Chlorine cas

Chlorine cas DEFAULT


HelpSubstance identity

The ‘Substance identity’ section is calculated from substance identification information from all ECHA databases. The substance identifiers displayed in the InfoCard are the best available substance name, EC number, CAS number and/or the molecular and structural formulas.

Some substance identifiers may have been claimed confidential, or may not have been provided, and therefore not be displayed.

EC (European Community) Number

The EC Number is the numerical identifier for substances in the EC Inventory. The EC Inventory is a combination of three independent European lists of substances from the previous EU chemicals regulatory frameworks (EINECS, ELINCS and the NLP-list). More information about the EC Inventory can be found here.

If the substance was not covered by the EC Inventory, ECHA attributes a list number in the same format, starting with the numbers 6, 7, 8 or 9.

The EC or list number is the primary substance identifier used by ECHA.

CAS (Chemical Abstract Service) registry number

The CAS number is the substance numerical identifier assigned by the Chemical Abstracts Service, a division of the American Chemical Society, to substances registered in the CAS registry database. A substance identified primarily by an EC or list number may be linked with more than one CAS number, or with CAS numbers that have been deleted. More information about CAS and the CAS registry can be found here.

Molecular formula

The molecular formula identifies each type of element by its chemical symbol and identifies the number of atoms of each element found in one discrete molecule of the substance. This information is only displayed if the substance is well–defined, its identity is not claimed confidential and there is sufficient information available in ECHA’s databases for ECHA’s algorithms to generate a molecular structure.

Molecular structure

The molecular structure is based on structures generated from information available in ECHA’s databases. If generated, an InChI string will also be generated and made available for searching. This information is only displayed if the substance is well-defined, its identity is not claimed confidential and there is sufficient information available in ECHA’s databases for ECHA’s algorithms to generate a molecular structure.

More help available here.

EC / List no.: 231-959-5

CAS no.:7782-50-5

Mol. formula:Cl2

HelpHazard classification and labelling

The ‘Hazard classification and labelling’ section shows the hazards of a substance based on the standardised system of statements and pictograms established under the CLP (Classification Labelling and Packaging) Regulation. The CLP Regulation makes sure that the hazards presented by chemicals are clearly communicated to workers and consumers in the European Union. The CLP Regulation uses the UN Global Harmonised System (GHS) and European Union Specific Hazard Statements (EUH).

This section is based on three sources for information (harmonised classification and labelling (CLH), REACH registrations and CLP notifications). The source of the information is mentioned in the introductory sentence of the hazard statements. When information is available in all sources, the first two are displayed as a priority.

Please note:

The purpose of the information provided under this section is to highlight the substance hazardousness in a readable format. It does not represent a new labelling, classification or hazard statement, neither reflect other factors that affect the susceptibility of the effects described, such as duration of exposure or substance concentration (e.g. in case of consumer and professional uses). Other relevant information includes the following:

  • Substances may have impurities and additives that lead to different classifications. If at least one company has indicated that the substance classification is affected by impurities or additives, this will be indicated by an informative sentence. However, substance notifications in the InfoCard are aggregated independently of the impurities and additives.
  • Hazard statements were adapted to improve readability and may not correspond textually to the hazard statements codes description in the European Union Specific Hazard Statements (EUH) or the UN Global Harmonised System (GHS).

To see the full list of notified classifications and to get more information on impurities and additives relevant to classification please consult the C&L Inventory.

More information about Classification and Labelling is available in the Regulations section of ECHA website.

More help available here.

Harmonised classification and labelling (CLH)

Harmonised classification and labelling is a legally binding classification and labelling for a substance, agreed at European Community level. Harmonisation is based on the substance’s physical, toxicological and eco-toxicological hazard assessment.

The ‘Hazard classification’ and labelling section uses the signal word, pictogram(s) and hazard statements of the substance under the harmonised classification and labelling (CLH) as its primary source of information.

If the substance is covered by more than one CLH entry (e.g. disodium tetraborate EC no. 215–540–4, is covered by three harmonisations: 005–011–00–4; 005–011–01–1 and 005–011–02–9), CLH information cannot be displayed in the InfoCard as the difference between the CLH classifications requires manual interpretation or verification. If a substance is classified under multiple CLH entries, a link to the C&L Inventory is provided to allow users to view CLH information associated with the substance and no text is automatically generated for the InfoCard.

It is possible that a harmonisation is introduced through an amendment to the CLP Regulation. In that case, the ATP (Adaptation to Technical Progress) number is displayed.

More info on CLH can be found here.

Classification and labelling under REACH

If available, additional information on classification and labelling (C&L) is derived from REACH registration dossiers submitted by industry. This information has not been reviewed or verified by ECHA, and may change without prior notice. REACH registration dossiers have greater data requirements (such as supporting studies) than do notifications under CLP.

Notifications under the Classification Labelling and Packaging (CLP) Regulation

If no EU harmonised classification and labelling exists and the substance was not registered under REACH, information derived from classification and labelling (C&L) notifications to ECHA under CLP Regulation is displayed under this section. These notifications can be provided by manufacturers, importers and downstream users. ECHA maintains the C&L Inventory, but does not review or verify the accuracy of the information.

Note that for readability purposes, only the pictograms, signal words and hazard statements referred in more than 5% of the notifications under CLP are displayed.

GHS03: OxidisingGHS04: Gas under PressureGHS06: Acute ToxicityGHS09: Hazardous to the Environment

Danger! According to the harmonised classification and labelling (ATP01corr) approved by the European Union, this substance is toxic if inhaled, is very toxic to aquatic life, may cause or intensify fire (oxidiser), causes serious eye irritation, causes skin irritation and may cause respiratory irritation.

Additionally, the classification provided by companies to ECHA in REACH registrations identifies that this substance is fatal if inhaled, is very toxic to aquatic life with long lasting effects and contains gas under pressure and may explode if heated.

Helpful informationAbout this substance

This section provides an overview of the calculated volume at which the substance is manufactured or imported to the European Economic Area (EU28 + Iceland, Liechtenstein and Norway). Additionally, if available, information on the use of the substance and how consumers and workers are likely to be exposed to it can also be displayed here.

The use information is displayed per substance life cycle stage (consumer use, in articles, by professional workers (widespread uses), in formulation or re-packing, at industrial sites or in manufacturing). The information is aggregated from the data coming from REACH substance registrations provided by industry.

For a detailed overview on identified uses and environmental releases, please consult the registered substance factsheet.

Use descriptors are adapted from ECHA guidance to improve readability and may not correspond textually to descriptor codes described in Chapter R.12: Use Descriptor system of ECHA Guidance on information requirements and chemical safety assessment.

The examples provided are generic examples and may not apply to the specific substance you are viewing. A substance may have its use restricted to certain articles or products and therefore not all the examples may apply to the specific substance. Furthermore, some substances can be found in an article, but with unlikely exposure (e.g. inside a watch) or with very low concentrations considered not to pose risks to human health or the environment.

Please note:

For readability purpose, only non-confidential use descriptors occurring in more than 5% of total occurrences are displayed.

The described Product category (i.e. the products in which the substance may be used) may refer to uses as intermediate and under controlled conditions, for which there is no consumer exposure.

More help is available here.

This substance is registered under the REACH Regulation and is manufactured in and / or imported to the European Economic Area, at ≥ 1 000 000 tonnes per annum.

This substance is used in articles, by professional workers (widespread uses), in formulation or re-packing, at industrial sites and in manufacturing.

Biocidal Uses

This substance is approved for use as a biocide in the EEA and/or Switzerland, for: human hygiene, disinfection, disinfection, veterinary hygiene, food and animals feeds, drinking water, drinking water.

This substance is being reviewed for use as a biocide in the EEA and/or Switzerland, for: disinfection, disinfection, food and animals feeds, preservation for liquid systems, preservation for liquid systems, preservation for liquid systems, preservation for liquid systems, controlling slimes.

Consumer Uses

ECHA has no public registered data indicating whether or in which chemical products the substance might be used. ECHA has no public registered data on the routes by which this substance is most likely to be released to the environment.

Article service life

Other release to the environment of this substance is likely to occur from: outdoor use in long-life materials with low release rate (e.g. metal, wooden and plastic construction and building materials).
ECHA has no public registered data indicating whether or into which articles the substance might have been processed.

Widespread uses by professional workers

This substance is used in the following products: laboratory chemicals, water treatment chemicals, biocides (e.g. disinfectants, pest control products) and washing & cleaning products.
This substance is used in the following areas: municipal supply (e.g. electricity, steam, gas, water) and sewage treatment and scientific research and development.
This substance is used for the manufacture of: rubber products.
Other release to the environment of this substance is likely to occur from: indoor use (e.g. machine wash liquids/detergents, automotive care products, paints and coating or adhesives, fragrances and air fresheners), outdoor use as processing aid and indoor use in close systems with minimal release (e.g. cooling liquids in refrigerators, oil-based electric heaters).

Formulation or re-packing

This substance is used in the following products: laboratory chemicals and semiconductors.
This substance has an industrial use resulting in manufacture of another substance (use of intermediates).
Release to the environment of this substance can occur from industrial use: formulation of mixtures and manufacturing of the substance.

Uses at industrial sites

This substance is used in the following products: semiconductors, metal surface treatment products, laboratory chemicals, paper chemicals and dyes and water treatment chemicals.
This substance has an industrial use resulting in manufacture of another substance (use of intermediates).
This substance is used for the manufacture of: chemicals, electrical, electronic and optical equipment, pulp, paper and paper products, metals, textile, leather or fur and mineral products (e.g. plasters, cement).
Release to the environment of this substance can occur from industrial use: as an intermediate step in further manufacturing of another substance (use of intermediates), as processing aid, in processing aids at industrial sites and manufacturing of the substance.


Release to the environment of this substance can occur from industrial use: manufacturing of the substance, formulation of mixtures, in processing aids at industrial sites, as processing aid and as an intermediate step in further manufacturing of another substance (use of intermediates).

HelpHow to use it safely

This section provides links to the list of precautions (precautionary statements) and to the guidance on safe use, if they have been provided in REACH registration dossiers.

  • Precautionary statements - describe recommended measures to minimise or prevent adverse effects resulting from exposure to a hazardous product or improper storage or handling of a hazardous product.
  • Guidance on safe use - recommendations by substance registrant on the proper use of the substance in various situations. Examples include recommended measures on fire-fighting, transport and recycling and disposal.

Please note: Precautionary measures and guidance on safe use concern the use and handling of the specific substance as such, not of the presence of the substance in other articles or mixtures. The precautionary measures and guidance on safe use are as submitted to ECHA by registrants under the REACH Regulation. Information on precautionary measures and the safe use is submitted by the registrant of a substance and the registrant is solely responsible for its accuracy and completeness.

More help available here.

about INFOCARD - Last updated: 13/10/2021 InfoCard

The InfoCard summarises the non-confidential data of a substance held in the databases of the European Chemicals Agency (ECHA). InfoCards are generated automatically based on the data available at the time of generation.

The quality and correctness of the information submitted to ECHA remains the responsibility of the data submitter. The type of uses and classifications may vary between different submissions to ECHA and for a full understanding it is recommended to consult the source data. Information on applicable regulatory frameworks is also automatically generated and may not be complete or up to date. It is the responsibility of the substance manufacturers and importers to consult official publications, e.g. the electronic edition of the Official Journal of the European Union.

