Sodium

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 entry: 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 entry 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 (WHO background document). 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 (WHO); 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).

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 (FAO 29). 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 (WHO).

Solubility

Sodium chloride 357 g/L and sodium carbonate 71 g/L at 0 C (WHO); 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.

4 · Role in treatment

5 · Removal and control

6 · Analytics

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.

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 (WHO). 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 (WHO). 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 (FAO 29).

Flags

• 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.

Gaps

• 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.

Sources

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

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 (Jefferson Lab figures via PubChem)
• 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 (CAK BREF, PDF p40). EU-27 and EFTA caustic consumption was 9,611 kt in 2012 (PDF p32).

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 (USGS, printed pp. 164 to 165); 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 (USGS); EU-27 and EFTA caustic soda 2012: organics 30.1 percent, miscellaneous 15.8, other inorganics 12.5 (CAK BREF, PDF p32); US soda ash 2024: chemicals 29 percent (USGS)
• 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 (CWW BREF, PDF p203 and p237); 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.