Boron

group 13 · period 2 · p-block · metalloid

fullBoron has a WHO guideline of 2.4 mg/L and an EU parametric value of 1.5 mg/L, sits in seawater at over 4 mg/L so that every desalination plant must design for it, passes conventional treatment and slips through reverse osmosis at neutral pH as uncharged boric acid, and is the classic irrigation toxicant above 0.7 mg/L; the treatment story is one acid base equilibrium at pH 9.2.

Typical wastewaters

seawater desalination (reverse osmosis brine and permeate) uncharged boric acid B(OH)₃ at seawater pH 8.1, 4.4 mg/L in the feed, concentrated in the reject and passing a single membrane pass only in part WHO: the guideline will be difficult to achieve in some desalinated supplies; the EU allows 2.4 mg/L where desalinated water predominates
municipal sewage and detergent, glass, soap and flame retardant manufacture borate from perborate detergents and manufacturing discharges, falling as perborate use declines
textile finishing boric acid, diboron trioxide and sodium borates as total boron; 100 µg/L reporting limit (ZDHC Table 1J), borate zinc salt also reported as boron
borax and boric acid production borate in residual brine and depleted liquor returned to the source water body; no other process wastewater discharge

1 · Identity

Symbol, number: B, 5
Oxidation states in water: +3 only, as boric acid B(OH)₃ (an uncharged, monomeric weak Lewis acid) below about pH 9 and as the borate anion B(OH)₄⁻ above it; polyborates appear only at concentrations far above natural water. No redox chemistry in water.
Note: The element entry carries the borate minerals, the Turkish and Californian deposits and the glass, detergent and flame retardant uses. This chapter is boron as a dissolved weak acid.

2 · Occurrence in water

Natural sources: Leaching from rocks and soils containing borates and borosilicates, the primary route into groundwater (WHO); orthoboric acid in volcanic spring waters (element entry); seawater, where boron is a major minor constituent at about 4.4 mg/L; concentrations vary widely with the surrounding geology (WHO).
Anthropogenic sources: Wastewater discharges of borate from detergents, glass, soap and flame retardant manufacture, although perborate use in detergents has fallen and boron in wastewater discharges continues to fall (WHO); desalinated water, where boron rejection is incomplete (EU DWD note); irrigation with borate rich water; borax and boric acid plants, which in the US may return only residual brine to the source water body.

matrix	typical range	note
drinking water, world	below 0.5 mg/Lregion-dependent	judged to be below 0.5 mg/L for most of the world; varies widely with geology and wastewater discharges
groundwater, US public water systems	median 0.0514 mg/L US surveys quoted in the 2008 health advisory	81.9 percent of groundwater systems had detections at or above 0.005 mg/L; 4.3 percent of surveyed systems above 0.7 mg/L (half the 1.4 mg/L health reference level); 7 of 228 samples in a second survey above 1.4 mg/L
surface water, US public water systems	median 0.029 mg/LUS survey	same survey
seawater	4.44 mg/Lsingle figure	oceanic abundance figure, Jefferson Lab via PubChem, quoted in the element entry; the feed concentration every seawater reverse osmosis plant designs against
high borate regions	2.05 to 29 mg/Lregion-dependent; study areas	drinking water in a high borate region against 0.03 to 0.4 mg/L in a low borate region; well water 1.2 to 25.1 mg/L in a boron area (epidemiological study areas quoted by EPA)
municipal and industrial wastewater	not read	WHO says discharges raise surface water borate but gives no concentration; no effluent survey read

3 · Speciation

Boric acid is a weak acid with pKa about 9.2 at 25 C, and it dissociates not by losing a proton but by taking up a hydroxide to form the tetrahedral borate ion. Below pH 8 nearly all boron is the uncharged, small B(OH)₃ molecule; at pH 9.2 half is B(OH)₄⁻; above pH 10.5 nearly all is borate. Ionic strength and temperature shift the pKa, seawater to about 8.6 (Stumm and Morgan). Everything in boron treatment follows: the uncharged acid is not rejected well by membranes, not adsorbed by conventional media and not precipitated; the anion is.