InfoCards are updated when new information is available. The date of the last update corresponds to the publication date of the InfoCard and not necessarily to the date in which the update occurred in the source data.

More help available here.


Regulatory context

Here you can find all of the regulations and regulatory lists in which this substance appears, according to the data available to ECHA. This substance has been found in the following regulatory activities (directly, or inheriting the regulatory context of a parent substance):


REACH - Registration, Evaluation, Authorisation and Restriction of Chemicals Regulation


CLP - Classification, Labelling and Packaging

  • Substances for which an agreed set of classification and labelling data has been agreed at EU level by Member States.

  • Substances for which classification and labelling data have been notified to ECHA by manufacturers or importers. Such notifications are required for hazardous substances, mixtures, or articles, manufactured or imported at over 1 kg per annum.

  • Substances for which industrial accident prevention and reporting requirements have been established.


BPR - Biocidal Products Regulation

  • Substances listed in the EINECS, ELINCS, or NLP inventories.

RegionLegislationLong-term Exposure Limit (LTEL) ValuesShort-term Exposure Limit (STEL) ValuesSkin DesignationDermal SensitizationRespiratory SensitizationWork SectorEffective DateExpiration DateMiscellaneous Notes
European UnionOELs - Occupational Exposure Limits - 2nd list1.50.5
RegionLegislationEmission mediumProcess/equipmentMaximum emissionsAverage emissionsNotes
Maximum limitMaximum sampling durationAverage limitAverage sampling duration
European UnionIndustrial Emissions DirectiveWaste gasesInstallations producing titanium dioxide40.0 mg/Nm35.0 mg/Nm31.0 dThe emission limit values which are expressed as concentrations in mass per cubic meter (Nm3) shall be calculated at a temperature of 27315 K and a pressure of 1013 kPa


Active Implantable Medical Devices Directive

  • This list contains hazardous substances for purposes of Directive 90/385/EEC on active implantable medical devices, particularly as regards Article 3 and Annex I (Essential Requirements). This non-exhaustive database is derived from: Table 3 of Annex VI to CLP, REACH Candidate List of SVHCs, and Directive 2000/54/EC's Annex III (Biological Agents list). Please note that Regulation (EU) 2017/745 on medical devices has repealed Directive 90/385/EEC and begun to apply from 26 May 2021. Nevertheless, Article 120 of the Regulation provides for a transitional period allowing medical devices, under specified conditions (e.g., placed on the market prior to 26 May 2021), to continue to comply with the Directive. In accordance with the fourth paragraph of Article 120, this period ends 26 May 2025.


CAD - Chemical Agents Directive

  • This list contains a non-exhaustive inventory based on the list of substances with harmonised classification and labelling (i.e., Table 3 of Annex VI to the CLP Regulation 1272/2008/EC). While the harmonised list covers many hazardous substances, others not listed may also meet the classification criteria in accordance with the CLP Regulation.


Construction Products Regulation

  • This list contains a non-exhaustive inventory of substances taken from: (1) Table 3 of Annex VI to CLP; (2) the Candidate List of SVHCs; (3) Annex XIV of REACH (Authorisation List); (4) Annex XVII of REACH (Restrictions List); (5) F-gases subject to emission limits/reporting per Regulation 517/2014/EU; and (6) volatile organic compounds (VOCs) listed in the Ambient Air Directive 2008/50/EC. The basis of the list is Annex I(3) of the Construction Products Regulation 305/2011/EC, which stipulates that construction works must not have a high impact on human health or the environment as a result of: giving off toxic gas; emissions of dangerous substances, volatile organic compounds (VOC), greenhouse gases or dangerous particles into indoor or outdoor air; release of dangerous substances into drinking water, ground water, marine waters, surface waters or soil.

  • This list contains a non-exhaustive inventory of substances originating from: (1) Table 3 of Annex VI to CLP (i.e., the list of harmonised substances); (2) the Candidate List of Substances of Very High Concern (SVHCs); and REACH Annex XIV (Authorisation List). This list is compiled on the basis of Article 6(5) of Regulation 305/2011/EC on Marketing of Construction Products. This provision requires SDSs and information on hazardous substances (i.e., SVHCs) contained in construction products be provided with the declaration of performance.


Cosmetic Products Regulation

  • This list contains substances which are banned from use in any cosmetic products marketed for sale or use in the European Union.



  • This list contains a non-exhaustive inventory of substances based on the list of hazardous substances with harmonised classification and labelling (i.e. Table 3 of Annex VI to the CLP Regulation), and the Candidate List of substances of very high concern (SVHCs). Pursuant to Article 6(6) of the EU Ecolabel Regulation, the ecolabel must not be awarded to goods containing substances or mixtures classified according to the CLP as toxic; hazardous to the environment; and carcinogenic, mutagenic, or toxic for reproduction (CMRs). Nor are products allowed the ecolabel award when they contain SVHCs (per Article 57 of REACH). While the CLP's harmonised list contains many such substances, other ones not listed in Table 3 may also meet the criteria specified for classification under the CLP.


End-of-Life Vehicles Directive

  • This list contains a non-exhaustive inventory of hazardous substances as defined by Article 2(11) of the End-of-Life Vehicles Directive 2000/53/EC. It is based on the relevant subset of substances with harmonised classification listed in Table 3 of Annex VI to the CLP Regulation 1272/2008/EC.


Food Contact Recycled Plastic Materials and Articles Regulation

  • This list contains the Annex I Plastic Food Contact Materials (FCMs) authorised for use in the European Union under Regulation 10/2011/EU. Pursuant to Art. 4(b) of Directive 282/2008/EC on recycled plastic FCMs, plastic recycling processes can only be authorised if input originates from plastic materials and articles manufactured in accordance with EU legislation on plastic food contact materials and articles.



General Product Safety Directive

  • This list contains a non-exhaustive inventory of substances that fall within the European Union's hazardous substance definitions, as provided on: (1) Table 3 of Annex VI to the CLP Regulation 1272/2008/EC; (2) Annex III of Directive 2000/54/EC (Biological Agents); Candidate List of SVHCs; and REACH Annexes XIV and XVII (Authorisation and Restriction lists). They can be considered hazardous for purposes of the General Product Safety Directive 2001/95/EC.


In Vitro Diagnostic Medical Devices Directive

  • This list contains a non-exhaustive inventory of hazardous substances for purposes of essential requirements (Article 3 and Annex I) for general safety, design, manufacture and hazard communication of in vitro diagnostic medical devices. It is derived from: Table 3 of Annex VI to CLP, REACH Candidate List of SVHCs, and Directive 2000/54/EC's Annex III (Biological Agents list).


Industrial Emissions Directive

  • This list contains the polluting substances for which emission limit values are assigned under Directive 2010/75/EU on Industrial Emissions (Integrated Pollution Prevention and Control - IPPC). Member States must permit all qualifying facilities in order to ensure that they minimize impact on the environment. The permit issued must provide emission limit values for pollutants on this list.

  • This list contains emission limit values for polluting substances in waste gases and waste water, assigned according to facility type (i.e., combustion plants (Annex V), waste incineration/co-incineration plants (Annex VI), and installations producing titanium dioxide (Annex VIII)), under Directive 2010/75/EU on Industrial Emissions (Integrated Pollution Prevention and Control - IPPC). For this list, if a substance presents 2 values in the ''Average sampling duration'' field, these indicate minimum and maximum average sampling period.



Inland Transport of Dangerous Goods Directive

  • This list contains the ADR Dangerous Goods List, as implemented by the European Union's Directive 2008/68/EC. This Directive applies the European Agreements on the international transport of dangerous goods by road (ADR) and inland waterways (ADN), and the regulations concerning the international carriage of dangerous goods by rail (RID).

  • This list contains the RID Dangerous Goods List, as implemented by the European Union's Directive 2008/68/EC. This Directive applies the European Agreements on the international transport of dangerous goods by road (ADR) and inland waterways (ADN), and the regulations concerning the international carriage of dangerous goods by rail (RID).

  • This list contains the ADN Dangerous Goods List, as implemented by the European Union's Directive 2008/68/EC. This Directive establishes rules for the safe transport of dangerous goods between EU countries by road (ADR) and inland waterways (ADN), and the regulations concerning the international carriage of dangerous goods by rail (RID).



Marine Environmental Policy Framework Directive

  • This list contains a non-exhaustive inventory of hazardous substances for purposes of the Marine Strategy Framework Directive, especially as it concerns Art. 3(8), and Annexes I and III. The listed substances meet the European Union's definitions as hazardous, as provided on: (1) Table 3 of Annex VI to the CLP Regulation (1272/2008/EC); (2) Annex III of Directive 2000/54/EC (Biological Agents); Candidate List of SVHCs; and REACH Annex XIV (Authorisation List).



Eye/Face Protection: Wear chemical safety goggles. A face shield (with safety goggles) may also be necessary.

Skin Protection: Wear chemical protective clothing e.g. gloves, aprons, boots. Coveralls or long sleeve shirts and pants in some operations. Wear a chemical protective, full-body encapsulating suit and self-contained breathing apparatus (SCBA). Suitable materials include: butyl rubber, neoprene rubber, Viton®, Viton®/butyl rubber, Barrier® - PE/PA/PE, Silver Shield® - PE/EVAL/PE, Trellchem® HPS, Trellchem® VPS, Saranex®™, Tychem® BR/LV, Tychem® Responder® CSM, Tychem® TK. The following materials should NOT be used: natural rubber, polyvinyl chloride. Recommendations are NOT valid for very thin neoprene rubber gloves (0.3 mm or less).

Respiratory Protection:

Up to 5 ppm:

(APF = 10) Any chemical cartridge respirator with cartridge(s) providing protection against chlorine*; or Any supplied-air respirator*.

*Reported to cause eye irritation or damage; may require eye protection.

APF = Assigned Protection Factor

Recommendations apply only to National Institute for Occupational Safety and Health (NIOSH) approved respirators. Refer to the NIOSH Pocket Guide to Chemical Hazards for more information.

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Chlorine: Lung Damaging Agent

EPA [1996]. SW-846 Method 0050: [PDF Format 188 KB] Isokinetic HCl/Cl2 emission sampling train. Washington, DC: U.S. Environmental Protection Agency.EPA [1996]. SW-846 Method 0051: [PDF Format 115 KB] Midget impinger HCl/Cl2 emission sampling train. Washington, DC: U.S. Environmental Protection Agency.EPA [1996]. SW-846 Method 9057: [PDF Format 44 KB] Determination of chloride from HCl/Cl2 emission sampling train (Methods 0050 and 0051) by anion chromatography. Washington, DC: U.S. Environmental Protection Agency.

NIOSH [1994]. NMAN Method 6011: Chlorine and bromine. In: NIOSH manual of analytical methods. 4th ed. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 94-113.

OSHA [1990]. Chlorine; OSHA Method ID-101; fully validated. Salt Lake City, UT: U.S. Department of Labor, Occupational Safety and Health Administration, Organic Methods Evaluation Branch, OSHA Analytical Laboratory.

