Chlorine

Typical wastewaters

• chlorinated municipal wastewater effluent free and combined chlorine residual (HOCl, OCl⁻, chloramines) after a 5 to 20 mg/L disinfection dose, removed by sulfite before discharge
• power plant once-through cooling water and cooling tower blowdown free available chlorine from intermittent biofouling control, 0.5 mg/L maximum and 0.2 mg/L average, discharged for no more than two hours a day
• municipal sewage, industrial effluent and road de-icing runoff chloride Cl⁻, the stable end state, conservative
• pulp bleaching chlorate ClO₃⁻
• rocket fuel, fireworks and flare manufacture and use perchlorate ClO₄⁻, not removed by conventional treatment
• textile dyeing and finishing and leather tanning total chlorine measured during sampling and reported, no limit value

1 · Identity

Symbol, number

Cl, 17

Oxidation states in water

-1 chloride (Cl⁻, the stable end state of all chlorine chemistry in water); 0 molecular chlorine, present only below about pH 4 in dilute solution; +1 hypochlorous acid, hypochlorite and the inorganic chloramines (free and combined chlorine); +3 chlorite; +4 chlorine dioxide; +5 chlorate; +7 perchlorate.

Note

Free chlorine means HOCl plus OCl⁻; combined chlorine means the chloramines NH₂Cl, NHCl₂ and NCl₃ and organic chloramines; total chlorine is their sum. The element entry already carries the hydrolysis and bleach equations; this chapter extends them to the treatment plant.

2 · Occurrence in water

Natural sources

Chloride from halite and other evaporites, seawater (19,400 mg/L), saline intrusion and rain; no oxidised chlorine occurs naturally except perchlorate, which forms in the atmosphere and is deposited in rain and in arid soils and nitrate deposits (WHO perchlorate document).

Anthropogenic sources

Free chlorine from disinfection of drinking water, wastewater, swimming pools and cooling water; chloride from sewage, industrial effluent and de-icing salt (WHO); chlorate and perchlorate carried into water by aged hypochlorite and by chlorine dioxide generation; chlorate from pulp bleaching and herbicide use; perchlorate from rocket fuel, fireworks and flares (WHO); trihalomethanes and haloacetic acids formed when chlorine meets natural organic matter.

3 · Speciation

Chlorine gas hydrolyses at once to hypochlorous acid and chloride (equilibrium constant 3.94 x 10^4 per mol per litre at 25 C; complete above pH 3 in dilute solution). Hypochlorous acid is a weak acid, pKa 7.5 (7.6 in the EPA chloramine chapter): HOCl and OCl⁻ are equal at pH 7.5 and 25 C, HOCl dominates below pH 6.5 and OCl⁻ above pH 8.5. HOCl is the germicide, 70 to 80 times more effective than OCl⁻ against bacteria, so chlorination works best at lower pH. With ammonia, HOCl forms monochloramine below a Cl₂ to N weight ratio of 5 to 1, passes through the breakpoint between 5 to 1 and 7.6 to 1 where the residual falls to a minimum and nitrogen gas, nitrate and nitrogen trichloride form, and leaves free chlorine above 7.6 to 1; dichloramine dominates at low pH.

Solubility

Chlorine gas dissolves at 14.6 g/L at 0 C (WHO) but hydrolyses rather than staying as Cl₂. All chlorides of the common cations are freely soluble, so chloride is conservative: it passes every treatment except desalination. Chlorate and perchlorate salts are freely soluble and both anions are stable and mobile in water.

Hydrolysis

Cl₂ to HOCl and Cl⁻ with release of a proton, so chlorine gas lowers pH; sodium and calcium hypochlorite release hydroxide and raise it.

Complexation

Chloride forms weak complexes with metals (not quantified in the sources read) and raises the corrosion rate of metals in distribution systems, releasing metals into the supply (WHO chloride fact sheet).