condition	dominant species	note
fresh water, pH below 8	B(OH)₃	uncharged; passes reverse osmosis in part and conventional treatment entirely
pH 8.5 to 10	B(OH)₃ and B(OH)₄⁻ together	seawater at pH 8.1 has boron mostly as boric acid
pH above 10.5	B(OH)₄⁻	the condition for high rejection in a second reverse osmosis pass and for anion exchange
borate brines and evaporites	polyborates such as B₄O₇²⁻, precipitating as borax and colemanite	not met in treatment; the element entry has the minerals

Solubility: Boric acid and the alkali borates are soluble; calcium and magnesium borates are sparingly soluble but do not control boron at treatment concentrations. No solubility is quoted because none was read.
Hydrolysis: Boric acid does not hydrolyse further; the tetraborate of borax hydrolyses in dilute solution to boric acid and borate.
Complexation: Boric acid complexes with polyols (cis diols), which is the chemistry of boron selective resins carrying N-methyl-D-glucamine groups and of the classical mannitol titration; it also esterifies with the diol groups of natural organic matter (general chemistry, not quantified in the sources read).
Precipitates: None of significance in natural or treated water; co-precipitation with Mg(OH)₂ in lime softening at high pH is reported in the literature but was not read this session.

\ce{B(OH)3 + H2O <=> B(OH)4^- + H+}

pKa about 9.2 at 25 C and zero ionic strength, lower in seawater; boric acid acts as a Lewis acid, accepting hydroxide rather than donating a proton

\ce{B(OH)3 + OH- -> B(OH)4^-}

caustic dosing before a second reverse osmosis pass or an anion exchanger; converts the poorly rejected acid to the well rejected anion

\ce{Na2B4O7 + 7 H2O -> 2 B(OH)3 + 2 B(OH)4^- + 2 Na^+}

dissolution of borax (sodium tetraborate) in dilute water; the reason a borax solution sits near pH 9.2 and is a buffer standard

\ce{B(OH)4^- + H+ -> B(OH)3 + H2O}

the reverse of the pKa 9.2 dissociation: acidification, or the acid elution step of a boron selective resin, returns borate to the uncharged acid; the reason boron in any water at neutral pH, and in every acidified sample bottle, is B(OH)3

4 · Role in treatment

as a problem

boron in desalinated water

seawater carries 4.4 mg/L boron mostly as uncharged boric acid at pH 8.1, which a single reverse osmosis pass rejects only in part

the EU allows 2.4 mg/L instead of 1.5 mg/L where desalinated water is the predominant source; WHO notes the guideline will be difficult to achieve in some desalinated supplies and in areas of high natural boron

conventional treatment does not remove boron

no precipitate, no adsorption on floc, no biological uptake of consequence

WHO: coagulation, sedimentation and filtration do not significantly remove boron

irrigation toxicity

boron accumulates in leaves of sensitive crops from irrigation water

FAO 29: no restriction below 0.7 mg/L, slight to moderate 0.7 to 3.0, severe above 3.0 mg/L; recycled water for agriculture must meet the crop, not the drinking water guideline

reverse osmosis brine and ion exchange regenerant

boron removed from product water is concentrated in the reject or in the regenerant

WHO: ion exchange and reverse osmosis are likely to be prohibitively expensive; blending with low boron supplies may be the only economical method

textile effluent

boric acid and borates from finishing chemistry

ZDHC reporting limit 100 µg/L as total boron for boric acid, boron oxide and sodium borates in textile wastewater, Table 1J

as a reagent

none as a treatment reagent

boron is a contaminant, not a reagent; borax is a pH buffer standard in the laboratory

5 · Removal and control

reverse osmosis, single pass

membrane rejection of B(OH)₃ is far lower than of salts because the molecule is small and uncharged; rejection rises with pH as borate forms

seawater at pH 8.1 in the first pass; WHO lists reverse osmosis among the special methods that may enable substantial reduction; no rejection percentage was read this session

Efficiency: substantial but incomplete (WHO); not quantified in the sources read
Interferences: low feed pH, high temperature and high recovery all lower boron rejection (general membrane practice, not from a source read)

reverse osmosis, second pass at high pH

caustic soda raises the first pass permeate to pH 10 to 11, converting boric acid to borate, which the second pass rejects like a salt