OSHA [1990]. Chlorine; OSHA Method ID-126SGX; partially validated. Salt Lake City, UT: U.S. Department of Labor, Occupational Safety and Health Administration, Organic Methods Evaluation Branch, OSHA Analytical Laboratory.

Laboratory Preparation of Chlorine


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Chemical Datasheet

2.3 - Poisonous gas5.1 - Oxidizer8 - Corrosive
Chemical Identifiers| Hazards| Response Recommendations| Physical Properties| Regulatory Information| Alternate Chemical Names

Chemical Identifiers

NFPA 704

General Description

A greenish yellow gas with a pungent suffocating odor. Toxic by inhalation. Slightly soluble in water. Liquefies at -35°C and room pressure. Readily liquefied by pressure applied at room temperature. Density (as a liquid) 13.0 lb / gal. Contact with unconfined liquid can cause frostbite by evaporative cooling. Does not burn but, like oxygen, supports combustion. Long-term inhalation of low concentrations or short-term inhalation of high concentrations has ill effects. Vapors are much heavier than air and tend to settle in low areas. Contact CHEMTREC to activate chlorine response team 800-424-9300. Used to purify water, bleach wood pulp, and to make other chemicals.

Rate of onset: Immediate to hours

Persistence: Minutes to hours

Odor threshold: 3.5 ppm

Source/use/other hazard: Cleaner/disinfectant in many industries; water treatment; WWI war gas; irritating corr fumes heavier than air.


Reactivity Alerts

  • Strong Oxidizing Agent
  • Water-Reactive

Air & Water Reactions

Water dissolves about twice its volume of chlorine gas, forming a mixture of hydrochloric acid and hypochlorous acids. Will be corrosive due to acidity and oxidizing potential.

Fire Hazard

May ignite other combustible materials (wood, paper, oil, etc.). Mixture with fuels may cause explosion. Container may explode in heat of fire. Vapor explosion and poison hazard indoors, outdoors or in sewers. Hydrogen and chlorine mixtures (5-95%) are exploded by almost any form of energy (heat, sunlight, sparks, etc.). May combine with water or steam to produce toxic and corrosive fumes of hydrochloric acid. Emits highly toxic fumes when heated. Avoid plastics and rubber. Avoid heat and contact with hydrogen gas or powdered metals. (EPA, 1998)

Health Hazard

Poisonous; may be fatal if inhaled. Contact may cause burns to skin and eyes. Bronchitis or chronic lung conditions. (EPA, 1998)

Reactivity Profile

CHLORINE reacts explosively with or supports the burning of numerous common materials. Ignites steel at 100° C in the presence of soot, rust, carbon, or other catalysts. Ignites dry steel wool at 50° C. Reacts as either a liquid or gas with alcohols (explosion), molten aluminum (explosion), silane (explosion), bromine pentafluoride, carbon disulfide (explosion catalyzed by iron), 1-chloro-2-propyne (excess chlorine causes an explosion), dibutyl phthalate (explosion at 118° C), diethyl ether (ignition), diethyl zinc (ignition), glycerol (explosion at 70-80° C), methane over yellow mercury oxide (explosion), acetylene (explosion initiated by sunlight or heating), ethylene over mercury, mercury(I) oxide, or silver(I) oxide (explosion initiated by heat or light), gasoline (exothermic reaction then detonation), naphtha-sodium hydroxide mixture (violent explosion), zinc chloride (exothermic reaction), wax (explosion), hydrogen (explosion initiated by light). Reacts as either a liquid or gas with carbides of iron, uranium and zirconium, with hydrides of potassium sodium and copper, with tin, aluminum powder, vanadium powder, aluminum foil, brass foil, copper foil, calcium powder, iron wire, manganese powder, potassium, antimony powder, bismuth, germanium, magnesium, sodium, and zinc. Causes ignition and a mild explosion when bubbled through cold methanol. Explodes or ignites if mixed in excess with ammonia and warmed. Causes ignition in contact with hydrazine, hydroxylamine, and calcium nitride. Forms explosive nitrogen trichloride from biuret contaminated with cyanuric acid. Readily forms an explosive N-chloro derivative with aziridine. Ignites or explodes with arsine, phosphine, silane, diborane, stibine, red phosphorus, white phosphorus, boron, active carbon, silicon, arsenic. Ignites sulfides at ambient temperature. Ignites (as a liquid) synthetic and natural rubber. Ignites trialkylboranes and tungsten dioxide.

Belongs to the Following Reactive Group(s)

Potentially Incompatible Absorbents

Use caution: Liquids with this reactive group classification have been known to react with the absorbents listed below. More info about absorbents, including situations to watch out for...

  • Cellulose-Based Absorbents
  • Mineral-Based & Clay-Based Absorbents
  • Expanded Polymeric Absorbents
  • Dirt/Earth

Response Recommendations

Isolation and Evacuation

Excerpt from ERG Guide 124 [Gases - Toxic and/or Corrosive - Oxidizing]:

As an immediate precautionary measure, isolate spill or leak area for at least 100 meters (330 feet) in all directions.

SPILL: See ERG Tables 1 and 3 - Initial Isolation and Protective Action Distances on the UN/NA 1017 datasheet.

FIRE: If tank, rail car or tank truck is involved in a fire, ISOLATE for 800 meters (1/2 mile) in all directions; also, consider initial evacuation for 800 meters (1/2 mile) in all directions. (ERG, 2016)


Evacuate area endangered by gas. Stay upwind; keep out of low areas. Wear positive pressure breathing apparatus and full protective clothing. Move container from fire area if you can do so without risk. Spray cooling water on containers that are exposed to flames until well after fire is out. If it is necessary to stop the flow of gas, use water spray to direct escaping gas away from those effecting shut-off.

Will not burn, but most combustible materials will burn in chlorine as they do in oxygen; flammable gases will form explosive mixtures with chlorine. Dry chemical, carbon dioxide, water spray, fog or foam. (EPA, 1998)

Non-Fire Response

Excerpt from ERG Guide 124 [Gases - Toxic and/or Corrosive - Oxidizing]:

Fully encapsulating, vapor-protective clothing should be worn for spills and leaks with no fire. Do not touch or walk through spilled material. Keep combustibles (wood, paper, oil, etc.) away from spilled material. Stop leak if you can do it without risk. Use water spray to reduce vapors or divert vapor cloud drift. Avoid allowing water runoff to contact spilled material. Do not direct water at spill or source of leak. If possible, turn leaking containers so that gas escapes rather than liquid. Prevent entry into waterways, sewers, basements or confined areas. Isolate area until gas has dispersed. Ventilate the area. (ERG, 2016)

Protective Clothing

Skin: Wear appropriate personal protective clothing to prevent skin from becoming frozen from contact with the liquid or from contact with vessels containing the liquid.

Eyes: Wear appropriate eye protection to prevent eye contact with the liquid that could result in burns or tissue damage from frostbite.

Wash skin: No recommendation is made specifying the need for washing the substance from the skin (either immediately or at the end of the work shift).

Remove: No recommendation is made specifying the need for removing clothing that becomes wet or contaminated.

Change: No recommendation is made specifying the need for the worker to change clothing after the work shift.

Provide: Quick drench facilities and/or eyewash fountains should be provided within the immediate work area for emergency use where there is any possibility of exposure to liquids that are extremely cold or rapidly evaporating. (NIOSH, 2016)

DuPont Tychem® Suit Fabrics

Tychem® Fabric Legend

QS = Tychem 2000 SFR
QC = Tychem 2000
SL = Tychem 4000
C3 = Tychem 5000
TF = Tychem 6000
TP = Tychem 6000 FR
BR = Tychem 9000
TK = Tychem 10000
RF = Tychem 10000 FR

Testing Details

The fabric permeation data was generated for DuPont by independent testing laboratories using ASTM F739, EN369, EN 374-3, EN ISO 6529 (method A and B) or ASTM D6978 test methods. Normalized breakthrough times (the time at which the permeation rate is equal to 0.1 µg/cm2/min) reported in minutes. All liquid chemicals have been tested between approximately 20°C and 27°C unless otherwise stated. A different temperature may have significant influence on the breakthrough time; permeation rates typically increase with temperature. All chemicals have been tested at a concentration of greater than 95% unless otherwise stated. Unless otherwise stated, permeation was measured for single chemicals. The permeation characteristics of mixtures can deviate considerably from the permeation behavior of the individual chemicals. Chemical warfare agents (Lewisite, Sarin, Soman, Sulfur Mustard, Tabun and VX Nerve Agent) have been tested at 22°C and 50% relative humidity per military standard MIL-STD-282.

Chlorine (>95%, liquid, -70° C)7782-50-5Liquid>480>480>480
Chlorine (gas)7782-50-5Vaporimm.>480imm.>480>480>480>480>480>480
Chlorine (gas, 20 ppm)7782-50-5Vapor>480*

Special Warnings from DuPont

  1. Serged and bound seams are degraded by some hazardous liquid chemicals, such as strong acids, and should not be worn when these chemicals are present.
  2. CAUTION: This information is based upon technical data that DuPont believes to be reliable. It is subject to revision as additional knowledge and experience are gained. DuPont makes no guarantee of results and assumes no obligation or liability...

    ... in connection with this information. It is the user's responsibility to determine the level of toxicity and the proper personal protective equipment needed. The information set forth herein reflects laboratory performance of fabrics, not complete garments, under controlled conditions. It is intended for informational use by persons having technical skill for evaluation under their specific end-use conditions, at their own discretion and risk. Anyone intending to use this information should first verify that the garment selected is suitable for the intended use. In many cases, seams and closures have shorter breakthrough times and higher permeation rates than the fabric. Please contact DuPont for specific data. If fabric becomes torn, abraded or punctured, or if seams or closures fail, or if attached gloves, visors, etc. are damaged, end user should discontinue use of garment to avoid potential exposure to chemical. Since conditions of use are outside our control, we make no warranties, express or implied, including, without limitation, no warranties of merchantability or fitness for a particular use and assume no liability in connection with any use of this information. This information is not intended as a license to operate under or a recommendation to infringe any patent or technical information of DuPont or others covering any material or its use.

(DuPont, 2018)

First Aid

Warning: Effects may be delayed. Caution is advised. Chlorine is corrosive and may be converted to hydrochloric acid in the lungs.

Signs and Symptoms of Acute Chlorine Exposure: Signs and symptoms of acute exposure to chlorine may include tachycardia (rapid heart rate), hypertension (high blood pressure) followed by hypotension (low blood pressure), and cardiovascular collapse. Pulmonary edema and pneumonia are often seen. The eyes, nose, throat, and chest may sting or burn following exposure to chlorine. Cough with bloody sputum, a feeling of suffocation, dizziness, agitation, anxiety, nausea, and vomiting are common. Dermal exposure may result in sweating, pain, irritation, and blisters.

Emergency Life-Support Procedures: Acute exposure to chlorine may require decontamination and life support for the victims. Emergency personnel should wear protective clothing appropriate to the type and degree of contamination. Air-purifying or supplied-air respiratory equipment should also be worn, as necessary. Rescue vehicles should carry supplies such as chlorine-resistant plastic sheeting and disposable bags to assist in preventing spread of contamination.

