Sodium

group 1 · period 3 · s-block · alkali metal

full Sodium is the ion that ordinary treatment leaves exactly where it found it. It does not hydrolyse, it has no second oxidation state, and every one of its common salts is soluble, so it cannot be oxidised, precipitated or settled out; only a membrane, a still or an ion exchange bed in the hydrogen form will part water from it. That makes sodium the accountant of a water: it is the major cation of seawater and brackish water, it tracks salinity conservatively, and whatever a plant adds stays in. A great deal is added, because sodium is the counter-ion of the whole treatment chemical inventory, arriving with hypochlorite, caustic soda, soda ash, bicarbonate and softener brine, and a softener returns sodium to the water equivalent for equivalent as it takes the hardness out. In the finished water sodium is a matter of taste and of diet rather than of toxicity, which is why the EU sets it as an indicator parameter at 200 mg/L rather than as a health limit. Its sharpest consequence is not in the pipe at all but in the field, where sodium measured against calcium and magnesium as the sodium adsorption ratio decides whether irrigation water leaves a soil open or disperses its clay and closes it.

Typical wastewaters

municipal sewage
Na⁺, conservative; sewage effluent carries the sodium of treatment chemicals (fluoride, bicarbonate, hypochlorite up to 30 mg/L together) and of domestic softeners (over 300 mg/L)
textile wet processing
Na⁺ with chloride and sulfate from dyeing salt and neutralisation, reported as total dissolved solids; not removed by a standard effluent plant
desalination brine (reverse osmosis concentrate, thermal distillation)
Na⁺ with Cl⁻ concentrated from seawater or brackish water; the marine discharge specification exempts desalination brine
ion exchange regenerant (softener spent brine)
NaCl brine carrying the exchanged calcium and magnesium; spent brine disposal is the environmental cost of ion exchange
road de-icing runoff
NaCl salt runoff to surface water and groundwater

1 · Identity

Symbol, number: Na, 11
Oxidation states in water: +1 only, as the hydrated Na⁺ ion; no hydrolysis, no complexes of consequence, no insoluble salts in fresh water (the element chapter: almost every sodium salt is soluble). Sodium is what is left when everything else has been removed, which is why it leaves treatment plants as salinity.
Note: The element chapter carries the metal, halite, soda ash and caustic soda production. This chapter is sodium as a dissolved ion, as a reagent counter-ion and as sodicity. Sodium hypochlorite chemistry is in the chlorine chapter, the carbonate equilibria in the carbon chapter, and the dechlorination sulfite salts in the chlorine chapter.

2 · Occurrence in water

Natural sources: Sodium is ubiquitous in water because its salts are so soluble; it is leached from the terrestrial environment to groundwater and surface water, and seawater is the reservoir ¹. Saline intrusion, mineral deposits and sea spray raise it; sodium bicarbonate groundwater forms where calcium is exchanged for sodium on clays, giving soft, high sodium, high fluoride and high boron water (general geochemistry).
Anthropogenic sources: Sewage effluent, road de-icing salt, and treatment chemicals: sodium fluoride, sodium bicarbonate and sodium hypochlorite together can add up to 30 mg/L and domestic water softeners over 300 mg/L ²; industrial brines, reverse osmosis concentrate, ion exchange regenerant, caustic soda and soda ash dosing, and the sodium salts of every industry (textile dyeing salt, chlor-alkali, pulp, food).