Precipitates

None of significance for chloride, chlorate or perchlorate. AgCl is the analytical precipitate for chloride titration.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

General practice, not from the sources read, except where noted: chlorine residual is measured on site at once: it is lost to light, agitation, warmth and reaction with the bottle contents. Samples for by-products need a quench (sulfite or ascorbic acid) or formation continues in the bottle. Chlorate and perchlorate samples need no preservation beyond cooling but hypochlorite stock must be diluted before injection. Chloride is conservative and easy; contamination from hands and glassware is the main error at low levels.

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

Free chlorine at drinking water levels shows no specific adverse effects in humans or animals; TDI 150 µg/kg body weight; IARC places hypochlorite in Group 3 (WHO). Chlorine gas is another matter and is in the element entry. Chlorate and perchlorate act on the thyroid: chlorate ADI 0 to 0.01 mg/kg from thyroid effects in rats, perchlorate PMTDI 0.01 mg/kg from inhibition of iodide uptake (WHO). Chloroform and the brominated trihalomethanes carry individual guideline values (WHO).

Bioaccumulation

Not relevant for free and combined chlorine, which react to chloride within hours. Perchlorate is highly stable, poorly retained by soils (more than 90 percent stays in the water phase) and accumulates in arid soils; it is taken up by crops and appears in produce and milk (WHO perchlorate document).

Ecotoxicity

US EPA aquatic life criteria for chlorine: freshwater 19 µg/L acute and 11 µg/L chronic, saltwater 13 and 7.5 µg/L (1986). This is the basis of dechlorination requirements on chlorinated effluents and cooling water.

Flags

• The US perchlorate status is time sensitive: a proposed rule of January 2026 with a final rule due May 2027; check before reuse.
• The chlorate occurrence figures are a 1996 US survey quoted by WHO; European and GCC occurrence was not read.
• The breakpoint equation is balanced here from the EPA dose table (6.3 mg Cl₂ per mg NH₃), not copied from a printed equation.
• The hypochlorite decomposition stoichiometry (3 OCl⁻ to chlorate and chloride) is taken from the book's own element entry; the WHO 2016 document describes the pathway with chlorite as intermediate but prints no equation.
• The thiosulfate and activated carbon dechlorination equations are cited to Metcalf and Eddy chapter 12 and were not re-read this session.
• The chlorite reduction with Fe(II) is an electron balance; WHO names the reagent only.
• Abu Dhabi values cover two media (marine outfall free residual chlorine 0.5 mg/L; sewer chloride 1000 mg/L); other GCC states not read.
• The ISO 7393-2 working range is quoted from the ISO catalogue abstract, the standard itself was not read.
• No free chlorine parametric value exists at EU level; national values (for example the 0.1 to 0.5 mg/L residuals that many Member States require) were not read and are not quoted.

Gaps

• No source read gives chloride, free chlorine or chlorate concentrations in industrial wastewater; the CWW BREF text on free chlorine and AOX was not re-read.
• The WHO monochloramine guideline value and the WHO haloacetic acid values were not read (the 2022 fact sheet URLs returned 404); the US and EU values stand in.
• Detection limits for the trihalomethane methods were not read.
• Chloride removal (reverse osmosis, electrodialysis, evaporation) is not written up because no read source covers it; it belongs with the sodium and TDS chapters.
• Cyanide oxidation by alkaline chlorination and chlorine in cooling water biocide chemistry beyond the CFR limits were not sourced.
• No kinetic data for THM formation or for chlorate formation rates in hypochlorite beyond the qualitative WHO statements.
• EU national free chlorine residual requirements and other GCC states were not read.

Sources

Identity

Name and symbol

Chlorine, Cl

Atomic number

17 protons

Position

group 17 · period 3 · p-block · diatomic nonmetal

CAS number

Cl₂: 7782-50-5

Atomic structure

Atomic mass

35.4527 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁵
[Ne] 3s²³p⁵

Electrons per shell

2, 8, 7

Valence electrons

7 outer shell

Physical properties

State at room temperature

Gas

Melting point

171.65 K (-101.5 °C)

Boiling point

239.11 K (-34.04 °C)

Density

0.0032 g/cm3 (gas at STP, so 3.214 g/L)

Appearance

pale yellow-green gas

Thermal conductivity

8.9-3 W/(m·K)