Inhalation Exposure:
1. Move victims to fresh air. Emergency personnel should avoid self-exposure to chlorine.
2. Evaluate vital signs including pulse and respiratory rate, and note any trauma. If no pulse is detected, provide CPR. If not breathing, provide artificial respiration. If breathing is labored, administer oxygen or other respiratory support.
3. Obtain authorization and/or further instructions from the local hospital for administration of an antidote or performance of other invasive procedures.
4. Transport to a health care facility.

Dermal/Eye Exposure:
1. Remove victims from exposure. Emergency personnel should avoid self- exposure to chlorine.
2. Evaluate vital signs including pulse and respiratory rate, and note any trauma. If no pulse is detected, provide CPR. If not breathing, provide artificial respiration. If breathing is labored, administer oxygen or other respiratory support.
3. Remove contaminated clothing as soon as possible.
4. If eye exposure has occurred, eyes must be flushed with lukewarm water for at least 15 minutes.
5. Wash exposed skin areas for at least 15 minutes with soap and water.
6. Obtain authorization and/or further instructions from the local hospital for administration of an antidote or performance of other invasive procedures.
7. Transport to a health care facility.

Ingestion Exposure: No information is available. (EPA, 1998)

Physical Properties

Flash Point: data unavailable

Lower Explosive Limit (LEL): data unavailable

Upper Explosive Limit (UEL): data unavailable

Autoignition Temperature: Not flammable (USCG, 1999)

Melting Point: -150 ° F (EPA, 1998)

Vapor Pressure: 7600 mm Hg at 86 ° F (EPA, 1998)

Vapor Density (Relative to Air): 2.49 (EPA, 1998)

Specific Gravity: 1.424 at 59 ° F (USCG, 1999)

Boiling Point: -30.3 ° F at 760 mm Hg (EPA, 1998)

Molecular Weight: 70.91 (EPA, 1998)

Water Solubility: 0.7 % (NIOSH, 2016)

Ionization Potential: 11.48 eV (NIOSH, 2016)

IDLH: 10 ppm (NIOSH, 2016)

AEGLs (Acute Exposure Guideline Levels)

Exposure PeriodAEGL-1AEGL-2AEGL-3
10 minutes0.5 ppm 2.8 ppm 50 ppm
30 minutes0.5 ppm 2.8 ppm 28 ppm
60 minutes0.5 ppm 2 ppm 20 ppm
4 hours0.5 ppm 1 ppm 10 ppm
8 hours0.5 ppm 0.71 ppm 7.1 ppm

(NAC/NRC, 2017)

ERPGs (Emergency Response Planning Guidelines)

Chlorine (7782-50-5) 1 ppm star-in-circle icon indicates that odor should be detectable near ERPG-1.3 ppm 20 ppm

(AIHA, 2016)

PACs (Protective Action Criteria)

Chlorine (7782-50-5) 0.5 ppm 2 ppm 20 ppm

(DOE, 2016)

Regulatory Information

The Regulatory Information fields include information from the U.S. Environmental Protection Agency's Title III Consolidated List of Lists, the U.S. Department of Homeland Security's Chemical Facility Anti-Terrorism Standards, and the U.S. Occupational Safety and Health Administration's Process Safety Management of Highly Hazardous Chemicals Standard List (see more about these data sources).

EPA Consolidated List of Lists

Regulatory NameCAS Number/
313 Category Code
CAA 112(r)
Chlorine7782-50-5100 pounds10 pounds10 pounds3132500 pounds

(EPA List of Lists, 2015)

DHS Chemical Facility Anti-Terrorism Standards (CFATS)

Chemical of InterestCAS NumberMin ConcSTQSecurity
Min ConcSTQSecurity
Min ConcSTQSecurity
Chlorine 7782-50-51.00 %2500 poundstoxic9.77 %500 poundsWME

(DHS, 2007)

OSHA Process Safety Management (PSM) Standard List

Chemical NameCAS NumberThreshold Quantity (TQ)
Chlorine7782-50-51500 pounds

(OSHA, 2011)

Alternate Chemical Names

This section provides a listing of alternate names for this chemical, including trade names and synonyms.


Cas chlorine


Not to be confused with chloride.

This article is about the chemical element. For other uses, see Chlorine (disambiguation).

"Cl" and "Cl2" redirect here. For other uses, see CL (disambiguation) and CL2 (disambiguation).

Chemical element with atomic number 17

Chemical element, symbol Cl and atomic number 17

A glass container filled with chlorine gas
Pronunciation ​(KLOR-een, -⁠yne)
Appearancepale yellow-green gas
Standard atomic weight Ar, std(Cl)[35.446, 35.457] conventional: 35.45[1]
Atomic number(Z)17
Groupgroup 17 (halogens)
Periodperiod 3
Block p-block
Electron configuration[Ne] 3s2 3p5
Electrons per shell2, 8, 7
Phaseat STPgas
Melting point(Cl2) 171.6 K ​(−101.5 °C, ​−150.7 °F)
Boiling point(Cl2) 239.11 K ​(−34.04 °C, ​−29.27 °F)
Density(at STP)3.2 g/L
when liquid (at b.p.)1.5625 g/cm3[2]
Critical point416.9 K, 7.991 MPa
Heat of fusion(Cl2) 6.406 kJ/mol
Heat of vaporisation(Cl2) 20.41 kJ/mol
Molar heat capacity(Cl2)
33.949 J/(mol·K)
Vapour pressure
P (Pa)1 10 100 1 k 10 k 100 k
at T (K)128 139 153 170 197 239
Oxidation states−1, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)
ElectronegativityPauling scale: 3.16
Ionisation energies
  • 1st: 1251.2 kJ/mol
  • 2nd: 2298 kJ/mol
  • 3rd: 3822 kJ/mol
  • (more)
Covalent radius102±4 pm
Van der Waals radius175 pm
Color lines in a spectral range
Spectral lines of chlorine
Natural occurrenceprimordial
Crystal structure ​orthorhombic
Orthorhombic crystal structure for chlorine
Speed of sound206 m/s (gas, at 0 °C)
Thermal conductivity8.9×10−3 W/(m⋅K)
Electrical resistivity>10 Ω⋅m (at 20 °C)
Magnetic orderingdiamagnetic[3]
Molar magnetic susceptibility−40.5×10−6 cm3/mol[4]
CAS NumberCl2: 7782-50-5
Discovery and first isolationCarl Wilhelm Scheele (1774)
Recognized as an element byHumphry Davy (1808)
Category Category: Chlorine
| references

Chlorine is a chemical element with the symbolCl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the revised Pauling scale, behind only oxygen and fluorine. On several scales other than the revised Pauling scale, nitrogen's electronegativity is also listed as greater than chlorine's, such as on the Allen, Allred-Rochow, Martynov-Batsanov, Mulliken-Jaffe, Nagle, and Noorizadeh-Shakerzadeh electronegativity scales.

Chlorine played an important role in the experiments conducted by medieval alchemists, which commonly involved the heating of chloride salts like ammonium chloride (sal ammoniac) and sodium chloride (common salt), producing various chemical substances containing chlorine such as hydrogen chloride, mercury(II) chloride (corrosive sublimate), and hydrochloric acid (in the form of aqua regia). However, the nature of free chlorine gas as a separate substance was only recognised around 1630 by Jan Baptist van Helmont. Carl Wilhelm Scheele wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element. In 1809, chemists suggested that the gas might be a pure element, and this was confirmed by Sir Humphry Davy in 1810, who named it after the Ancient Greekχλωρός (khlōrós, "pale green") because of its colour.

Because of its great reactivity, all chlorine in the Earth's crust is in the form of ionicchloride compounds, which includes table salt. It is the second-most abundanthalogen (after fluorine) and twenty-first most abundant chemical element in Earth's crust. These crustal deposits are nevertheless dwarfed by the huge reserves of chloride in seawater.

Elemental chlorine is commercially produced from brine by electrolysis, predominantly in the chlor-alkali process. The high oxidising potential of elemental chlorine led to the development of commercial bleaches and disinfectants, and a reagent for many processes in the chemical industry. Chlorine is used in the manufacture of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride (PVC), many intermediates for the production of plastics, and other end products which do not contain the element. As a common disinfectant, elemental chlorine and chlorine-generating compounds are used more directly in swimming pools to keep them sanitary. Elemental chlorine at high concentration is extremely dangerous, and poisonous to most living organisms. As a chemical warfare agent, chlorine was first used in World War I as a poison gas weapon.

In the form of chloride ions, chlorine is necessary to all known species of life. Other types of chlorine compounds are rare in living organisms, and artificially produced chlorinated organics range from inert to toxic. In the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Small quantities of elemental chlorine are generated by oxidation of chloride to hypochlorite in neutrophils as part of an immune system response against bacteria.


The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 BC and brine as early as 6000 BC.[5] Its importance in food was very well known in classical antiquity and was sometimes used as payment for services for Roman generals and military tribunes.

Early discoveries

Around 900, the authors of the Arabic writings attributed to Jabir ibn Hayyan (Latin: Geber) and the Persian physician and alchemist Abu Bakr al-Razi (c. 865–925, Latin: Rhazes) were experimenting with sal ammoniac (ammonium chloride), which when it was distilled together with vitriol (hydrated sulfates of various metals) produced hydrogen chloride.[6] However, it appears that in these early experiments with chloride salts, the gaseous products were discarded, and hydrogen chloride may have been produced many times before it was discovered that it can be put to chemical use.[7] One of the first such uses was the synthesis of mercury(II) chloride (corrosive sublimate), whose production from the heating of mercury either with alum and ammonium chloride or with vitriol and sodium chloride was first described in the De aluminibus et salibus ("On Alums and Salts", an eleventh- or twelfth century Arabic text falsely attributed to Abu Bakr al-Razi and translated into Latin in the second half of the twelfth century by Gerard of Cremona, 1144–1187).[8] Another important development was the discovery by pseudo-Geber (in the De inventione veritatis, "On the Discovery of Truth", after c. 1300) that by adding ammonium chloride to nitric acid, a strong solvent capable of dissolving gold (i.e., aqua regia) could be produced.[9] Although aqua regia is an unstable mixture that continually gives off fumes containing free chlorine gas, this chlorine gas appears to have been ignored until c. 1630, when its nature as a separate gaseous substance was recognised by the Flemish chemist and physician Jan Baptist van Helmont.[10][note 1]


The element was first studied in detail in 1774 by Swedish chemist Carl Wilhelm Scheele, and he is credited with the discovery.[11][12] Scheele produced chlorine by reacting MnO2 (as the mineral pyrolusite) with HCl:[10]

4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2

Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow-green color, and the smell similar to aqua regia.[13] He called it "dephlogisticated muriatic acid air" since it is a gas (then called "airs") and it came from hydrochloric acid (then known as "muriatic acid").[12] He failed to establish chlorine as an element.[12]

Common chemical theory at that time held that an acid is a compound that contains oxygen (remnants of this survive in the German and Dutch names of oxygen: sauerstoff or zuurstof, both translating into English as acid substance), so a number of chemists, including Claude Berthollet, suggested that Scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum.[14][15]

In 1809, Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to decompose dephlogisticated muriatic acid air by reacting it with charcoal to release the free element muriaticum (and carbon dioxide).[12] They did not succeed and published a report in which they considered the possibility that dephlogisticated muriatic acid air is an element, but were not convinced.[16]