matrix	typical range	note
drinking water, typical	below 20 mg/Lregion-dependent; old surveys	most supplies; can exceed 250 mg/L in some countries; a 1963 to 1966 US survey of 2100 samples ranged 0.4 to 1900 mg/L with 42 percent above 20 mg/L and 5 percent above 250 mg/L; a later survey of 630 systems ranged below 1 to 402 mg/L
groundwater supplies, US (989 community systems)	median 16.4; 99th percentile 517 mg/LUS survey	37 percent above 30 mg/L and 13 percent above 120 mg/L in the National Inorganic and Radionuclide Survey
surface water, US rivers	20 (Mississippi) to 120,000 (Great Salt Lake) mg/Lextremes, not a distribution	the range the EPA advisory quotes for US surface waters
seawater	10,800 mg/Lsingle figure	oceanic abundance figure, Jefferson Lab via PubChem, quoted in the element chapter
drinking water after domestic softening	over 300 mg/L	can reach this; usually much lower; the stoichiometry adds 0.46 mg sodium per mg of hardness as CaCO₃ exchanged
municipal and industrial wastewater	not recorded	no effluent survey is quoted here; textile effluent salt is high enough that ZDHC requires TDS to be reported ahead of a future limit

3 · Speciation

Sodium is Na⁺ everywhere in water: no hydrolysis, no redox, no precipitation, ion pairs with sulfate and carbonate that matter only in brines. It is conservative, so it tracks salinity, and its ratio to calcium and magnesium (the sodium adsorption ratio, SAR) rather than its concentration decides its effect on soil: a high sodium, low calcium water disperses clay and destroys infiltration ³. Its taste threshold depends on the anion: about 20 mg/L as sodium carbonate, 150 as chloride, 190 as nitrate, 220 as sulfate and 420 as bicarbonate at room temperature ².

condition	dominant species	note
all natural and treated water	Na⁺ (hydrated)	conservative; passes every process except membranes, distillation and ion exchange
irrigation water with a high ratio of sodium to calcium plus magnesium	Na⁺ exchanging onto soil clay for Ca²⁺ and Mg²⁺	sodic soil, reduced infiltration; evaluate SAR together with electrical conductivity ³
brines and reverse osmosis concentrate	Na⁺ with NaSO₄⁻ and NaCO₃⁻ ion pairs	not relevant below a few g/L

Solubility: Sodium chloride 357 g/L and sodium carbonate 71 g/L at 0 C ²; all common sodium salts dissolve freely, so sodium has no solubility control in water and is never removed by precipitation.
Hydrolysis: None; Na⁺ is neither an acid nor a base in water. The pH of a sodium salt solution is that of its anion (carbonate alkaline, chloride neutral).
Complexation: Negligible; weak outer sphere ion pairs with sulfate and carbonate only at high ionic strength (general; not quantified in the sources read).
Precipitates: None from water. Sodium is the counter-ion left in solution when calcium, magnesium and metals are precipitated with sodium reagents.

\ce{2 RNa + Ca^2+ -> R2Ca + 2 Na^+}

cation exchange softening on a strong acid resin in the sodium form (R is the resin site); the same for Mg^2+; every equivalent of hardness removed adds one equivalent of sodium, 0.46 mg Na per mg CaCO3 ⁴

\ce{R2Ca + 2 NaCl -> 2 RNa + CaCl2}

regeneration with brine at 80 to 320 g NaCl per litre of resin ⁵; the spent brine carries the hardness plus the excess salt

\ce{NaOH -> Na^+ + OH-}

caustic soda dosing for pH elevation and metal hydroxide precipitation; adds 0.58 mg sodium per mg NaOH by the atomic weights and no hardness, the reason it replaces lime where sludge or scaling matter

\ce{Na2CO3 + Ca^2+ -> CaCO3 (s) + 2 Na^+}

soda ash removal of non-carbonate hardness after lime, and carbonate precipitation of heavy metals in industrial effluent (element chapter); the carbonate equilibria are in the carbon chapter

\ce{NaHCO3 -> Na^+ + HCO3^-}

sodium bicarbonate dosing for alkalinity in nitrification and for corrosion control without a pH jump

\ce{NaOCl + H2O -> HOCl + Na^+ + OH-}

sodium hypochlorite dosing, typically 12.5 percent available chlorine; the sodium stays; the chlorine chapter carries the chemistry