\ce{B(OH)3 + OH- -> B(OH)4^-}

the permeate has little hardness so scaling at high pH is limited; the design of every large seawater plant that must meet 1.5 mg/L or less (general practice built on the pKa, not from a source read)

Efficiency: to below 1 mg/L in practice; no figure read
Interferences: carbonate scaling if hardness remains

boron selective ion exchange

resin with N-methyl-D-glucamine groups complexes boric acid through its diol; regenerated with acid then caustic

polishing of reverse osmosis permeate or groundwater; WHO lists ion exchange among the special methods; the resin chemistry is general knowledge, not from a source read

Efficiency: not quantified

blending

dilution with a low boron source

WHO: may be the only economical method where concentrations are high

Efficiency: arithmetic

conventional treatment

none

coagulation, sedimentation and filtration do not significantly remove boron (WHO); the US BAT table of 40 CFR 141.62 has no boron row because boron is unregulated

Efficiency: nil

6 · Analytics

method	standard	detection limit	note
ICP-MS	EPA 200.8; ISO 17294-2	0.15 µg/L (WHO)	memory effects from boron in the sample introduction system; use a rinse with ammonia or mannitol (general practice)
ICP-OES	EPA 200.7; Standard Methods 3120 B; ISO 11885	6 to 10 µg/L (WHO, ICP-AES)	the ZDHC method for total boron
colorimetry (curcumin, carmine, azomethine-H)	Standard Methods 4500-B B and C; ISO 9390 (azomethine-H)	not read	field and small laboratory methods

Sampling pitfalls: Never use borosilicate glass for boron samples or standards; boron leaches from the glass. Use polyethylene bottles. No acidification is needed for dissolved boron, and filtration is unnecessary because boron is not particulate; acidified metal samples in glass bottles give false boron. Total boron and boric acid are the same number at any pH the laboratory sees.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

drinking water

body	limit	note
WHO GDWQ 4th ed. with addenda (2022)	2.4 mg/L	TDI 0.17 mg/kg body weight from a BMDL₀₅ of 10.3 mg/kg per day for decreased fetal body weight in rats and an uncertainty factor of 60; 40 percent allocation to water because other intake is low; 60 kg, 2 L/day; assessment 2009; where the value is difficult to achieve in desalinated or naturally high supplies, authorities should consider a value above 2.4 mg/L by assessing other exposure
EU DWD 2020/2184	1.5 mg/L	Annex I Part B; a parametric value of 2.4 mg/L applies when desalinated water is the predominant source of the supply system or in regions where geological conditions could lead to high boron in groundwater
US EPA	not regulated	no MCL; 2008 health advisory: one-day and ten-day 3.0 mg/L (child), longer-term 2.0 mg/L (child) and 5 mg/L (adult), lifetime 5 mg/L from a DWEL of 7.0 mg/L and RfD 0.2 mg/kg per day; CCL health reference level 1.4 mg/L

discharge

body	limit	note
EU CWW BREF BAT-AEL (Decision 2016/902)	not set	boron is not among the BAT 12 parameters
US EPA 40 CFR 415.272, borax production (BPT)	no limit	no discharge of process wastewater pollutants to navigable waters except return of residual brine and depleted liquor to the source water body; boron itself is not a limited parameter
Abu Dhabi ADS 23/2017, discharge to the marine environment (EAD)	1.0 mg/L region-dependent; marine discharge only; below ambient seawater boron	Table 1 maximum allowable concentration at the point of discharge; below the seawater concentration, which the specification exempts by excluding desalination brine
Abu Dhabi DoE Trade Effluent Control Regulations 2022, discharge to sewer	5 mg/L region-dependent; sewer discharge	Table A₄

industry thresholds

sector	body	limit	note
textile	ZDHC Wastewater Guidelines v₂.1 (2022), Table 1J boric acid, diboron trioxide, disodium octaborate, disodium tetraborate	100 µg/L	reporting limit for textile wastewater, determined as total boron by ICP; the limit refers to elemental boron, not the salt; borate zinc salt 100 µg/L each as boron and zinc (Table 1N)
irrigation	FAO Irrigation and Drainage Paper 29 (1985), Table 1	below 0.7 no restriction; 0.7 to 3.0 slight to moderate; above 3.0 severe mg/L	specific ion toxicity to sensitive crops; the usual normal range in irrigation water is 0 to 2 mg/L

