# Borate
**Borate** refers to boron-containing oxyanions and their salts, formed because boron has a high affinity for oxygen, and by extension to borate esters, the organic compounds in which boron is bound to oxygen through alcohol-derived groups.[1](https://pubs.acs.org/doi/10.1021/bk-2016-1236.ch001) The term covers inorganic species such as the tetrahydroxyborate ion \( \mathrm{B(OH)_4^-} \) and the polyborate anions of concentrated solutions, mineral salts such as borax, covalent borate esters \( \mathrm{B(OR)_3} \), and the borate diester bridges that cross-link pectin in every vascular plant cell wall.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full)
| Key fact | Detail |
|---|---|
| Core chemistry | Boron's high affinity for oxygen leads to borate oxyanions; the simplest aqueous equilibrium is \( \mathrm{B(OH)_3 + H_2O \rightleftharpoons B(OH)_4^- + H^+} \) with pKa = 9.24 at 25 °C[1](https://pubs.acs.org/doi/10.1021/bk-2016-1236.ch001) • [3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis) |
| Industrial minerals | Four minerals account for 90 percent of borates used by industry worldwide: the sodium borates tincal and kernite, the calcium borate colemanite, and the sodium-calcium borate ulexite[4](https://www.usgs.gov/publications/borates) |
| Borax | Sodium tetraborate decahydrate (Na2B4O7·10H2O), a white crystalline solid found naturally as the mineral tincal; its solutions have a pH of about 9.24[4](https://www.usgs.gov/publications/borates) • [5](https://npic.orst.edu/factsheets/archive/borictech.html) |
| Borate esters | Formed from boric acid and alcohols, \( \mathrm{B(OH)_3 + 3\,ROH \rightleftharpoons B(OR)_3 + 3\,H_2O} \); the equilibrium favors hydrolysis and is driven right by azeotropic removal of water[6](https://citeseerx.ist.psu.edu/document?doi=f15a042e89c90f51e53cc050707b21bebfa227cc&repid=rep1&type=pdf) |
| Plant biology | More than 90 percent of the pectic polysaccharide rhamnogalacturonan II is dimerized by borate diester bonds under boron-sufficient conditions, the only experimentally proven molecular function of boron as an essential trace element[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full) |
| Acute toxicity | Boric acid oral LD50 in rats is 3450 mg/kg (males) and 4080 mg/kg (females); borax 4550 and 4980 mg/kg, both rated low in acute oral toxicity[5](https://npic.orst.edu/factsheets/archive/borictech.html) |
## Aqueous chemistry
[Boric acid](https://www.edgepedia.org/boric-acid), more precisely written \( \mathrm{B(OH)_3} \), is a very weak acid that does not dissociate as a Brønsted acid. It acts as a Lewis acid, accepting hydroxide from water to form the tetrahedral tetrahydroxyborate ion \( \mathrm{B(OH)_4^-} \), with \( K_a = 5.8 \times 10^{-10} \) and pKa = 9.24 at 25 °C.[3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis) • [7](https://acikerisim.ege.edu.tr/server/api/core/bitstreams/bc4db6d8-a64d-483a-b8c5-869706897f0a/content) Below pH 9.24 the neutral trigonal species predominates; above it, the tetrahedral anion takes over.[7](https://acikerisim.ege.edu.tr/server/api/core/bitstreams/bc4db6d8-a64d-483a-b8c5-869706897f0a/content) In dilute solution (below 0.1 M) boric acid buffers in the pH range 8 to 10.[3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis)
**Polyborates.** Concentrated solutions behave differently. Potentiometric and Raman studies identified polymeric anions including triborate \( \mathrm{B_3O_3(OH)_4^-} \) and \( \mathrm{B_3O_3(OH)_5^{2-}} \), tetraborate \( \mathrm{B_4O_5(OH)_4^{2-}} \), and pentaborate \( \mathrm{B_5O_6(OH)_4^-} \).[3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis) Their dissociation constants exceed that of boric acid by a factor of 210 to 440, which explains why a 0.65 M boric acid solution measures pH 3.90 with a glass electrode against a calculated 4.71.[3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis) This polyborate chemistry underlies the Winkler modification of the Kjeldahl nitrogen determination, in which distilled ammonia is captured in boric acid as ammonium borate and titrated with hydrochloric acid; the method is recommended by ISO, IDF, and EPA.[3](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis)
## Borate esters
Borate esters, \( \mathrm{B(OR)_3} \), form when boric acid reacts with alcohols. The equilibrium favors the hydrolysis products, so synthesis requires removing water, typically by azeotropic distillation with benzene or toluene; no catalyst is needed and the reaction is endothermic.[6](https://citeseerx.ist.psu.edu/document?doi=f15a042e89c90f51e53cc050707b21bebfa227cc&repid=rep1&type=pdf) Adding water reverses the reaction and releases the alcohol, with boric acid filtered off as a powder, a property used to separate alcohols.[6](https://citeseerx.ist.psu.edu/document?doi=f15a042e89c90f51e53cc050707b21bebfa227cc&repid=rep1&type=pdf) Trimethyl borate boils at about 68.7 °C and forms a low-boiling azeotrope with methanol.[6](https://citeseerx.ist.psu.edu/document?doi=f15a042e89c90f51e53cc050707b21bebfa227cc&repid=rep1&type=pdf) • [8](https://patents.google.com/patent/US2739979A/en)