\ce{Na^+ + SO4^2- <=> NaSO4^-}

the outer sphere ion pair; it is negligible in fresh water and matters in reverse osmosis concentrate and brine, where it lowers the free sulfate activity and so raises the sulfate a scaling index will tolerate before barite or gypsum appears

\ce{2 NaCl + 2 H2O -> 2 NaOH + Cl2 (g) + H2 (g)}

the chlor-alkali cell: every sodium reagent a plant buys, caustic soda, hypochlorite, soda ash, starts as salt, and the sodium arrives in the water with whatever anion it was paired with. This is why sodium is the counter-ion of the whole treatment chemical inventory and why it accumulates in a recycled loop

\ce{NaCl + H2O -> NaOCl + H2 (g)}

on-site hypochlorite generation by electrolysis of softened brine; the hydrogen is vented and both the sodium and the chloride stay in the treated water, so the salinity cost of on-site generation is the same as that of delivered hypochlorite. No salt consumption figure was read

4 · Role in treatment

as a problem

taste

sodium salts taste salty or soda like above thresholds that depend on the anion

WHO: above about 200 mg/L unacceptable taste; EPA: 30 to 60 mg/L is the threshold for taste sensitive individuals, with individual thresholds from about 30 to 460 mg/L

dietary sodium for restricted patients

water at 20 mg/L gives about 40 mg/day, small beside a 2 to 8 g/day diet but not for people limited to 500 mg/day

EPA guidance level 20 mg/L for those individuals, not to be extrapolated to the population; the US requires reporting of sodium to health authorities

softener sodium

ion exchange softening trades every calcium and magnesium for two sodium

WHO: domestic softeners can give over 300 mg/L; hard water of 300 mg/L as CaCO₃ fully softened adds 138 mg/L sodium by the stoichiometry

sodicity of irrigation water and reclaimed water

sodium exchanges onto clay, dispersing soil structure and cutting infiltration; the effect is worse at low salinity

FAO 29 Table 1: with SAR 0 to 3, no restriction above ECw 0.7 dS/m and severe below 0.2; the thresholds rise with SAR (3 to 6: 1.2 and 0.3; 6 to 12: 1.9 and 0.5; 12 to 20: 2.9 and 1.3; 20 to 40: 5.0 and 2.9 dS/m); sodium toxicity to sensitive crops by surface irrigation: SAR below 3 none, 3 to 9 slight to moderate, above 9 severe; sprinkler: below 3 meq/L none, above 3 slight to moderate

\ce{2 Na^+ + XCa -> XNa2 + Ca^2+}

X is a soil clay exchange site; two sodium replace one calcium, which is why the sodium adsorption ratio and not the sodium concentration governs, and why the same sodium level is harmless in a calcium rich water and destroys infiltration in a soft one. Gypsum dosing runs the exchange backwards

salinity from sodium reagents

every sodium chemical dosed (hypochlorite, caustic, soda ash, bicarbonate, sulfite, aluminate, silicate, brine) leaves its sodium in the water or the effluent

the element chapter: whatever sodium industry adds stays dissolved and leaves as salinity; ZDHC will limit TDS in textile effluent for this reason

corrosion and aggressiveness of sodium waters

sodium bicarbonate waters low in calcium do not form a protective carbonate film; high chloride and sulfate sodium waters raise corrosion rates

EU DWD: water should not be aggressive or corrosive, and demineralised or softened water may need calcium and magnesium salts added

sodium hydroxide and hypochlorite hazards

caustic burns, heat of dilution, hypochlorite decomposition to chlorate

chlorine chapter for hypochlorite storage

as a reagent

sodium hydroxide (caustic soda)

strong base for pH elevation, metal hydroxide precipitation, membrane cleaning, boron rejection in second pass reverse osmosis, regeneration of anion resins and activated alumina