8 · Health and environmental effects

Toxicity: Boron is essential for plant cell walls and probably not for humans; daily intake about 2 mg from food (element entry). In animals the male reproductive tract is the consistent target (testicular lesions in rats, mice and dogs) and developmental toxicity is demonstrated in rats, mice and rabbits; boric acid and borax are not genotoxic and caused no tumours in long term studies (WHO). EPA's RfD of 0.2 mg/kg per day rests on the BMDL for decreased fetal body weight with a data derived adjustment factor of 66.
Bioaccumulation: Not addressed in the sources read; boron accumulates in the leaves of irrigated crops, which is the basis of the FAO limits.
Ecotoxicity: US EPA national recommended criteria list boron with a narrative statement in the 1986 Gold Book and no numeric criterion; the numeric guidance in use is the FAO irrigation threshold.

Flags

The pKa of boric acid and the shift in seawater are cited to Stumm and Morgan from memory of the text, not re-read this session.
Two-pass reverse osmosis at high pH and the glucamine resin are general practice; WHO names ion exchange and reverse osmosis without describing them, and no rejection percentages were read.
US occurrence medians come from surveys quoted in the 2008 EPA health advisory, not from a current dataset.
The EPA health advisory values are transcribed from the 2008 document sections 5.1 to 5.4; the child longer-term value of 2.0 mg/L is taken from the summary read, the derivation itself was not checked line by line.
Abu Dhabi values cover two media (marine outfall 1.0 mg/L, sewer 5 mg/L); other GCC states not read.

Gaps

No reverse osmosis boron rejection figures, resin capacities or full scale performance data were read.
No municipal or industrial effluent boron concentrations were read; the ZDHC reporting limit stands in for textile.
The WHO 2009 boron background document was not reachable at the guessed URL and was not read.
No solubility or polyborate constants are quoted.
Other GCC discharge standards were not read; Gulf desalination permits treat boron case by case and were not sourced.
The loading step of a boron selective resin is a diol chelation with the N-methylglucamine group; no source read prints a stoichiometry for it, so only the acid elution step is written as an equation.

Sources

WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), chapter 12 chemical fact sheet, Boron (pp. 350 to 351)
US EPA, Drinking Water Health Advisory for Boron, EPA 822-R-08-013 (May 2008), sections 2 (occurrence) and 5 (health advisories)
Directive (EU) 2020/2184 on the quality of water intended for human consumption (recast), Annex I Parts B and C
Commission Implementing Decision (EU) 2016/902 establishing BAT conclusions for common waste water and waste gas treatment/management systems in the chemical sector (CWW), BAT 12 Tables 1 and 2 with footnotes
40 CFR 415.272, Borax production subcategory, BPT effluent limitations
Abu Dhabi Specification ADS 23/2017, Environmental Specifications for Land-Based Liquid Discharges to the Marine Environment (Environment Agency Abu Dhabi), Table 1
Abu Dhabi Department of Energy, Trade Effluent Control Regulations 2022 (DoE/PD/R01/005, effective 1 January 2022), Schedule A Tables A1, A2 and A4
ZDHC Wastewater Guidelines Version 2.1 (November 2022), Tables 1J (flame retardants, boron compounds) and 1N
Ayers, R. S. and Westcot, D. W., Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1 (1985), chapter 1, Table 1 and Table 2
US EPA, National Recommended Water Quality Criteria, Aquatic Life Criteria Table
Stumm, W. and Morgan, J. J., Aquatic Chemistry, 3rd ed. (Wiley, 1996), chapter 3 (acids and bases, boric acid) and chapter 4 (seawater acid base constants)
Standard Methods for the Examination of Water and Wastewater (online edition), 4500-B Boron, 3120 B
The Element Book, entries for boron (seawater abundance, borate minerals, detergents and glass) (data/elements/B.json, data/reference/text/B.json)

Identity

Name and symbol: Boron, B
Atomic number: 5 protons
Position: group 13 · period 2 · p-block · metalloid
CAS number: 7440-42-8