**Diol complexes.** Boric acid and borate bind 1,2- and 1,3-diols to form cyclic esters, and the stability of the complex depends strongly on the diol's geometry: when the hydroxyl groups match the structural parameters required by tetrahedrally coordinated boron, a strong complex forms.[9](https://www.sciencedirect.com/topics/chemistry/borate-ester) Adding polyols such as glycerol or mannitol increases the apparent acidity of boric acid through cyclic borate ester formation, a effect used to permit its titration.[9](https://www.sciencedirect.com/topics/chemistry/borate-ester) The most stable borate esters form with cis-diols on a furanoid ring, structures rare in nature and limited to apiose and ribose.[9](https://www.sciencedirect.com/topics/chemistry/borate-ester) Esters form and dissociate spontaneously on a rapid time scale in pH-dependent equilibria.[9](https://www.sciencedirect.com/topics/chemistry/borate-ester)
## Role in plants
Boron is an essential micronutrient for vascular plants, and its cross-linking role is structural. The pectic polysaccharide rhamnogalacturonan II (RG-II), first identified in 1978 in sycamore cell walls, exists in primary walls as a dimer covalently cross-linked by a borate diester between apiosyl residues of side chain A on two RG-II monomers.[10](https://www.annualreviews.org/content/journals/10.1146/annurev.arplant.55.031903.141750) • [2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full) This dimerization cross-links the two homogalacturonan chains on which the RG-II molecules are built and is required for the three-dimensional pectic network that gives the primary wall its mechanical properties; decreased borate cross-linking produces many symptoms of boron deficiency.[10](https://www.annualreviews.org/content/journals/10.1146/annurev.arplant.55.031903.141750) Boron deficiency impairs cell elongation rather than cell division, with symptoms including inhibited root cell elongation, leaf cell expansion, and pollen tube elongation.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full)
Plants regulate boron supply with two families of membrane transporters: NIPs (nodulin-26-like intrinsic proteins), which facilitate boric acid permeation, and BORs, which export borate from cells.[11](https://academic.oup.com/pcp/article/62/4/590/6133239) Because boric acid is a small uncharged molecule at physiological pH, it permeates lipid membranes relatively well, and under limited boron conditions these transporters are needed for efficient RG-II cross-linking.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full) • [11](https://academic.oup.com/pcp/article/62/4/590/6133239) In the [Arabidopsis](https://www.edgepedia.org/arabidopsis) bor1-1 mutant, the proportion of borate-dimerized RG-II falls to 40 percent against 90 percent in wild type.[2](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full)
## Polymers and materials
Borate cross-linking of diol-bearing polymers is widely used. Poly(vinyl alcohol) forms charged complexes with borate ions from borax or from boric acid plus sodium hydroxide, and at sufficient polymer concentration undergoes a thermoreversible sol-gel transition.[12](https://www.sciencedirect.com/science/article/pii/0032386192908862) The familiar school demonstration of slime is PVA cross-linked with borax: the tetrahydroxyborate anion reacts with a diol on one PVA chain to form a monodiol complex, then its remaining hydroxyl-bearing positions react with a diol on an adjacent chain.[13](https://www.nature.com/articles/s40494-020-00447-3) Boric acid cross-linking raises the glass transition temperature of PVA films and inhibits recrystallization on melting.[14](https://www.sciencedirect.com/science/article/pii/S0032386110008311) [Boronate ester](https://www.edgepedia.org/boronate-ester) hydrogels built on the same reversible chemistry are under development for biomedical applications.[15](https://pubs.acs.org/doi/10.1021/acs.chemmater.4c00507)