\ce{NaOH -> Na^+ + OH-}

50 percent solution freezes near 12 C, 25 percent is the usual winter strength (general practice); no sludge and no hardness, unlike lime

sodium carbonate (soda ash)

removes non-carbonate (permanent) hardness after lime softening, precipitates heavy metals as carbonates, raises alkalinity

\ce{Na2CO3 + Ca^2+ -> CaCO3 (s) + 2 Na^+}

lime soda softening ⁶; the element chapter names soda ash for softening and for metal precipitation

sodium bicarbonate

alkalinity supplement for nitrification (7.14 g as CaCO₃ per g N, nitrogen chapter) and for corrosion control

\ce{NaHCO3 -> Na^+ + HCO3^-}

gentle, self limiting near pH 8.3

sodium chloride brine

regenerant for sodium form cation exchangers (softeners) and chloride form anion exchangers (nitrate, perchlorate); feed for on-site hypochlorite generation (hydrogen chapter)

\ce{R2Ca + 2 NaCl -> 2 RNa + CaCl2}

spent brine disposal is the environmental cost of ion exchange

sodium hypochlorite

disinfectant and oxidant, 10 to 16 percent commercial strength

\ce{NaOCl + H2O -> HOCl + Na^+ + OH-}

chlorine chapter

sodium sulfite, bisulfite, metabisulfite and thiosulfate

dechlorination and oxygen scavenging

\ce{Na2SO3 + HOCl -> Na2SO4 + HCl}

chlorine and oxygen chapters; 1.46 parts NaHSO3 or 1.34 parts Na2S2O5 per part chlorine; the chlorine chapter of this book carries the same reaction with sulfur dioxide and sulfite; the sulfur is oxidised from S(IV) to S(VI) and the chlorine reduced to chloride, and an overdose goes on to consume dissolved oxygen at four parts sulfite per part oxygen

sodium aluminate, sodium silicate, sodium nitrate, sodium fluoride, sodium chlorite

coagulant aid and alkaline aluminium source; corrosion inhibitor and coagulant aid; electron acceptor for sewer odour control; fluoridation; chlorine dioxide precursor

\ce{NaAlO2 + 2 H2O -> Al(OH)3 (s) + Na^+ + OH-}

each under its own element; the sodium is the common residue; sodium aluminate is the one coagulant that raises alkalinity instead of consuming it, one equivalent of hydroxide per aluminium, which is why it is paired with alum in soft water

5 · Removal and control

reverse osmosis

membrane rejection of Na⁺ with Cl⁻; the process that desalinates seawater and brackish water

EPA secondary standards guidance: distillation, reverse osmosis and electrodialysis are effective for chloride, TDS and other inorganics but fairly expensive and may be impractical for small systems

Efficiency: rejection above 99 percent for seawater membranes (general, not from a source read)
Interferences: scaling by calcium carbonate and sulfate in the concentrate; boron and CO₂ pass (boron and carbon chapters)

electrodialysis and electrodialysis reversal

sodium migrates through cation membranes under a direct current

brackish water; EPA names it with reverse osmosis and distillation

Efficiency: not quantified

distillation and thermal desalination

sodium stays in the brine

multistage flash and multiple effect plants of the Gulf; the marine discharge specification exempts their brine (ADS 23)

Efficiency: essentially complete

cation exchange in hydrogen form (demineralisation)

strong acid resin in H+ form takes Na⁺ and releases acid, followed by an anion bed; regenerated with acid

\ce{RH + Na^+ -> RNa + H+}

boiler feed and process water ⁴; sodium leakage sets the mixed bed polishing need

Efficiency: to µg/L in mixed beds (general)

conventional treatment and softening

none; sodium form softening adds sodium and lime softening leaves it

Efficiency: nil

sodicity management for irrigation

not removal but correction: gypsum or calcium chloride raises calcium, lowering SAR; blending with low sodium water