Atomic structure

Atomic mass: 10.811 u
Electron configuration: 1s² 2s² 2p¹
[He] 2s²²p¹
Electrons per shell: 2, 3
Valence electrons: 3 outer shell

isotope	mass (u)	abundance
¹⁰B	10.01293695(41)	19.9 %
¹¹B	11.00930536(45)	80.1 %

natural isotopic composition, mole fraction

Physical properties

State at room temperature: Solid
Melting point: 2,348 K (2,074.85 °C)
Boiling point: 4,273 K (3,999.85 °C)
Density: 2.37 g/cm3
Appearance: black-brown
Thermal conductivity: 27.4 W/(m·K)
Electrical resistivity: ~10⁶ Ω·m at 20 °C
Electrical conductivity: 1.00e-1 S/m
Crystal structure: rhombohedral
Molar heat capacity: 11.087 J/(mol·K)

Chemical properties

Oxidation states: +3
Electronegativity: 2.04 (Pauling Scale)
Ionisation energy: 8.298 eV
1st 800.6, 2nd 2,427.1, 3rd 3,659.7 kJ/mol
Electron affinity: 0.277 eV
Atomic radius: empirical 84, covalent 84, van der Waals 192 pm
Ionic radius: B³⁺ 27 pm

Reactivity: A metalloid with three valence electrons that builds covalent networks; crystalline boron is chemically inert at room temperature, and boron chemistry is done from borates rather than from the element.
with water: Does not react with water under ordinary conditions.
with oxygen, air: Protected by a thin oxide or hydroxide film in air at room temperature; burns at high temperature to boron trioxide: $\ce{4 B + 3 O2 -> 2 B2O3}$
with acids: Resists boiling hydrochloric and hydrofluoric acid; finely divided boron is attacked slowly by hot concentrated nitric acid, hot sulfuric acid or hot sulfuric and chromic acid mixtures.
with halogens: Forms the complete series of planar trihalides BF3, BCl3, BBr3 and BI3, Lewis acids that hydrolyse to boric acid; industrially they are made from borates and boron oxide, not from the element.

Typical compounds

H₃BO₃ boric acid textile fibreglass, flame retardant, mild antiseptic
Na₂B₄O₇.10H₂O borax sodium borate decahydrate; laundry products, glass, the ore tincal
B₂O₃ boron trioxide boric oxide; makes borosilicate glass tough and heat resistant
BN boron nitride cubic form as hard as diamond; insulator that conducts heat
B₂H₆ diborane simplest borane; hydroboration reagent, ignites in air
BF₃ boron trifluoride planar Lewis acid; gives tetrafluoroborate with fluoride

Occurrence, production and use

Crustal abundance: 1.0×101 milligrams per kilogram
Oceanic abundance: 4.44 milligrams per liter

Occurrence and sources

The element is not found free in nature, but occurs as orthoboric acid usually found in certain volcanic spring waters and as borates in boron and colemantie.

Important sources of boron are ore rasorite (kernite) and tincal (borax ore). Both of these ores are found in the Mojave Desert. Tincal is the most important source of boron from the Mojave. Extensive borax deposits are also found in Turkey.

Boron exists naturally as 19.78% 10B isotope and 80.22% 11B isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder.

Boron of 99.9999% purity has been produced and is available commercially. Elemental boron has an energy band gap of 1.50 to 1.56 eV, which is higher than that of either silicon or germanium.

dissolved in seawater about 4.44 mg/L; crustal estimate 10 mg/kg (Jefferson Lab figures via PubChem)
tincal (borax), kernite, ulexite, borate brines Mojave Desert, California; in 2024 the leading US producer mined kernite for boric acid, tincal for sodium borate and ulexite for specialty glass and ceramics, with two other companies solution-mining borate brines
colemanite and other borates Turkey (about 70 percent colemanite, used for heat-resistant glass), Chile and Bolivia (ulexite), Peru, Argentina, Russia (datolite), China
orthoboric acid volcanic spring waters

Extraction, production: Open-pit and solution mining of borate ores; refining to boric acid, sodium borates and boric oxide
Kernite goes to boric acid, tincal to sodium borate (USGS, printed pp. 48 to 49). No reaction equations are printed in the sources.
Elemental boron by hydrogen reduction of boron trichloride or tribromide on heated filaments; amorphous boron by heating the trioxide with magnesium
$\ce{2 BCl3 (g) + 3 H2 (g) -> 2 B (s) + 6 HCl (g)}$
Described in words by the RSC and Los Alamos; elemental boron has limited commercial application and the industry trades in borates, not the element.