In 1810, Sir Humphry Davy tried the same experiment again, and concluded that the substance was an element, and not a compound.[12] He announced his results to the Royal Society on 15 November that year.[10] At that time, he named this new element "chlorine", from the Greek word χλωρος (chlōros, "green-yellow"), in reference to its color.[17] The name "halogen", meaning "salt producer", was originally used for chlorine in 1811 by Johann Salomo Christoph Schweigger.[18] This term was later used as a generic term to describe all the elements in the chlorine family (fluorine, bromine, iodine), after a suggestion by Jöns Jakob Berzelius in 1826.[19][20] In 1823, Michael Faraday liquefied chlorine for the first time,[21][22][23] and demonstrated that what was then known as "solid chlorine" had a structure of chlorine hydrate (Cl2·H2O).[10]

Later uses

Chlorine gas was first used by French chemist Claude Berthollet to bleach textiles in 1785.[24][25] Modern bleaches resulted from further work by Berthollet, who first produced sodium hypochlorite in 1789 in his laboratory in the town of Javel (now part of Paris, France), by passing chlorine gas through a solution of sodium carbonate. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of sodium hypochlorite. This process was not very efficient, and alternative production methods were sought. Scottish chemist and industrialist Charles Tennant first produced a solution of calcium hypochlorite ("chlorinated lime"), then solid calcium hypochlorite (bleaching powder).[24] These compounds produced low levels of elemental chlorine and could be more efficiently transported than sodium hypochlorite, which remained as dilute solutions because when purified to eliminate water, it became a dangerously powerful and unstable oxidizer. Near the end of the nineteenth century, E. S. Smith patented a method of sodium hypochlorite production involving electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite.[26] This is known as the chloralkali process, first introduced on an industrial scale in 1892, and now the source of most elemental chlorine and sodium hydroxide.[27] In 1884 Chemischen Fabrik Griesheim of Germany developed another chloralkali process which entered commercial production in 1888.[28]

Elemental chlorine solutions dissolved in chemically basic water (sodium and calcium hypochlorite) were first used as anti-putrefaction agents and disinfectants in the 1820s, in France, long before the establishment of the germ theory of disease. This practice was pioneered by Antoine-Germain Labarraque, who adapted Berthollet's "Javel water" bleach and other chlorine preparations (for a more complete history, see below).[29] Elemental chlorine has since served a continuous function in topical antisepsis (wound irrigation solutions and the like) and public sanitation, particularly in swimming and drinking water.[13]

Chlorine gas was first used as a weapon on April 22, 1915, at Ypres by the German Army.[30][31] The effect on the allies was devastating because the existing gas masks were difficult to deploy and had not been broadly distributed.[32][33]


Chlorine, liquefied under a pressure of 7.4 bar at room temperature, displayed in a quartz ampule embedded in acrylic glass.
Solid chlorine at −150 °C

Chlorine is the second halogen, being a nonmetal in group 17 of the periodic table. Its properties are thus similar to fluorine, bromine, and iodine, and are largely intermediate between those of the first two. Chlorine has the electron configuration [Ne]3s23p5, with the seven electrons in the third and outermost shell acting as its valence electrons. Like all halogens, it is thus one electron short of a full octet, and is hence a strong oxidising agent, reacting with many elements in order to complete its outer shell.[34] Corresponding to periodic trends, it is intermediate in electronegativity between fluorine and bromine (F: 3.98, Cl: 3.16, Br: 2.96, I: 2.66), and is less reactive than fluorine and more reactive than bromine. It is also a weaker oxidising agent than fluorine, but a stronger one than bromine. Conversely, the chloride ion is a weaker reducing agent than bromide, but a stronger one than fluoride.[34] It is intermediate in atomic radius between fluorine and bromine, and this leads to many of its atomic properties similarly continuing the trend from iodine to bromine upward, such as first ionisation energy, electron affinity, enthalpy of dissociation of the X2 molecule (X = Cl, Br, I), ionic radius, and X–X bond length. (Fluorine is anomalous due to its small size.)[34]

All four stable halogens experience intermolecular van der Waals forces of attraction, and their strength increases together with the number of electrons among all homonuclear diatomic halogen molecules. Thus, the melting and boiling points of chlorine are intermediate between those of fluorine and bromine: chlorine melts at −101.0 °C and boils at −34.0 °C. As a result of the increasing molecular weight of the halogens down the group, the density and heats of fusion and vaporisation of chlorine are again intermediate between those of bromine and fluorine, although all their heats of vaporisation are fairly low (leading to high volatility) thanks to their diatomic molecular structure.[34] The halogens darken in colour as the group is descended: thus, while fluorine is a pale yellow gas, chlorine is distinctly yellow-green. This trend occurs because the wavelengths of visible light absorbed by the halogens increase down the group.[34] Specifically, the colour of a halogen, such as chlorine, results from the electron transition between the highest occupied antibonding πg molecular orbital and the lowest vacant antibonding σu molecular orbital.[35] The colour fades at low temperatures, so that solid chlorine at −195 °C is almost colourless.[34]

Like solid bromine and iodine, solid chlorine crystallises in the orthorhombic crystal system, in a layered lattice of Cl2 molecules. The Cl–Cl distance is 198 pm (close to the gaseous Cl–Cl distance of 199 pm) and the Cl···Cl distance between molecules is 332 pm within a layer and 382 pm between layers (compare the van der Waals radius of chlorine, 180 pm). This structure means that chlorine is a very poor conductor of electricity, and indeed its conductivity is so low as to be practically unmeasurable.[34]


Main article: Isotopes of chlorine

Chlorine has two stable isotopes, 35Cl and 37Cl. These are its only two natural isotopes occurring in quantity, with 35Cl making up 76% of natural chlorine and 37Cl making up the remaining 24%. Both are synthesised in stars in the oxygen-burning and silicon-burning processes.[36] Both have nuclear spin 3/2+ and thus may be used for nuclear magnetic resonance, although the spin magnitude being greater than 1/2 results in non-spherical nuclear charge distribution and thus resonance broadening as a result of a nonzero nuclear quadrupole moment and resultant quadrupolar relaxation. The other chlorine isotopes are all radioactive, with half-lives too short to occur in nature primordially. Of these, the most commonly used in the laboratory are 36Cl (t1/2 = 3.0×105 y) and 38Cl (t1/2 = 37.2 min), which may be produced from the neutron activation of natural chlorine.[34]

The most stable chlorine radioisotope is 36Cl. The primary decay mode of isotopes lighter than 35Cl is electron capture to isotopes of sulfur; that of isotopes heavier than 37Cl is beta decay to isotopes of argon; and 36Cl may decay by either mode to stable 36S or 36Ar.[37]36Cl occurs in trace quantities in nature as a cosmogenic nuclide in a ratio of about (7–10) × 10−13 to 1 with stable chlorine isotopes: it is produced in the atmosphere by spallation of 36Ar by interactions with cosmic rayprotons. In the top meter of the lithosphere, 36Cl is generated primarily by thermal neutron activation of 35Cl and spallation of 39K and 40Ca. In the subsurface environment, muon capture by 40Ca becomes more important as a way to generate 36Cl.[38][39]

Chemistry and compounds

F 159 574 645 582 456
Cl 243 428 444 427 327
Br 193 363 368 360 272
I 151 294 272 285 239

Chlorine is intermediate in reactivity between fluorine and bromine, and is one of the most reactive elements. Chlorine is a weaker oxidising agent than fluorine but a stronger one than bromine or iodine. This can be seen from the standard electrode potentials of the X2/X couples (F, +2.866 V; Cl, +1.395 V; Br, +1.087 V; I, +0.615 V; At, approximately +0.3 V). However, this trend is not shown in the bond energies because fluorine is singular due to its small size, low polarisability, and inability to show hypervalence. As another difference, chlorine has a significant chemistry in positive oxidation states while fluorine does not. Chlorination often leads to higher oxidation states than bromination or iodination but lower oxidation states than fluorination. Chlorine tends to react with compounds including M–M, M–H, or M–C bonds to form M–Cl bonds.[35]

Given that E°(1/2O2/H2O) = +1.229 V, which is less than +1.395 V, it would be expected that chlorine should be able to oxidise water to oxygen and hydrochloric acid. However, the kinetics of this reaction are unfavorable, and there is also a bubble overpotential effect to consider, so that electrolysis of aqueous chloride solutions evolves chlorine gas and not oxygen gas, a fact that is very useful for the industrial production of chlorine.[40]

Hydrogen chloride

Structure of solid deuterium chloride, with D···Cl hydrogen bonds

The simplest chlorine compound is hydrogen chloride, HCl, a major chemical in industry as well as in the laboratory, both as a gas and dissolved in water as hydrochloric acid. It is often produced by burning hydrogen gas in chlorine gas, or as a byproduct of chlorinating hydrocarbons. Another approach is to treat sodium chloride with concentrated sulfuric acid to produce hydrochloric acid, also known as the "salt-cake" process:[41]

NaCl + H2SO4150 °C⟶  NaHSO4 + HCl
NaCl + NaHSO4540–600 °C⟶  Na2SO4 + HCl

In the laboratory, hydrogen chloride gas may be made by drying the acid with concentrated sulfuric acid. Deuterium chloride, DCl, may be produced by reacting benzoyl chloride with heavy water (D2O).[41]

At room temperature, hydrogen chloride is a colourless gas, like all the hydrogen halides apart from hydrogen fluoride, since hydrogen cannot form strong hydrogen bonds to the larger electronegative chlorine atom; however, weak hydrogen bonding is present in solid crystalline hydrogen chloride at low temperatures, similar to the hydrogen fluoride structure, before disorder begins to prevail as the temperature is raised.[41] Hydrochloric acid is a strong acid (pKa = −7) because the hydrogen bonds to chlorine are too weak to inhibit dissociation. The HCl/H2O system has many hydrates HCl·nH2O for n = 1, 2, 3, 4, and 6. Beyond a 1:1 mixture of HCl and H2O, the system separates completely into two separate liquid phases. Hydrochloric acid forms an azeotrope with boiling point 108.58 °C at 20.22 g HCl per 100 g solution; thus hydrochloric acid cannot be concentrated beyond this point by distillation.[42]

Unlike hydrogen fluoride, anhydrous liquid hydrogen chloride is difficult to work with as a solvent, because its boiling point is low, it has a small liquid range, its dielectric constant is low and it does not dissociate appreciably into H2Cl+ and HCl
2 ions – the latter, in any case, are much less stable than the bifluoride ions (HF
2) due to the very weak hydrogen bonding between hydrogen and chlorine, though its salts with very large and weakly polarising cations such as Cs+ and NR+
4 (R = Me, Et, Bun) may still be isolated. Anhydrous hydrogen chloride is a poor solvent, only able to dissolve small molecular compounds such as nitrosyl chloride and phenol, or salts with very low lattice energies such as tetraalkylammonium halides. It readily protonates electrophiles containing lone-pairs or π bonds. Solvolysis, ligand replacement reactions, and oxidations are well-characterised in hydrogen chloride solution:[43]

Ph3SnCl + HCl ⟶ Ph2SnCl2 + PhH (solvolysis)
Ph3COH + 3 HCl ⟶ Ph
2 + H3O+Cl (solvolysis)
2 + BCl3 ⟶ Me
4 + HCl (ligand replacement)
PCl3 + Cl2 + HCl ⟶ PCl+
2 (oxidation)

Other binary chlorides

Nearly all elements in the periodic table form binary chlorides. The exceptions are decidedly in the minority and stem in each case from one of three causes: extreme inertness and reluctance to participate in chemical reactions (the noble gases, with the exception of xenon in the highly unstable XeCl2 and XeCl4); extreme nuclear instability hampering chemical investigation before decay and transmutation (many of the heaviest elements beyond bismuth); and having an electronegativity higher than chlorine's (oxygen and fluorine) so that the resultant binary compounds are formally not chlorides but rather oxides or fluorides of chlorine.[44] Even though nitrogen in NCl3 is bearing a negative charge, the compound is usually called nitrogen trichloride.