FAO 29 evaluates SAR with electrical conductivity

Efficiency: not applicable

6 · Analytics

method	standard	detection limit	note
flame atomic absorption or flame emission	Standard Methods 3111 B; 3500-Na B (flame emission); ISO 9964-1 and 9964-3	2 µg/L by flame AAS ²	the classical methods; ionisation suppression with potassium
ICP-OES	EPA 200.7; Standard Methods 3120 B; ISO 11885	40 µg/L by ICP-AES ²	sodium is among the routine ICP analytes
ion chromatography, cations	Standard Methods 4110 (anions) with cation columns; ISO 14911	not recorded	gives sodium, potassium, ammonium, calcium and magnesium together, the SAR set
SAR calculation	FAO 29 Figure 1	not applicable	SAR equals sodium divided by the square root of half the sum of calcium and magnesium, all in meq/L; adjusted SAR corrects for calcium precipitation with bicarbonate

Sampling pitfalls: Sodium is the easiest ion to contaminate and the hardest to lose: sweat, glass leaching, sodium containing preservatives and detergents raise it. Use plastic bottles rinsed with the sample; acidify with nitric acid only if metals share the bottle. Report the anion balance; sodium is often calculated by difference in old data and is then only as good as the other ions.

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)	no guideline	no firm conclusions on sodium in drinking water and hypertension, so no health based guideline; above 200 mg/L unacceptable taste; assessment 1993
EU DWD 2020/2184	200 mg/L	Annex I Part C indicator parameter
US EPA	not regulated	no primary or secondary standard; 2003 drinking water advisory recommends 30 to 60 mg/L for taste (not enforceable, a guideline for States) and keeps the 20 mg/L guidance level for people restricted to 500 mg/day sodium, not to be extrapolated to the whole population; sodium results are reported to the State health authority

discharge

body	limit	note
EU CWW BREF BAT-AEL ⁷	not set	sodium and salinity are not BAT 12 parameters; the BAT conclusions require monitoring of salts among the relevant pollutants in the waste water stream inventory
US EPA 40 CFR 133.102, secondary treatment	not set	no federal sodium or TDS effluent limit; salinity limits are permit specific
Abu Dhabi ADS 23/2017, discharge to the marine environment (EAD)	not set region-dependent; marine discharge only	no sodium, chloride or TDS row; desalination brine is exempt from the specification
Abu Dhabi DoE Trade Effluent Control Regulations 2022, discharge to sewer	TDS 2000; chloride 1000 mg/L region-dependent; sewer discharge	Tables A₁ and A₂; sodium itself is not listed, these two rows bracket it

industry thresholds

sector	body	limit	note
textile and leather	ZDHC Wastewater Guidelines v₂.1 (2022), total dissolved solids	sample and report only mg/L	Table 3; salt used in wet processing or formed by neutralisation is not removed by a standard effluent plant, so TDS is reported ahead of a future limit
irrigation	FAO Irrigation and Drainage Paper 29 (1985), Table 1	SAR below 3 none; 3 to 9 slight to moderate; above 9 severe (surface irrigation, sensitive crops); sprinkler below 3 meq/L none, above 3 slight to moderate SAR and meq/L	infiltration is evaluated from SAR and conductivity together; usual range of sodium in irrigation water 0 to 40 meq/L and SAR 0 to 15

8 · Health and environmental effects

Toxicity: Essential; adults need about 500 mg/day and Western diets supply 2 to 8 g/day, so drinking water at 20 mg/L (about 40 mg/day) is a minor source ². Acute effects only from gross overdoses of salt; infants with gastrointestinal fluid loss are vulnerable to hypernatraemia. The link between sodium and hypertension is established for diet but no firm conclusion can be drawn for drinking water sodium, hence no guideline ². EPA's 20 mg/L guidance protects people on 500 mg/day diets.
Bioaccumulation: Not applicable; regulated by the kidney.
Ecotoxicity: No US EPA aquatic criterion for sodium; salinity and chloride criteria apply. In soil the effect is physical: sodic dispersion of clay and loss of infiltration ³.