Uses

Boron is used in pyrotechnics and flares to produce a green color. Boron has also been used in some rockets as an ignition source. Boron-10, one of the naturally occurring isotopes of boron, is a good absorber of neutrons and is used in the control rods of nuclear reactors, as a radiation shield and as a neutron detector. Boron filaments are used in the aerospace industry because of their high-strength and lightweight.

Boron forms several commercially important compounds. The most important boron compound is sodium borate pentahydrate (Na2B4O7·5H2O). Large amounts of this compound are used in the manufacture of fiberglass insulation and sodium perborate bleach. The second most important compound is boric acid (H3BO3), which is used to manufacture textile fiberglass and is used in cellulose insulation as a flame retardant. Sodium borate decahydrate (Na2B4O7·10H2O), better known as borax, is the third most important boron compound. Borax is used in laundry products and as a mild antiseptic. Borax is also a key ingredient in a substance known as Oobleck, a strange material 6th grade students experiment with while participating in Jefferson Lab's BEAMS program. Other boron compounds are used to make borosilicate glasses, enamels for covering steel and as a potential medicine for treating arthritis.

Amorphous boron is used in pyrotechnic flares to provide a distinctive green color, and in rockets as an igniter.

By far the most commercially important boron compound in terms of dollar sales is Na2B4O7 • 5H2O. This pentahydrate is used in very large quantities in the manufacture of insulation fiberglass and sodium perborate bleach.

Boric acid is also an important boron compound with major markets in textile products. Use of borax as a mild antiseptic is minor in economical terms. Boron compounds are also extensively used in the manufacture of borosilicate glasses. Other boron compounds show promise in treating arthritis.

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal.

Boron also has lubricating properties similar to graphite. The hydrides are easily oxidized with considerable energy liberation, and have been studied for use as rocket fuels. Demand is increasing for boron filaments, a high-strength, lightweight material chiefly employed for advanced aerospace structures.

Boron is similar to carbon in that it has a capacity to form stable covalently bonded molecular networks. Carbonates, metalloboranes, phosphacarboranes, and other families comprise thousands of compounds.

Glass and ceramics: borosilicate glass, insulation and textile fibreglass; tile glazes and enamels glass and ceramics were the leading US users in 2024; more than three quarters of world borate consumption goes to ceramics, detergents, fertilisers and glass
Chemicals: sodium perborate bleach and borates in detergents and cleaning products; boric acid, borax as mild antiseptic and in eye drops; sodium octaborate flame retardant; boron carbide abrasives
Agriculture: borate micronutrient fertilisers and insecticides; Turkey opened a 35,000 tonne per year granular boron plant for the fertiliser industry in 2024
Textiles: boric acid has major markets in textile products; fibreglass textiles are made from borosilicate glass
Nuclear and electronics: boron-10 for reactor control and neutron detection; boron as a dopant for semiconductors

Safety, toxicity

Elemental boron and the borates are not considered to be toxic, and they do not require special care in handling. However, some of the more exotic boron hydrogen compounds are definitely toxic and do require care.

GHS classification, signal word Warning

H302 Harmful if swallowed Acute toxicity, oral
H319 Causes serious eye irritation Serious eye damage/eye irritation
H332 Harmful if inhaled Acute toxicity, inhalation
H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
H412 Harmful to aquatic life with long lasting effects to the aquatic environment, long-term hazard
H371 May cause damage to organs Specific target organ toxicity, single exposure

Discovery and name

Discovered by: Joseph Louis Gay-Lussac and Louis Jacques Thénard
Discovered: 30 June 1808
First isolated: Humphry Davy
Named by: not in sources
Origin of the name: after borax, from which it was isolated; Ultimately from Arabic بَوْرَق (bawraq)

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.