Chlorination of metals with Cl2 usually leads to a higher oxidation state than bromination with Br2 when multiple oxidation states are available, such as in MoCl5 and MoBr3. Chlorides can be made by reaction of an element or its oxide, hydroxide, or carbonate with hydrochloric acid, and then dehydrated by mildly high temperatures combined with either low pressure or anhydrous hydrogen chloride gas. These methods work best when the chloride product is stable to hydrolysis; otherwise, the possibilities include high-temperature oxidative chlorination of the element with chlorine or hydrogen chloride, high-temperature chlorination of a metal oxide or other halide by chlorine, a volatile metal chloride, carbon tetrachloride, or an organic chloride. For instance, zirconium dioxide reacts with chlorine at standard conditions to produce zirconium tetrachloride, and uranium trioxide reacts with hexachloropropene when heated under reflux to give uranium tetrachloride. The second example also involves a reduction in oxidation state, which can also be achieved by reducing a higher chloride using hydrogen or a metal as a reducing agent. This may also be achieved by thermal decomposition or disproportionation as follows:[44]

EuCl3 + 1/2 H2 ⟶ EuCl2 + HCl
ReCl5at "bp"⟶  ReCl3 + Cl2
AuCl3160 °C⟶  AuCl + Cl2

Most of the chlorides the metals in groups 1, 2, and 3, along with the lanthanides and actinides in the +2 and +3 oxidation states, are mostly ionic, while nonmetals tend to form covalent molecular chlorides, as do metals in high oxidation states from +3 and above. Silver chloride is very insoluble in water and is thus often used as a qualitative test for chlorine.[44]

Polychlorine compounds

Although dichlorine is a strong oxidising agent with a high first ionisation energy, it may be oxidised under extreme conditions to form the Cl+
2 cation. This is very unstable and has only been characterised by its electronic band spectrum when produced in a low-pressure discharge tube. The yellow Cl+
3 cation is more stable and may be produced as follows:[45]

Cl2 + ClF + AsF5−78 °C⟶ Cl+

This reaction is conducted in the oxidising solvent arsenic pentafluoride. The trichloride anion, Cl
3, has also been characterised; it is analogous to triiodide.[46]

Chlorine fluorides

The three fluorides of chlorine form a subset of the interhalogen compounds, all of which are diamagnetic.[46] Some cationic and anionic derivatives are known, such as ClF
2, ClF
4, ClF+
2, and Cl2F+.[47] Some pseudohalides of chlorine are also known, such as cyanogen chloride (ClCN, linear), chlorine cyanate (ClNCO), chlorine thiocyanate (ClSCN, unlike its oxygen counterpart), and chlorine azide (ClN3).[46]

Chlorine monofluoride (ClF) is extremely thermally stable, and is sold commercially in 500-gram steel lecture bottles. It is a colourless gas that melts at −155.6 °C and boils at −100.1 °C. It may be produced by the direction of its elements at 225 °C, though it must then be separated and purified from chlorine trifluoride and its reactants. Its properties are mostly intermediate between those of chlorine and fluorine. It will react with many metals and nonmetals from room temperature and above, fluorinating them and liberating chlorine. It will also act as a chlorofluorinating agent, adding chlorine and fluorine across a multiple bond or by oxidation: for example, it will attack carbon monoxide to form carbonyl chlorofluoride, COFCl. It will react analogously with hexafluoroacetone, (CF3)2CO, with a potassium fluoride catalyst to produce heptafluoroisopropyl hypochlorite, (CF3)2CFOCl; with nitriles RCN to produce RCF2NCl2; and with the sulfur oxides SO2 and SO3 to produce ClSO2F and ClOSO2F respectively. It will also react exothermically and violently with compounds containing –OH and –NH groups, such as water:[46]

H2O + 2 ClF ⟶ 2 HF + Cl2O

Chlorine trifluoride (ClF3) is a volatile colourless molecular liquid which melts at −76.3 °C and boils at 11.8 °C. It may be formed by directly fluorinating gaseous chlorine or chlorine monofluoride at 200–300 °C. It is one of the most reactive known chemical compounds, reacting with many substances which in ordinary circumstances would be considered chemically inert, such as asbestos, concrete, and sand. It explodes on contact with water and most organic substances. The list of elements it sets on fire is diverse, containing hydrogen, potassium, phosphorus, arsenic, antimony, sulfur, selenium, tellurium, bromine, iodine, and powdered molybdenum, tungsten, rhodium, iridium, and iron. An impermeable fluoride layer is formed by sodium, magnesium, aluminium, zinc, tin, and silver, which may be removed by heating. When heated, even such noble metals as palladium, platinum, and gold are attacked and even the noble gasesxenon and radon do not escape fluorination. Nickel containers are usually used due to that metal's great resistance to attack by chlorine trifluoride, stemming from the formation of an unreactive nickel fluoride layer. Its reaction with hydrazine to form hydrogen fluoride, nitrogen, and chlorine gases was used in experimental rocket motors, but has problems largely stemming from its extreme hypergolicity resulting in ignition without any measurable delay. Today, it is mostly used in nuclear fuel processing, to oxidise uranium to uranium hexafluoride for its enriching and to separate it from plutonium. It can act as a fluoride ion donor or acceptor (Lewis base or acid), although it does not dissociate appreciably into ClF+
2 and ClF
4 ions.[48]

Chlorine pentafluoride (ClF5) is made on a large scale by direct fluorination of chlorine with excess fluorine gas at 350 °C and 250 atm, and on a small scale by reacting metal chlorides with fluorine gas at 100–300 °C. It melts at −103 °C and boils at −13.1 °C. It is a very strong fluorinating agent, although it is still not as effective as chlorine trifluoride. Only a few specific stoichiometric reactions have been characterised. Arsenic pentafluoride and antimony pentafluoride form ionic adducts of the form [ClF4]+[MF6] (M = As, Sb) and water reacts vigorously as follows:[49]

2 H2O + ClF5 ⟶ 4 HF + FClO2

The product, chloryl fluoride, is one of the five known chlorine oxide fluorides. These range from the thermally unstable FClO to the chemically unreactive perchloryl fluoride (FClO3), the other three being FClO2, F3ClO, and F3ClO2. All five behave similarly to the chlorine fluorides, both structurally and chemically, and may act as Lewis acids or bases by gaining or losing fluoride ions respectively or as very strong oxidising and fluorinating agents.[50]

Chlorine oxides

Yellow chlorine dioxide(ClO2) gas above a yellow chlorine dioxide solution. Solutions of pure chlorine dioxide are dark green: this solution is yellow due to impurities produced from its photodecomposition.

The chlorine oxides are well-studied in spite of their instability (all of them are endothermic compounds). They are important because they are produced when chlorofluorocarbons undergo photolysis in the upper atmosphere and cause the destruction of the ozone layer. None of them can be made from directly reacting the elements.[51]

Dichlorine monoxide (Cl2O) is a brownish-yellow gas (red-brown when solid or liquid) which may be obtained by reacting chlorine gas with yellow mercury(II) oxide. It is very soluble in water, in which it is in equilibrium with hypochlorous acid (HOCl), of which it is the anhydride. It is thus an effective bleach and is mostly used to make hypochlorites. It explodes on heating or sparking or in the presence of ammonia gas.[51]

Chlorine dioxide (ClO2) was the first chlorine oxide to be discovered in 1811 by Humphry Davy. It is a yellow paramagnetic gas (deep-red as a solid or liquid), as expected from its having an odd number of electrons: it is stable towards dimerisation due to the delocalisation of the unpaired electron. It explodes above −40 °C as a liquid and under pressure as a gas and therefore must be made at low concentrations for wood-pulp bleaching and water treatment. It is usually prepared by reducing a chlorate as follows:[51]

3 + Cl + 2 H+ ⟶ ClO2 + 1/2 Cl2 + H2O

Its production is thus intimately linked to the redox reactions of the chlorine oxoacids. It is a strong oxidising agent, reacting with sulfur, phosphorus, phosphorus halides, and potassium borohydride. It dissolves exothermically in water to form dark-green solutions that very slowly decompose in the dark. Crystalline clathrate hydrates ClO2·nH2O (n ≈ 6–10) separate out at low temperatures. However, in the presence of light, these solutions rapidly photodecompose to form a mixture of chloric and hydrochloric acids. Photolysis of individual ClO2 molecules result in the radicals ClO and ClOO, while at room temperature mostly chlorine, oxygen, and some ClO3 and Cl2O6 are produced. Cl2O3 is also produced when photolysing the solid at −78 °C: it is a dark brown solid that explodes below 0 °C. The ClO radical leads to the depletion of atmospheric ozone and is thus environmentally important as follows:[51]

Cl• + O3 ⟶ ClO• + O2
ClO• + O• ⟶ Cl• + O2

Chlorine perchlorate (ClOClO3) is a pale yellow liquid that is less stable than ClO2 and decomposes at room temperature to form chlorine, oxygen, and dichlorine hexoxide (Cl2O6).[51] Chlorine perchlorate may also be considered a chlorine derivative of perchloric acid (HOClO3), similar to the thermally unstable chlorine derivatives of other oxoacids: examples include chlorine nitrate (ClONO2, vigorously reactive and explosive), and chlorine fluorosulfate (ClOSO2F, more stable but still moisture-sensitive and highly reactive).[52] Dichlorine hexoxide is a dark-red liquid that freezes to form a solid which turns yellow at −180 °C: it is usually made by reaction of chlorine dioxide with oxygen. Despite attempts to rationalise it as the dimer of ClO3, it reacts more as though it were chloryl perchlorate, [ClO2]+[ClO4], which has been confirmed to be the correct structure of the solid. It hydrolyses in water to give a mixture of chloric and perchloric acids: the analogous reaction with anhydrous hydrogen fluoride does not proceed to completion.[51]

Dichlorine heptoxide (Cl2O7) is the anhydride of perchloric acid (HClO4) and can readily be obtained from it by dehydrating it with phosphoric acid at −10 °C and then distilling the product at −35 °C and 1 mmHg. It is a shock-sensitive, colourless oily liquid. It is the least reactive of the chlorine oxides, being the only one to not set organic materials on fire at room temperature. It may be dissolved in water to regenerate perchloric acid or in aqueous alkalis to regenerate perchlorates. However, it thermally decomposes explosively by breaking one of the central Cl–O bonds, producing the radicals ClO3 and ClO4 which immediately decompose to the elements through intermediate oxides.[51]