Read these with care

The WHO drinking water surveys are US data from the 1960s to 1980s; the EPA survey is from about 2001.
Ion exchange stoichiometry and regenerant doses are cited to MWH chapter 16 from memory of the text, not re-read.
Reverse osmosis rejection and mixed bed sodium leakage are general practice, not from sources read.
The sodium added per unit hardness (0.46 mg per mg CaCO₃) and per mg NaOH (0.58) are computed from atomic weights here.
Abu Dhabi has no sodium limit in either medium; the sewer TDS and chloride rows are quoted as the bracket; other GCC states not read.

What this chapter does not cover

No sewage or industrial effluent sodium survey was read; the ZDHC TDS reporting requirement stands in for textile.
No membrane rejection, resin capacity or brine volume figures are quoted.
The WHO chapter 10 acceptability text on sodium and TDS was not read; the fact sheets stand in.
The sodium content of reclaimed water for irrigation and the adjusted SAR calculation were not transcribed from FAO 29.
Other GCC discharge standards were not read; none of the Abu Dhabi documents limits sodium directly.
The chlor-alkali, on-site hypochlorite, sodium aluminate and soil exchange equations are written as the mass balances behind statements the sources make in words; no cell efficiency, salt consumption or soil exchange constant was read.

Notes

WHO background document
WHO
FAO 29
MWH chapter 16
MWH chapter 16, from the chapter
MWH chapter 22
Decision 2016/902

Sources

WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), chapter 12 chemical fact sheet, Sodium (pp. 462 to 463)
WHO, Sodium in Drinking-water, background document for development of WHO Guidelines for Drinking-water Quality, WHO/SDE/WSH/03.04/15 (2003; text of 1996)
US EPA, Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Sodium, EPA 822-R-03-006 (February 2003), executive summary and occurrence section
Directive (EU) 2020/2184 on the quality of water intended for human consumption (recast), Annex I Parts B and C
US EPA, Secondary Drinking Water Standards: Guidance for Nuisance Chemicals
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 133.102, Secondary treatment (BOD5, suspended solids, pH)
Abu Dhabi Specification ADS 23/2017, Environmental Specifications for Land-Based Liquid Discharges to the Marine Environment 18, 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), Table 3 conventional parameters and anions
Ayers, R. S. and Westcot, D. W., Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1 (1985), Tables 1 and 2 and Figure 1 (SAR)
US EPA, National Recommended Water Quality Criteria, Aquatic Life Criteria Table
US EPA, Alternative Disinfectants and Oxidants Guidance Manual, EPA 815-R-99-014 (April 1999), section 2.7.2.2 (sodium hypochlorite) and the dechlorination salts in the chlorine chapter
MWH, Water Treatment: Principles and Design, 3rd ed. 19, chapter 16 (ion exchange: softening, demineralisation, regeneration) and chapter 22 (lime soda softening)
Metcalf and Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed. (McGraw-Hill, 2014), chapter 7 (alkalinity supplements for nitrification)
Standard Methods for the Examination of Water and Wastewater (online edition), 3111 B, 3120 B, 3500-Na B
The Element Book, entries for sodium (seawater abundance, salt and soda ash production, conservative ion narrative)
Stumm, W. and Morgan, J. J., Aquatic Chemistry, 3rd ed. 20, chapter 6 (ion pairs at high ionic strength)

Identity

Name and symbol: Sodium, Na
Atomic number: 11 protons
Position: group 1 · period 3 · s-block · alkali metal
CAS number: 7440-23-5