Electrical resistivity

>10 Ω·m at 20 °C

Electrical conductivity

1.00e-1 S/m

Crystal structure

orthorhombic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+7, +5, +1, -1

Electronegativity

3.16 (Pauling Scale)

Ionisation energy

12.968 eV
1st 1,251.2, 2nd 2,298, 3rd 3,822 kJ/mol

Electron affinity

3.617 eV

Atomic radius

empirical 102, covalent 102, van der Waals 175 pm

Ionic radius

Cl⁻ 181; Cl⁵⁺ 12 (3py-coordinate); Cl⁷⁺ 27 pm

Reactivity

The second halogen ([Ne] 3s2 3p5), one electron short of an octet; a yellow-green gas that is one of the most reactive elements and a strong oxidiser, weaker than fluorine and stronger than bromine, combining directly with nearly all elements.

with water

Moderately soluble (about 3 volumes per volume at 10 C) and reacts reversibly and slowly to hydrochloric and hypochlorous acids, the basis of water disinfection: $\ce{Cl2 + H2O -> HCl + HOCl}$

with oxygen, air

Does not combine directly with oxygen; none of the chlorine oxides (Cl2O, ClO2, Cl2O7) can be made from the elements.

with acids

Does not react with acids; chlorine is itself liberated from hydrochloric acid by oxidisers such as manganese dioxide.

with halogens

Combines with fluorine to the interhalogens: chlorine monofluoride from the elements at 225 C, the trifluoride at 200 to 300 C and the pentafluoride with excess fluorine at 350 C and 250 atm: $\ce{Cl2 + 3 F2 -> 2 ClF3}$

Typical compounds

• NaCl sodium chloride halite; brine electrolysis makes chlorine, hydrogen and caustic soda
• HCl hydrogen chloride hydrochloric acid; from burning hydrogen in chlorine
• NaOCl sodium hypochlorite bleach and disinfectant
• NaClO₃ sodium chlorate source of chlorine dioxide for pulp bleaching
• C₂H₃Cl vinyl chloride monomer of PVC, about a fifth of chlorine use
• CCl₄ carbon tetrachloride chlorinated solvent

Occurrence, production and use

Crustal abundance

1.45×102 milligrams per kilogram

Oceanic abundance

1.94×104 milligrams per liter

Occurrence and sources

In nature it is found in the combined state only, chiefly with sodium as common salt (NaCl), carnallite, and sylvite.

• dissolved in seawater about 19,400 mg/L; crustal estimate 145 mg/kg (Jefferson Lab figures via PubChem)
• halite, rock salt (NaCl) and brine evaporite beds, salt domes, solution-mined brine and solar salt; the chemical industry took 39 percent of US salt in 2024, 91 percent of it as brine, chiefly for chlorine and caustic soda
• carnallite and sylvite (potassium chlorides) evaporite deposits

Extraction, production

Chlor-alkali electrolysis of brine (membrane, diaphragm or mercury cell)

$\ce{2 NaCl + 2 H2O -> 2 NaOH + H2 + Cl2}$

Anode, all techniques: $\ce{2 Cl- -> Cl2 + 2 e-}$. Cathode, membrane and diaphragm cells: $\ce{2 Na+ + 2 e- + 2 H2O -> 2 NaOH + H2}$; mercury cells form sodium amalgam and decompose it with water. Products come in fixed ratio, 1,070 to 1,128 kg NaOH and about 28 kg H2 per tonne Cl2. Side reactions in the anolyte make hypochlorite and chlorate and are repressed by low pH (CAK BREF, PDF p40). Mercury cells were about 6 to 7 percent of world capacity in 2012 (PDF p30).

Uses

Chlorine is commonly used as an antiseptic and is used to make drinking water safe and to treat swimming pools. Large amounts of chlorine are used in many industrial processes, such as in the production of paper products, plastics, dyes, textiles, medicines, antiseptics, insecticides, solvents and paints.