Chlorine oxoacids and oxyanions

E°(couple)a(H+) = 1
E°(couple)a(OH) = 1

Chlorine forms four oxoacids: hypochlorous acid (HOCl), chlorous acid (HOClO), chloric acid (HOClO2), and perchloric acid (HOClO3). As can be seen from the redox potentials given in the adjacent table, chlorine is much more stable towards disproportionation in acidic solutions than in alkaline solutions:[40]

Cl2 + H2O⇌ HOCl + H+ + ClKac = 4.2 × 10−4 mol2 l−2
Cl2 + 2 OH⇌ OCl + H2O + ClKalk = 7.5 × 1015 mol−1 l

The hypochlorite ions also disproportionate further to produce chloride and chlorate (3 ClO ⇌ 2 Cl + ClO
3) but this reaction is quite slow at temperatures below 70 °C in spite of the very favourable equilibrium constant of 1027. The chlorate ions may themselves disproportionate to form chloride and perchlorate (4 ClO
3 ⇌ Cl + 3 ClO
4) but this is still very slow even at 100 °C despite the very favourable equilibrium constant of 1020. The rates of reaction for the chlorine oxyanions increases as the oxidation state of chlorine decreases. The strengths of the chlorine oxyacids increase very quickly as the oxidation state of chlorine increases due to the increasing delocalisation of charge over more and more oxygen atoms in their conjugate bases.[40]

Most of the chlorine oxoacids may be produced by exploiting these disproportionation reactions. Hypochlorous acid (HOCl) is highly reactive and quite unstable; its salts are mostly used for their bleaching and sterilising abilities. They are very strong oxidising agents, transferring an oxygen atom to most inorganic species. Chlorous acid (HOClO) is even more unstable and cannot be isolated or concentrated without decomposition: it is known from the decomposition of aqueous chlorine dioxide. However, sodium chlorite is a stable salt and is useful for bleaching and stripping textiles, as an oxidising agent, and as a source of chlorine dioxide. Chloric acid (HOClO2) is a strong acid that is quite stable in cold water up to 30% concentration, but on warming gives chlorine and chlorine dioxide. Evaporation under reduced pressure allows it to be concentrated further to about 40%, but then it decomposes to perchloric acid, chlorine, oxygen, water, and chlorine dioxide. Its most important salt is sodium chlorate, mostly used to make chlorine dioxide to bleach paper pulp. The decomposition of chlorate to chloride and oxygen is a common way to produce oxygen in the laboratory on a small scale. Chloride and chlorate may comproportionate to form chlorine as follows:[53]

3 + 5 Cl + 6 H+ ⟶ 3 Cl2 + 3 H2O

Perchlorates and perchloric acid (HOClO3) are the most stable oxo-compounds of chlorine, in keeping with the fact that chlorine compounds are most stable when the chlorine atom is in its lowest (−1) or highest (+7) possible oxidation states. Perchloric acid and aqueous perchlorates are vigorous and sometimes violent oxidising agents when heated, in stark contrast to their mostly inactive nature at room temperature due to the high activation energies for these reactions for kinetic reasons. Perchlorates are made by electrolytically oxidising sodium chlorate, and perchloric acid is made by reacting anhydrous sodium perchlorate or barium perchlorate with concentrated hydrochloric acid, filtering away the chloride precipitated and distilling the filtrate to concentrate it. Anhydrous perchloric acid is a colourless mobile liquid that is sensitive to shock that explodes on contact with most organic compounds, sets hydrogen iodide and thionyl chloride on fire and even oxidises silver and gold. Although it is a weak ligand, weaker than water, a few compounds involving coordinated ClO
4 are known.[53]

Organochlorine compounds

Main article: Organochlorine compound

Suggested mechanism for the chlorination of a carboxylic acid by phosphorus pentachloride to form an acyl chloride

Like the other carbon–halogen bonds, the C–Cl bond is a common functional group that forms part of core organic chemistry. Formally, compounds with this functional group may be considered organic derivatives of the chloride anion. Due to the difference of electronegativity between chlorine (3.16) and carbon (2.55), the carbon in a C–Cl bond is electron-deficient and thus electrophilic. Chlorination modifies the physical properties of hydrocarbons in several ways: chlorocarbons are typically denser than water due to the higher atomic weight of chlorine versus hydrogen, and aliphatic organochlorides are alkylating agents because chloride is a leaving group.[54]

Alkanes and aryl alkanes may be chlorinated under free-radical conditions, with UV light. However, the extent of chlorination is difficult to control: the reaction is not regioselective and often results in a mixture of various isomers with different degrees of chlorination, though this may be permissible if the products are easily separated. Aryl chlorides may be prepared by the Friedel-Crafts halogenation, using chlorine and a Lewis acid catalyst.[54] The haloform reaction, using chlorine and sodium hydroxide, is also able to generate alkyl halides from methyl ketones, and related compounds. Chlorine adds to the multiple bonds on alkenes and alkynes as well, giving di- or tetra-chloro compounds. However, due to the expense and reactivity of chlorine, organochlorine compounds are more commonly produced by using hydrogen chloride, or with chlorinating agents such as phosphorus pentachloride (PCl5) or thionyl chloride (SOCl2). The last is very convenient in the laboratory because all side products are gaseous and do not have to be distilled out.[54]

Many organochlorine compounds have been isolated from natural sources ranging from bacteria to humans.[55][56] Chlorinated organic compounds are found in nearly every class of biomolecules including alkaloids, terpenes, amino acids, flavonoids, steroids, and fatty acids.[55][57] Organochlorides, including dioxins, are produced in the high temperature environment of forest fires, and dioxins have been found in the preserved ashes of lightning-ignited fires that predate synthetic dioxins.[58] In addition, a variety of simple chlorinated hydrocarbons including dichloromethane, chloroform, and carbon tetrachloride have been isolated from marine algae.[59] A majority of the chloromethane in the environment is produced naturally by biological decomposition, forest fires, and volcanoes.[60]

Some types of organochlorides, though not all, have significant toxicity to plants or animals, including humans. Dioxins, produced when organic matter is burned in the presence of chlorine, and some insecticides, such as DDT, are persistent organic pollutants which pose dangers when they are released into the environment. For example, DDT, which was widely used to control insects in the mid 20th century, also accumulates in food chains, and causes reproductive problems (e.g., eggshell thinning) in certain bird species.[61] Due to the ready homolytic fission of the C–Cl bond to create chlorine radicals in the upper atmosphere, chlorofluorocarbons have been phased out due to the harm they do to the ozone layer.[51]

Occurrence and production

Main articles: Chlorine production and Chloralkali process

Chlorine is too reactive to occur as the free element in nature but is very abundant in the form of its chloride salts. It is the twenty-first most abundant element in Earth's crust and makes up 126 parts per million of it, through the large deposits of chloride minerals, especially sodium chloride, that have been evaporated from water bodies. All of these pale in comparison to the reserves of chloride ions in seawater: smaller amounts at higher concentrations occur in some inland seas and underground brine wells, such as the Great Salt Lake in Utah and the Dead Sea in Israel.[62]

Small batches of chlorine gas are prepared in the laboratory by combining hydrochloric acid and manganese dioxide, but the need rarely arises due to its ready availability. In industry, elemental chlorine is usually produced by the electrolysis of sodium chloride dissolved in water. This method, the chloralkali process industrialized in 1892, now provides most industrial chlorine gas.[27] Along with chlorine, the method yields hydrogen gas and sodium hydroxide, which is the most valuable product. The process proceeds according to the following chemical equation:[63]

2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH

The electrolysis of chloride solutions all proceed according to the following equations:

Cathode: 2 H2O + 2 e → H2 + 2 OH
Anode: 2 Cl → Cl2 + 2 e

In diaphragm cell electrolysis, an asbestos (or polymer-fiber) diaphragm separates a cathode and an anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathode.[64] The salt solution (brine) is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the causticalkali is produced and the brine is partially depleted. Diaphragm methods produce dilute and slightly impure alkali, but they are not burdened with the problem of mercury disposal and they are more energy efficient.[27]

Membrane cell electrolysis employs permeable membrane as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration. This method also produces very pure sodium (or potassium) hydroxide but has the disadvantage of requiring very pure brine at high concentrations.[65]

Membrane cell process for chloralkali production

In the Deacon process, hydrogen chloride recovered from the production of organochlorine compounds is recovered as chlorine. The process relies on oxidation using oxygen:

4 HCl + O2 → 2 Cl2 + 2 H2O

The reaction requires a catalyst. As introduced by Deacon, early catalysts were based on copper. Commercial processes, such as the Mitsui MT-Chlorine Process, have switched to chromium and ruthenium-based catalysts.[66] The chlorine produced is available in cylinders from sizes ranging from 450 g to 70 kg, as well as drums (865 kg), tank wagons (15 tonnes on roads; 27–90 tonnes by rail), and barges (600–1200 tonnes).[67]


Sodium chloride is the most common chlorine compound, and is the main source of chlorine for the demand by the chemical industry. About 15000 chlorine-containing compounds are commercially traded, including such diverse compounds as chlorinated methane, ethanes, vinyl chloride, polyvinyl chloride (PVC), aluminium trichloride for catalysis, the chlorides of magnesium, titanium, zirconium, and hafnium which are the precursors for producing the pure form of those elements.[13]

Quantitatively, of all elemental chlorine produced, about 63% is used in the manufacture of organic compounds, and 18% in the manufacture of inorganic chlorine compounds. About 15,000 chlorine compounds are used commercially. The remaining 19% of chlorine produced is used for bleaches and disinfection products.[67] The most significant of organic compounds in terms of production volume are 1,2-dichloroethane and vinyl chloride, intermediates in the production of PVC. Other particularly important organochlorines are methyl chloride, methylene chloride, chloroform, vinylidene chloride, trichloroethylene, perchloroethylene, allyl chloride, epichlorohydrin, chlorobenzene, dichlorobenzenes, and trichlorobenzenes. The major inorganic compounds include HCl, Cl2O, HOCl, NaClO3, chlorinated isocyanurates, AlCl3, SiCl4, SnCl4, PCl3, PCl5, POCl3, AsCl3, SbCl3, SbCl5, BiCl3, S2Cl2, SCl2, SOCl2, ClF3, ICl, ICl3, TiCl3, TiCl4, MoCl5, FeCl3, ZnCl2, and so on.[67]

Sanitation, disinfection, and antisepsis

Main articles: Water chlorination and Bleach

Combating putrefaction

In France (as elsewhere), animal intestines were processed to make musical instrument strings, Goldbeater's skin and other products. This was done in "gut factories" (boyauderies), and it was an odiferous and unhealthy process. In or about 1820, the Société d'encouragement pour l'industrie nationale offered a prize for the discovery of a method, chemical or mechanical, for separating the peritoneal membrane of animal intestines without putrefaction.[70][71] The prize was won by Antoine-Germain Labarraque, a 44-year-old French chemist and pharmacist who had discovered that Berthollet's chlorinated bleaching solutions ("Eau de Javel") not only destroyed the smell of putrefaction of animal tissue decomposition, but also actually retarded the decomposition.[71][29]

Labarraque's research resulted in the use of chlorides and hypochlorites of lime (calcium hypochlorite) and of sodium (sodium hypochlorite) in the boyauderies. The same chemicals were found to be useful in the routine disinfection and deodorization of latrines, sewers, markets, abattoirs, anatomical theatres, and morgues.[72] They were successful in hospitals, lazarets, prisons, infirmaries (both on land and at sea), magnaneries, stables, cattle-sheds, etc.; and they were beneficial during exhumations,[73]embalming, outbreaks of epidemic disease, fever, and blackleg in cattle.[70]


Labarraque's chlorinated lime and soda solutions have been advocated since 1828 to prevent infection (called "contagious infection", presumed to be transmitted by "miasmas"), and to treat putrefaction of existing wounds, including septic wounds.[74] In his 1828 work, Labarraque recommended that doctors breathe chlorine, wash their hands in chlorinated lime, and even sprinkle chlorinated lime about the patients' beds in cases of "contagious infection". In 1828, the contagion of infections was well known, even though the agency of the microbe was not discovered until more than half a century later.