Atomic structure

Atomic mass: 22.989 u
Electron configuration: 1s² 2s² 2p⁶ 3s¹
[Ne] 3s¹
Electrons per shell: 2, 8, 1
Valence electrons: 1 outer shell

isotope	mass (u)	abundance
²³Na	22.989 769 28(2)	100 %

natural isotopic composition, mole fraction

Physical properties

State at room temperature: Solid
Melting point: 370.95 K (97.8 °C)
Boiling point: 1,156 K (882.85 °C)
Density: 0.97 g/cm3
Appearance: silvery white metallic
Thermal conductivity: 142 W/(m·K)
Electrical resistivity: 47.7 nΩ·m at 20 °C
Electrical conductivity: 20.96 MS/m
Crystal structure: body-centered cubic
Molar heat capacity: 28.23 J/(mol·K)

Chemical properties

Oxidation states: +1
Electronegativity: 0.93 (Pauling Scale)
Ionisation energy: 5.139 eV
1st 495.8, 2nd 4,562, 3rd 6,910.3 kJ/mol
Electron affinity: 0.548 eV
Atomic radius: empirical 166, covalent 166, van der Waals 227 pm
Ionic radius: Na⁺ 102 pm

Reactivity: An alkali metal with one 3s electron and a strongly negative reduction potential (about -2.7 V); highly reactive, more so than lithium and less than potassium, never found free in nature.
with water: Reacts vigorously with cold water, melting into a ball that skates over the surface, to sodium hydroxide and hydrogen, which may catch fire: $\ce{2 Na + 2 H2O -> 2 NaOH + H2}$
with oxygen, air: Tarnishes within seconds in air; heated in air or oxygen it burns with an orange flame, mainly to sodium peroxide with some oxide: $\ce{2 Na + O2 -> Na2O2}$
with acids: Reacts violently with dilute acids to the sodium salt and hydrogen.
with halogens: Reacts vigorously with all the halogens to the ionic sodium halides: $\ce{2 Na + Cl2 -> 2 NaCl}$

Typical compounds

NaCl sodium chloride
rock salt and sea salt; source of nearly all sodium chemistry
NaOH sodium hydroxide
caustic soda, from brine electrolysis; soap and paper
Na₂CO₃ sodium carbonate
soda ash; glass making and water softening
NaHCO₃ sodium hydrogen carbonate
baking soda
NaNO₃ sodium nitrate
Chile saltpeter; fertiliser
Na₂S₂O₃.5H₂O sodium thiosulfate pentahydrate
hypo, the photographic fixer

Occurrence, production and use

Crustal abundance: 2.36×104 milligrams per kilogram
Oceanic abundance: 1.08×104 milligrams per liter

Occurrence and sources

Sodium is present in fair abundance in the sun and stars. The D lines of sodium are among the most prominent in the solar spectrum. Sodium is the fourth most abundant element on earth, comprising about 2.6% of the earth's crust; it is the most abundant of the alkali group of metals.

It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride. This method is much cheaper than that of electrolyzing sodium hydroxide, as was used several years ago.

dissolved in seawater
about 10,800 mg/L; crustal estimate 23,600 mg/kg ¹
halite, rock salt (NaCl); salt in brine; solar salt
evaporite beds and domes, wells and saline lakes; US 2024 output by type: salt in brine 42 percent, rock salt 40, solar 9, vacuum pan 9; the salt content of the oceans is nearly unlimited
trona and sodium carbonate brines
Green River Basin, Wyoming, the world's largest trona deposit; Searles and Owens Lakes, California; Turkey, Botswana, Kenya, Ethiopia

Extraction, production: Mining of rock salt, solution mining as brine, solar evaporation of seawater and lake brines
Physical; brine is the form the chemical industry buys, 91 percent of chemical-feedstock salt in the United States in 2024.
Chlor-alkali electrolysis of brine to sodium hydroxide, chlorine and hydrogen
$\ce{2 NaCl + 2 H2O -> 2 NaOH + H2 + Cl2}$
Anode, all cell types: $\ce{2 Cl- -> Cl2 + 2 e-}$ . Cathode, membrane and diaphragm cells: $\ce{2 Na+ + 2 e- + 2 H2O -> 2 NaOH + H2}$ . Mercury cells make an amalgam first ². EU-27 and EFTA caustic consumption was 9,611 kt in 2012 ³.
Natural soda ash from trona; synthetic soda ash from salt and limestone
$\ce{2 NaCl (aq) + CaCO3 (s) -> Na2CO3 (s) + CaCl2 (aq)}$
Synthetic soda ash from salt and limestone costs more energy and releases more CO2 than natural ⁴; its stoichiometry is not printed in the cached sources.