Two of the most familiar chlorine compounds are sodium chloride (NaCl) and hydrogen chloride (HCl). Sodium chloride, commonly known as table salt, is used to season food and in some industrial processes. Hydrogen chloride, when mixed with water (H2O), forms hydrochloric acid, a strong and commercially important acid. Other chlorine compounds include: chloroform (CHCl3), carbon tetrachloride (CCl4), potassium chloride (KCl), lithium chloride (LiCl), magnesium chloride (MgCl2) and chlorine dioxide (ClO2).

Chlorine is a very dangerous material. Liquid chlorine burns the skin and gaseous chlorine irritates the mucus membranes. Concentrations of the gas as low as 3.5 parts per million can be detected by smell while concentrations of 1000 parts per million can be fatal after a few deep breaths.

Chlorine is widely used in making many everyday products. It is used for producing safe drinking water the world over. Even the smallest water supplies are now usually chlorinated.

It is also extensively used in the production of paper products, dyestuffs, textiles, petroleum products, medicines, antiseptics, insecticides, food, solvents, paints, plastics, and many other consumer products.

Most of the chlorine produced is used in the manufacture of chlorinated compounds for sanitation, pulp bleaching, disinfectants, and textile processing. Further use is in the manufacture of chlorates, chloroform, carbon tetrachloride, and in the extraction of bromine.

Organic chemistry demands much from chlorine, both as an oxidizing agent and in substitution, since it often brings many desired properties in an organic compound when substituted for hydrogen, as in one form of synthetic rubber.

• PVC: direct chlorination of ethylene, $\ce{C2H4 + Cl2 -> C2H4Cl2 (EDC)}$, -180 kJ/mol; EDC cracking, $\ce{C2H4Cl2 -> CH2CHCl (VCM) + HCl}$, +71 kJ/mol; oxychlorination that reuses the HCl, C2H4 + 1/2 $\ce{O2 + 2 HCl -> C2H4Cl2 + H2O}$, -239 kJ/mol; about 90 percent of plants run this balanced unit (LVOC BREF, PDF p528) PVC 33.3 percent (3,245 kt) of EU-27 and EFTA chlorine in 2012 (CAK BREF, PDF p31); the RSC's undated figure is about 20 percent
• Isocyanates, epichlorohydrin, chloromethanes and solvents: isocyanates and oxygenates for polyurethane foams, insulation and paints; epichlorohydrin for epoxy resins; chloromethanes for silicone rubbers and PTFE; phosphorus trichloride, $\ce{P4 + 6 Cl2 -> 4 PCl3 (SIC BREF, PDF}$ p217); chlorinated solvents for degreasing and dry cleaning EU-27 and EFTA 2012: isocyanates and oxygenates 30.3 percent, other organics 9.3, epichlorohydrin 5.2, chloromethanes 4.6, solvents 2.9 (CAK BREF, PDF p31)
• Water treatment and disinfection: chlorination of drinking water and swimming pools worldwide; sodium hypochlorite from scrubbing chlorine in caustic; chlorine, hypochlorite and chlorine dioxide as oxidants in waste water treatment (CWW BREF, PDF p247) inorganics, including disinfectants, water treatment and pigments, 14.3 percent of EU-27 and EFTA chlorine in 2012
• Pharmaceuticals: chlorine as oxidant and in substitution reactions; 85 percent of pharmaceuticals use chlorine or its compounds at some stage
• Pulp, paper and textiles: pulp bleaching, textile processing, dyestuffs

Safety, toxicity

Chlorine is a respiratory irritant. The gas irritates the mucus membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. In fact, chlorine was used as a war gas in 1915.

Exposure to chlorine should not exceed 0.5 ppm (8-hour time-weighted average - 40 hour week).

GHS classification, signal word Warning

• H317 May cause an allergic skin reaction Sensitization, Skin

Discovery and name

Discovered by

Carl Wilhelm Scheele

Discovered

1774

First isolated

not in sources

Named by

not in sources

Origin of the name

after the Ancient Greek χλωρός ("pale green") because of its colour

It is a member of the halogen (salt-forming) group of elements and is obtained from chlorides by the action of oxidizing agents and more often by electrolysis; it is a greenish-yellow gas, combining directly with nearly all elements. At 10°C one volume of water dissolves 3.10 volumes of chlorine, at 30°C only 1.77 volumes.