Chlorine

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17 sulfur ← chlorine → argon F ↑ Cl ↓ Br Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	chlorine, Cl, 17
Chemical series	halogens
Group, Period, Block	17, 3, p
Appearance	yellowish green
Standard atomic weight	35.453(2) g·mol−1
Electron configuration	[Ne] 3s2 3p5
Electrons per shell	2, 8, 7
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 3.2 g/L
Melting point	171.6 K (-101.5 °C, -150.7 °F)
Boiling point	239.11 K (-34.04 °C, -29.27 °F)
Critical point	416.9 K, 7.991 MPa
Heat of fusion	(Cl2) 6.406 kJ·mol−1
Heat of vaporization	(Cl2) 20.41 kJ·mol−1
Heat capacity	(25 °C) (Cl2) 33.949 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 128 139 153 170 197 239
Atomic properties
Crystal structure	orthorhombic
Oxidation states	±1, 3, 5, 7 (strongly acidic oxide)
Electronegativity	3.16 (Pauling scale)
Ionization energies (more)	1st: 1251.2 kJ·mol−1
	2nd: 2298 kJ·mol−1
	3rd: 3822 kJ·mol−1
Atomic radius	100 pm
Atomic radius (calc.)	79 pm
Covalent radius	99 pm
Van der Waals radius	175 pm
Miscellaneous
Magnetic ordering	nonmagnetic
Electrical resistivity	(20 °C) > 10Ω·m
Thermal conductivity	(300 K) 8.9 m W·m−1·K−1
Speed of sound	(gas, 0 °C) 206 m/s
CAS registry number	7782-50-5
Selected isotopes
Main article: Isotopes of chlorine iso NA half-life DM DE (MeV) DP 35Cl 75.77% Cl is stable with 18 neutrons 36Cl syn 301×103 y β- 0.709 36Ar ε - 36S 37Cl 24.23% Cl is stable with 20 neutrons
References
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Chlorine (IPA: /ˈklɔːriːn/, Greek: χλωρóς chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its elemental form under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm^[1] and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. It is also used in swimming pools to keep them clean. In the upper atmosphere, chlorine has been implicated in destruction of the ozone layer.

Contents 1 Notable characteristics 2 History 3 Occurrence 4 Isotopes 4.1 36Cl 5 Chlorine gas extraction 5.1 Mercury cell electrolysis 5.2 Diaphragm cell electrolysis 5.3 Membrane cell electrolysis 5.4 Other methods 6 Applications and Uses 6.1 Purification and disinfection 6.2 Oxidizing agent 6.3 World War I 6.4 Iraq War 6.5 Other uses 7 Compounds 7.1 Oxidation States 8 Safety 9 See also 10 References 11 External links

[edit] Notable characteristics

Chlorine gas is diatomic with the formula Cl₂. It combines readily with nearly all other elements, although it is not as extremely reactive as fluorine. At 10 °C one litre of water dissolves 3.10 litres of gaseous chlorine and at 30 °C only 1.77 litres.^[2]

This element is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. As the chloride ion, Cl ^– , it is also the most abundant dissolved species in ocean water.

[edit] History

Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it dephlogisticated muriatic acid (see phlogiston theory) and mistakenly thought it contained oxygen. Chlorine was given its current name in 1810 by Sir Humphry Davy, who insisted that it was in fact an element.

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. As described by the soldiers it had a distinctive smell of a mixture between pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. It was pioneered by a German scientist later to be a Nobel laureate, Fritz Haber. It is alleged that his role in the use of chlorine as a deadly weapon drove his wife to suicide. After its first use, it was utilized by both sides as a chemical weapon.

[edit] Occurrence

In nature, chlorine is found mainly as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans — about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits. Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Common chloride minerals include halite (sodium chloride), sylvite (potassium chloride), and carnallite (potassium magnesium chloride hexahydrate).

Industrially, elemental chlorine is usually produced by the electrolysis of sodium chloride dissolved in water. Along with chlorine, this chloralkali process yields hydrogen gas and sodium hydroxide, according to the chemical equation

2 NaCl + 2 H₂O → Cl₂ + H₂ + 2 NaOH

See also Halide minerals.

[edit] Isotopes

Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, ³⁵Cl (75.77%) and ³⁷Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.

[edit] ³⁶Cl

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about 7x10^-13 to 1 with stable isotopes. ³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the subsurface environment, ³⁶Cl is generated primarily as a result of neutron capture by ³⁵Cl or muon capture by ⁴⁰Ca. ³⁶Cl decays to ³⁶S and to ³⁶Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of ³⁶Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years before the present. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

[edit] Chlorine gas extraction

Chlorine can be manufactured by electrolysis of a sodium chloride solution (brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H₂). These two products, as well as chlorine are highly reactive. There are three industrial methods for the extraction of chlorine by electrolysis.

[edit] Mercury cell electrolysis

Mercury cell electrolysis, also known as the Castner-Kellner process, was the first method used to produce chlorine on an industrial scale. Titanium or graphite anodes are located above a liquid mercury cathode. Slate baffles divide the cell into two chambers, in which the anode is in contact with just one. The baffles do not go all the way to the bottom of the cell, but allow the mercury cathode (but not the electrolyte) to flow beneath them. Sodium chloride solution is placed in the anode chamber and water in the other chamber. When an electrical current is applied, chlorine is released at the anodes and sodium dissolves into the mercury cathode forming an amalgam. By rocking the entire cell, the mercury amalgam is exposed to the water chamber, where it reacts to form sodium hydroxide and hydrogen gas as byproducts.^[3][4]

This method consumes vast amounts of energy and there are also concerns about mercury emissions.