**Catalysis.** Borate esters catalyze direct amidation of carboxylic acids with amines, with reaction kinetics showing a positive dependence on catalyst concentration (0.8th order); boric acid itself is used industrially as a low-cost amidation catalyst but only for relatively reactive acid/amine combinations.[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC5609808/)
## Minerals and industry
Deposits of borates are associated with volcanic activity and arid climates, with the largest economically viable deposits in the [Mojave Desert](https://www.edgepedia.org/mojave-desert) of the United States near Boron, California, the Alpide belt in southern Asia, and the Andean belt of South America.[4](https://www.usgs.gov/publications/borates) Boric acid is sold as a colorless crystalline solid in technical, national formulary, and special quality grades.[4](https://www.usgs.gov/publications/borates) Boric acid and borate salts serve as insecticides (stomach poisons, with most borate salts also abrasive to insect exoskeletons), fungistats, herbicides, soil amendments for boron-deficient soils, and components of fertilizers, cleaners, detergents, glass, fiberglass, porcelain enamel, ceramic glazes, and fire retardants.[5](https://npic.orst.edu/factsheets/archive/borictech.html) In the body, over 90 percent of absorbed inorganic borates exist as undissociated boric acid, which is rapidly excreted in urine with an average human urinary elimination half-life of approximately 21 hours.[5](https://npic.orst.edu/factsheets/archive/borictech.html)
## References
1. [Boron Chemistry: An Overview, ACS Symposium Series, Boron Reagents in Synthesis](https://pubs.acs.org/doi/10.1021/bk-2016-1236.ch001)
2. [Funakawa H, Miwa K (2015). Synthesis of borate cross-linked rhamnogalacturonan II. Frontiers in Plant Science](https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2015.00223/full)
3. [Cruz G (2013). Boric Acid in Kjeldahl Analysis. Journal of Chemical Education 90(12):1645–1648](https://-pubs-acs-org-443.webvpn1.xju.edu.cn/jceda8/article/90/12/1645/840664/Boric-Acid-in-Kjeldahl-Analysis)
4. [Crangle RD (2013). Borates. U.S. Geological Survey, Mining Engineering](https://www.usgs.gov/publications/borates)
5. [Boric Acid Technical Fact Sheet, National Pesticide Information Center, Oregon State University](https://npic.orst.edu/factsheets/archive/borictech.html)
6. [The Separation of Alcohols, Stellenbosch University thesis](https://citeseerx.ist.psu.edu/document?doi=f15a042e89c90f51e53cc050707b21bebfa227cc&repid=rep1&type=pdf)
7. [Ege University graduate thesis, chemistry of boric acid in nature](https://acikerisim.ege.edu.tr/server/api/core/bitstreams/bc4db6d8-a64d-483a-b8c5-869706897f0a/content)
8. [US2739979A, Preparation of boric acid esters, Google Patents](https://patents.google.com/patent/US2739979A/en)
9. [Borate Ester, an overview, ScienceDirect Topics (Smoum & Srebnik, Contemporary Aspects of Boron, 2005)](https://www.sciencedirect.com/topics/chemistry/borate-ester)
10. [O'Neill MA, Ishii T, Albersheim P, Darvill AG (2004). Rhamnogalacturonan II: Structure and Function of a Borate Cross-Linked Cell Wall Pectic Polysaccharide. Annual Review of Plant Biology 55:109–139](https://www.annualreviews.org/content/journals/10.1146/annurev.arplant.55.031903.141750)
11. [Regulation, Diversity and Evolution of Boron Transporters in Plants. Plant and Cell Physiology 62(4):590 (2021)](https://academic.oup.com/pcp/article/62/4/590/6133239)
12. [Phase behaviour and sol-gel transition of poly(vinyl alcohol)-borate complex in aqueous solution. Polymer 33(10):2182–2188 (1992)](https://www.sciencedirect.com/science/article/pii/0032386192908862)
13. [Characterization of polyvinyl alcohol-borax/agarose double network hydrogel. npj Heritage Science (2020)](https://www.nature.com/articles/s40494-020-00447-3)
14. [Role of boric acid for a poly(vinyl alcohol) film as a cross-linking agent. Polymer 51(23):5539–5549 (2010)](https://www.sciencedirect.com/science/article/pii/S0032386110008311)
15. [Boronate Ester Hydrogels for Biomedical Applications: Challenges and Opportunities. Chemistry of Materials (2024)](https://pubs.acs.org/doi/10.1021/acs.chemmater.4c00507)
16. [Borate esters: Simple catalysts for the sustainable synthesis of complex amides. PMC (2017)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5609808/)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Carboxylic acid derivatives › Esters › Phosphate, sulfate, and other oxoacid esters › Borate and boronate esters*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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