During the Paris cholera outbreak of 1832, large quantities of so-called chloride of lime were used to disinfect the capital. This was not simply modern calcium chloride, but chlorine gas dissolved in lime-water (dilute calcium hydroxide) to form calcium hypochlorite (chlorinated lime). Labarraque's discovery helped to remove the terrible stench of decay from hospitals and dissecting rooms, and by doing so, effectively deodorised the Latin Quarter of Paris.[75] These "putrid miasmas" were thought by many to cause the spread of "contagion" and "infection" – both words used before the germ theory of infection. Chloride of lime was used for destroying odors and "putrid matter". One source claims chloride of lime was used by Dr. John Snow to disinfect water from the cholera-contaminated well that was feeding the Broad Street pump in 1854 London,[76] though three other reputable sources that describe that famous cholera epidemic do not mention the incident.[77][78][79] One reference makes it clear that chloride of lime was used to disinfect the offal and filth in the streets surrounding the Broad Street pump – a common practice in mid-nineteenth century England.[77]: 296 

Semmelweis and experiments with antisepsis

Perhaps the most famous application of Labarraque's chlorine and chemical base solutions was in 1847, when Ignaz Semmelweis used chlorine-water (chlorine dissolved in pure water, which was cheaper than chlorinated lime solutions) to disinfect the hands of Austrian doctors, which Semmelweis noticed still carried the stench of decomposition from the dissection rooms to the patient examination rooms. Long before the germ theory of disease, Semmelweis theorized that "cadaveric particles" were transmitting decay from fresh medical cadavers to living patients, and he used the well-known "Labarraque's solutions" as the only known method to remove the smell of decay and tissue decomposition (which he found that soap did not). The solutions proved to be far more effective antiseptics than soap (Semmelweis was also aware of their greater efficacy, but not the reason), and this resulted in Semmelweis's celebrated success in stopping the transmission of childbed fever ("puerperal fever") in the maternity wards of Vienna General Hospital in Austria in 1847.[80]

Much later, during World War I in 1916, a standardized and diluted modification of Labarraque's solution containing hypochlorite (0.5%) and boric acid as an acidic stabilizer was developed by Henry Drysdale Dakin (who gave full credit to Labarraque's prior work in this area). Called Dakin's solution, the method of wound irrigation with chlorinated solutions allowed antiseptic treatment of a wide variety of open wounds, long before the modern antibiotic era. A modified version of this solution continues to be employed in wound irrigation in modern times, where it remains effective against bacteria that are resistant to multiple antibiotics (see Century Pharmaceuticals).[81]

Public sanitation

The first continuous application of chlorination to drinking U.S. water was installed in Jersey City, New Jersey, in 1908.[82] By 1918, the US Department of Treasury called for all drinking water to be disinfected with chlorine. Chlorine is presently an important chemical for water purification (such as in water treatment plants), in disinfectants, and in bleach. Even small water supplies are now routinely chlorinated.[83]

Chlorine is usually used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and public swimming pools. In most private swimming pools, chlorine itself is not used, but rather sodium hypochlorite, formed from chlorine and sodium hydroxide, or solid tablets of chlorinated isocyanurates. The drawback of using chlorine in swimming pools is that the chlorine reacts with the proteins in human hair and skin. Contrary to popular belief, the distinctive "chlorine aroma" associated with swimming pools is not the result of elemental chlorine itself, but of chloramine, a chemical compound produced by the reaction of free dissolved chlorine with amines in organic substances. As a disinfectant in water, chlorine is more than three times as effective against Escherichia coli as bromine, and more than six times as effective as iodine.[84] Increasingly, monochloramine itself is being directly added to drinking water for purposes of disinfection, a process known as chloramination.[85]

It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative methods of adding chlorine are used. These include hypochlorite solutions, which gradually release chlorine into the water, and compounds like sodium dichloro-s-triazinetrione (dihydrate or anhydrous), sometimes referred to as "dichlor", and trichloro-s-triazinetrione, sometimes referred to as "trichlor". These compounds are stable while solid and may be used in powdered, granular, or tablet form. When added in small amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule, forming hypochlorous acid (HOCl), which acts as a general biocide, killing germs, microorganisms, algae, and so on.

Use as a weapon

World War I

Main article: Chemical weapons in World War I

Chlorine gas, also known as bertholite, was first used as a weapon in World War I by Germany on April 22, 1915, in the Second Battle of Ypres.[88][89] As described by the soldiers, it had the distinctive smell of a mixture of pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. Chlorine reacts with water in the mucosa of the lungs to form hydrochloric acid, destructive to living tissue and potentially lethal. Human respiratory systems can be protected from chlorine gas by gas masks with activated charcoal or other filters, which makes chlorine gas much less lethal than other chemical weapons. It was pioneered by a German scientist later to be a Nobel laureate, Fritz Haber of the Kaiser Wilhelm Institute in Berlin, in collaboration with the German chemical conglomerate IG Farben, which developed methods for discharging chlorine gas against an entrenched enemy.[90] After its first use, both sides in the conflict used chlorine as a chemical weapon, but it was soon replaced by the more deadly phosgene and mustard gas.[91]


Main article: Chlorine bombings in Iraq

Chlorine gas was also used during the Iraq War in Anbar Province in 2007, with insurgents packing truck bombs with mortar shells and chlorine tanks. The attacks killed two people from the explosives and sickened more than 350. Most of the deaths were caused by the force of the explosions rather than the effects of chlorine since the toxic gas is readily dispersed and diluted in the atmosphere by the blast. In some bombings, over a hundred civilians were hospitalized due to breathing difficulties. The Iraqi authorities tightened security for elemental chlorine, which is essential for providing safe drinking water to the population.[92][93]

On 23 October 2014, it was reported that the Islamic State of Iraq and the Levant had used chlorine gas in the town of Duluiyah, Iraq.[94] Laboratory analysis of clothing and soil samples confirmed the use of chlorine gas against Kurdish Peshmerga Forces in a vehicle-borne improvised explosive device attack on 23 January 2015 at the Highway 47 Kiske Junction near Mosul.[95]


Main article: Use of chemical weapons in the Syrian Civil War

The Syrian government has used chlorine as a chemical weapon[96] delivered from barrel bombs and rockets.[97][98] In 2016, the OPCW-UN Joint Investigative Mechanism concluded that the Syrian government used chlorine as a chemical weapon in three separate attacks.[99] Later investigations from the OPCW's Investigation and Identification Team concluded that the Syrian Air Force was responsible for chlorine attacks in 2017 and 2018.[100]

Biological role

The chloride anion is an essential nutrient for metabolism. Chlorine is needed for the production of hydrochloric acid in the stomach and in cellular pump functions.[101] The main dietary source is table salt, or sodium chloride. Overly low or high concentrations of chloride in the blood are examples of electrolyte disturbances. Hypochloremia (having too little chloride) rarely occurs in the absence of other abnormalities. It is sometimes associated with hypoventilation.[102] It can be associated with chronic respiratory acidosis.[103]Hyperchloremia (having too much chloride) usually does not produce symptoms. When symptoms do occur, they tend to resemble those of hypernatremia (having too much sodium). Reduction in blood chloride leads to cerebral dehydration; symptoms are most often caused by rapid rehydration which results in cerebral edema. Hyperchloremia can affect oxygen transport.[104]


GHS pictogramsGHS03: OxidizingGHS06: ToxicGHS09: Environmental hazard
GHS Signal wordDanger

GHS hazard statements

H270, H315, H319, H331, H335, H400

GHS precautionary statements

P220, P244, P261, P304, P340, P312, P403, P233, P410, P403[105]
NFPA 704 (fire diamond)

Chemical compound

Chlorine is a toxic gas that attacks the respiratory system, eyes, and skin.[107] Because it is denser than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials.[108][109]

Chlorine is detectable with measuring devices in concentrations as low as 0.2 parts per million (ppm), and by smell at 3 ppm. Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gas.[13] The IDLH (immediately dangerous to life and health) concentration is 10 ppm.[110] Breathing lower concentrations can aggravate the respiratory system and exposure to the gas can irritate the eyes.[111] When chlorine is inhaled at concentrations greater than 30 ppm, it reacts with water within the lungs, producing hydrochloric acid (HCl) and hypochlorous acid (HClO).

When used at specified levels for water disinfection, the reaction of chlorine with water is not a major concern for human health. Other materials present in the water may generate disinfection by-products that are associated with negative effects on human health.[112][113]

In the United States, the Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit for elemental chlorine at 1 ppm, or 3 mg/m3. The National Institute for Occupational Safety and Health has designated a recommended exposure limit of 0.5 ppm over 15 minutes.[110]

In the home, accidents occur when hypochlorite bleach solutions come into contact with certain acidic drain-cleaners to produce chlorine gas.[114] Hypochlorite bleach (a popular laundry additive) combined with ammonia (another popular laundry additive) produces chloramines, another toxic group of chemicals.[115]

Chlorine-induced cracking in structural materials

Chlorine "attack" on an acetal resin plumbing joint resulting from a fractured acetal joint in a water supply system which started at an injection moldingdefect in the joint and slowly grew until the part failed; the fracture surface shows iron and calcium salts that were deposited in the leaking joint from the water supply before failure and are the indirect result of the chlorine attack

Chlorine is widely used for purifying water, especially potable water supplies and water used in swimming pools. Several catastrophic collapses of swimming pool ceilings have occurred from chlorine-induced stress corrosion cracking of stainless steel suspension rods.[116] Some polymers are also sensitive to attack, including acetal resin and polybutene. Both materials were used in hot and cold water domestic plumbing, and stress corrosion cracking caused widespread failures in the US in the 1980s and 1990s.[117]

Chlorine-iron fire

The element iron can combine with chlorine at high temperatures in a strong exothermic reaction, creating a chlorine-iron fire.[118][119] Chlorine-iron fires are a risk in chemical process plants, where much of the pipework that carries chlorine gas is made of steel.[118][119]

See also

Preparation of chlorine


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I could not be mistaken, the girl got to the sweet moment of the long-awaited orgasm. Continuing to kiss her fleshly, I slightly increased the pressure of my finger, and accelerated the amplitude of his immersion in depth. The measures taken immediately bore fruit.

The young woman shook shallowly, as if tiny shells had begun to explode inside her.

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