Uses

Sodium is used in the production of titanium, sodamide, sodium cyanide, sodium peroxide, and sodium hydride. Liquid sodium has been used as a coolant for nuclear reactors. Sodium vapor is used in streetlights and produces a brilliant yellow light.

Sodium also forms many useful compounds. Some of the most common are: table salt (NaCl), soda ash (Na2CO3), baking soda (NaHCO3), caustic soda (NaOH), Chile saltpeter (NaNO3) and borax (Na2B4O7·10H2O).

Metallic sodium is vital in the manufacture of esters and in the preparation of organic compounds. The metal may be used to improve the structure of certain alloys, descale metal, and purify molten metals.

An alloy of sodium with potassium, NaK, is an important heat transfer agent.

Chemicals
- salt as feedstock for chlorine and caustic soda
- caustic soda in organic synthesis, neutralisation of acids, gas scrubbing, phosphates and other inorganics
- soda ash in chemicals and detergents
- sodium metal for esters and organic synthesis
chemical industry 39 percent of US salt sales in 2024, chlor-alkali the main consumer ⁵; EU-27 and EFTA caustic soda 2012: organics 30.1 percent, miscellaneous 15.8, other inorganics 12.5 ⁶; US soda ash 2024: chemicals 29 percent ⁵
Glass: soda ash as the flux in glassmaking, a use dating from antiquity
glass 45 percent of US soda ash consumption in 2024
Food and beverage
- salt as flavouring and preservative
- caustic soda for fruit and vegetable peeling, ice cream, thickeners and wrappings
food processing 4 percent of US salt in 2024; food industries 5.3 percent of EU-27 and EFTA caustic soda in 2012
Pulp, paper and textiles
- caustic soda for pulp, paper and cellulose
- bleach for textiles
- rayon (viscose) spinning
EU-27 and EFTA caustic soda in 2012: pulp, paper and cellulose 13.4 percent, bleach 3.9, rayon 1.5
Water treatment
- sodium hydroxide to raise pH and precipitate heavy metals; sodium carbonate for precipitation ⁷
- caustic for flocculation and acidity control
water treatment 5.0 percent of EU-27 and EFTA caustic soda in 2012; 1 percent of US soda ash in 2024

Safety, toxicity

Sodium metal should be handled with great care. It cannot be maintained in an inert atmosphere and contact with water and other substances with which sodium reacts should be avoided.

GHS classification, signal word Danger

H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases
H314 Causes severe skin burns and eye damage Skin corrosion/irritation
H318 Causes serious eye damage Serious eye damage/eye irritation

Discovery and name

Discovered by: Humphry Davy
Discovered: 1807
First isolated: not in sources
Named by: not in sources
Origin of the name: possibly from Arabic suda, 'headache', for soda's use as an anticephalalgic

Sodium, like every reactive element, is never found free in nature. Sodium is a soft, bright, silvery metal which floats on water. Decomposition in water results in the evolution of hydrogen and the formation of the hydroxide. It may or may not ignite spontaneously on water, depending on the amount of oxide and metal exposed to the water. It normally does not ignite in air at temperatures below 115°C.

Notes

Jefferson Lab figures via PubChem
CAK BREF, PDF p40
PDF p32
USGS, printed pp. 164 to 165
USGS
CAK BREF, PDF p32
CWW BREF, PDF p203 and p237

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.