid and borax under buildings has been effective in controlling [[Coptotermes formosanus]] and [[Reticulitermes speratus]] populations. Decaying wood treated with 0.25 to 0.5 percent [[disodium octaborate]] ({{chem2|Na2B8O13*4H2O}}, commonly abbreviated DOT) is also effective for baiting [[Heterotermes 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."<ref>{{cite journal  | title = Review of recent research on the use of borates for termite prevention | journal = In Proceedings of the 2nd International Conference on Wood Protection with Diffusible Preservatives and Pesticides |date=1997 | url = http://manoa.hawaii.edu/ctahr/termite/aboutcontact/grace/pdfs/123.pdf  | pages = 85–92  }}</ref>

===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 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|date=June 2022}}

Boric acid is added to salt in the curing of cattle hides, [[calfskin]]s, and [[sheepskin]]s. This helps to control bacterial development and insects.{{citation needed|date=June 2022}}

===pH buffer===
[[File:Borate pH.gif|thumb|alt=Distribution between boric acid and borate ion versus pH assuming p''K''{{sub|a}} = 9.0 (e.g. salt-water swimming pool)|Boric acid predominates in solution below pH 9]]
[[File:Borate buffer.gif|thumb|alt=Buffer capacity of the boric acid - borate system versus pH assuming p''K''{{sub|a}} = 9.0 (e.g. salt-water swimming pool)|Boric 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&nbsp;ppm boron equivalents) as a primary or adjunct pH buffer system in [[swimming pools]]. Boric acid is a weak acid, with p''K''{{sub|a}} (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&nbsp;°C. But apparent p''K''{{sub|a}} 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 the [[bicarbonate]] system with p''K''{{sub|a{{sub|1}}}} = 6.0 and p''K''{{sub|a{{sub|2}}}} = 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 free [[Hypochlorous acid|HOCl]] concentration needed for pool sanitation. Still, it may add marginally to the photo-protective effects of [[cyanuric acid]] and confer other benefits through anti-corrosive activity or perceived water softness, depending on overall pool solute composition.<ref name=birch2013/>

===Lubrication===
Colloidal suspensions of nanoparticles of boric acid dissolved in petroleum or vegetable oil can form a remarkable lubricant on ceramic or metal surfaces<ref name=duzc2009/> with a coefficient of sliding friction that decreases with increasing pressure to a value ranging from 0.10 to 0.02. Self-lubricating {{chem2|B(OH)3}} films result from a spontaneous chemical reaction between water molecules and {{chem2|B2O3}} 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|date=June 2022}}

Boric acid is used to lubricate [[carrom]] and [[novuss]] boards, allowing for faster play.<ref name=singh2009/>

===Nuclear power===
Boric acid is used in some [[nuclear power plant]]s as a [[neutron poison]]. The boron in boric acid reduces the probability of [[nuclear fission|thermal fission]] by absorbing some [[thermal neutron]]s. [[Nuclear chain reaction|Fission chain reaction]]s 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 high [[nuclear cross section|cross-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 in [[pressurized water reactor]]s (PWRs), whereas [[boiling water reactor]]s (BWRs) employ [[control rod]] pattern and coolant flow for power control. However, BWRs can use an aqueous solution of boric acid and [[borax]] or [[sodium pentaborate]] for an emergency shutdown system if the control rods fail to insert. Boric acid may be dissolved in [[spent fuel pool]]s used to store [[Spent nuclear fuel|spent fuel]] elements. The concentration is high enough to keep neutron multiplication at a minimum. Boric acid was dumped over Reactor 4 of the [[Chernobyl disaster|Chernobyl]] nuclear power plant after its [[nuclear meltdown|meltdown]] to prevent another reaction from occurring.{{citation needed|date=October 2013}}

===Pyrotechnics===
Boron is used in [[pyrotechnics]] to prevent the [[amide]]-forming reaction between [[aluminium]] and [[nitrate]]s. 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 by [[torch (juggling)|fire jugglers]] and fire spinners to create a deep green flame much stronger than copper sulfate.<ref name=weing1947/>

===Agriculture===
Boric acid is used to treat or prevent [[Boron deficiency (plant disorder)|boron deficiencies]] in plants. It is also used in the preservation of grains such as rice and wheat.<ref name=CFS2019/>

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<ref name="Ainslie1998">{{cite journal | last1=Ainslie | first1=Michael A. | last2=McColm | first2=James G. | title=A simplified formula for viscous and chemical absorption in sea water | journal=The Journal of the Acoustical Society of America | volume=103 | issue=3 | date=1998-03-01 | issn=0001-4966 | doi=10.1121/1.421258 | pages=1671–1672 | bibcode=1998ASAJ..103.1671A | url=https://pubs.aip.org/jasa/article/103/3/1671/557895/A-simplified-formula-for-viscous-and-chemical | access-date=2025-04-21| url-access=subscription }}</ref>
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</references>

==Further reading==
* {{cite book | author = Jolly, W. L. | title = Modern Inorganic Chemistry | edition = 2nd | location = New York | publisher = McGraw-Hill | year = 1991 | isbn = 978-0-07-112651-9 }}
* {{cite book |author1=Goodman, L. |author2=Gilman, A. |author3=Brunton, L. |author4=Lazo, J. |author5=Parker, K. | location = New York | publisher = McGraw Hill | year = 2006 | title = Goodman & Gilman's The Pharmacological Basis of Therapeutics }}
* 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.

==External links==
* {{commons category-inline|Boric acid}}
* [http://npic.orst.edu/factsheets/archive/borictech.pdf Boric Acid Technical Fact Sheet – National Pesticide Information Center]
* [http://npic.orst.edu/factsheets/boricgen.pdf Boric Acid General Fact Sheet – National Pesticide Information Center]
* [http://www.inchem.org/documents/icsc/icsc/eics0991.htm International Chemical Safety Card 0991]
* [https://web.archive.org/web/20080406065032/http://www.epa.gov/oppsrrd1/REDs/factsheets/0024fact.pdf US EPA Pesticide Reregistration Eligibility Decision]
* [https://web.archive.org/web/20060209040519/http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory – Boron and compounds]
* [https://web.archive.org/web/20021212234630/http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/BORIC%20ACID.htm Boric acid at ChemicalLand21]
* [https://web.archive.org/web/20100401084438/http://echa.europa.eu/doc/press/pr_10_03_svhc_consultation_20100308.pdf European Chemicals Agency (ECHA) "New Public Consultation on Eight Potential Substances of Very High Concern" – includes Boric Acid. Closes 22 April 2010]
* [http://chemsub.online.fr/name/Boric_acid.html ChemSub Online: Boric acid]

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