[edit] Diaphragm cell electrolysis

In diaphragm cell electrolysis, an asbestos (or other pourous material) diaphragm separates cathode and anode, preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing. There are several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird cell (1901), the Gibbs cell (1908), and the Townsend cell (1904).^[5][6] The cells vary in construction and placement of the diaphragm, with some having the diaphragm in direct contact with the cathode. Each of these uses the same principle of allowing sodium ions to diffuse through the pourous diaphragm from the anode side containing the chloride to the cathode side containing the hydroxide. The concentration of sodium ions in the cathode side is kept low by constantly removing some hydroxide solution and replacing it with water. The sodium ion concentration on the anode side is kept high by adding sodium chloride to keep the solution saturated. Sodium ions are driven by the electric current to flow toward the cathode, whereas the chloride ions are driven in the opposite direction. Despite this some diffusion of chloride and hypochlorite ions through the diaphragm is unavoidable. As a result diaphragm methods produce alkali of less purity than do mercury cell methods. But diaphragm cells are not burdened with the cost of mercury and the problem of preventing mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings over the mercury cell method.^[6]

[edit] Membrane cell electrolysis

The electrolysis cell is divided into two by a cation permeable membrane acting as an ion exchanger. Saturated sodium chloride solution is passed through the anode compartment leaving a lower concentration. Sodium hydroxide solution is circulated through the cathode compartment exiting at a higher concentration. A portion of this concentrated sodium hydroxide solution is diverted as product while the remainder is diluted with deionized water and passed through the electrolyzer again.

This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution.

Cathode: 2 H⁺(aq) + 2 e^– → H_{2 (g)}

Anode: 2 Cl^– → Cl_{2 (g)} + 2 e^–

Overall equation: 2 NaCl + 2H₂0 → Cl₂ + H₂ + 2 NaOH

[edit] Other methods

Before electrolytic methods were used for chlorine production, the direct oxidation of hydrogen chloride with oxygen or air was exercised in the Deacon process:

4 HCl + O₂ → 2 Cl₂ + 2 H₂O

This reaction is accomplished with the use of CuCl₂ as a catalyst and is performed in 400°C. The amount of extracted chlorine is approximately at 80%. Due to the extremely corrosive reaction mixture, industrial use of this method is difficult.

Another earlier process to produce chlorine was to heat brine with acid and manganese dioxide.

2 NaCl + 2H₂SO₄ + MnO₂ → Na₂SO₄ + MnSO₄ + 2 H₂O + Cl₂

Using this process, chemist Carl Wilhelm Scheele was the first to isolate chlorine in a laboratory. The manganese can be recovered by the Weldon process.^[7]

The Downs process for producing sodium metal electrolytically from fused sodium chloride produces chlorine as a byproduct.

In a laboratory, small amounts of chlorine gas can be created by adding concentrated hydrochloric acid (typically about 5M) to sodium chlorate solution.

Small amounts of chlorine gas can also be made in the laboratory by putting concentrated hydrochloric acid in a flask with a side arm and rubber tubing attached. Manganese dioxide is then added and the flask stoppered. The reaction is not greatly exothermic. As chlorine is denser than air, it can be easily collected by placing the tube inside a flask where it will displace the air. Once full, the collecting flask can be stoppered.

[edit] Applications and Uses

[edit] Purification and disinfection

Chlorine is an important chemical for some processes of water purification, in disinfectants, and in bleach. Ozone can also be used for killing bacteria, and is preferred by many municipal drinking water systems because ozone does not form organochlorine compounds and does not remain in the water after treatment. Ozone is also more reactive and will kill organisms that chlorine will not.

Chlorine is also used widely in the manufacture of many every-day items, or to purify water in various forms.

• Used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and swimming pools. However, in most non-commercial swimming pools chlorine itself is not used, but rather sodium hypochlorite (household bleach), a compound of chlorine with sodium and oxygen. Calcium hypochlorite is also used as a cheaper alternative. Even small water supplies are now routinely chlorinated.^[8] (See also chlorination)
• Used widely in paper product production, antiseptic, dyestuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products.

[edit] Oxidizing agent

Chlorine is used extensively in organic and inorganic chemistry as an oxidizing agent and in substitution reactions because chlorine often imparts many desired properties to an organic compound when it is substituted for hydrogen (as in synthetic rubber production) because of its high electron affinity. Excess chlorine is removed from water with sulfur dioxide.

[edit] World War I

Main article: Use of poison gas in World War I

Chlorine became the first killing agent to be employed during World War I. German chemical conglomerate IG Farben had been producing chlorine as a by-product of their dye manufacturing. In cooperation with Fritz Haber of the Kaiser Wilhelm Institute for Chemistry in Berlin, they developed methods of discharging chlorine gas against an entrenched enemy.

[edit] Iraq War

Chlorine gas has been used by insurgents in the Iraq War as a chemical weapon to increase the capability to threaten coalition forces in their attacks. On March 17, 2007, three chlorine filled trucks were detonated in the Anbar province killing 2 and sickening over 350.^[9]

[edit] Other uses

It is also used in the production of chlorates, chloroform, carbon tetrachloride, and in bromine extraction.

[edit] Compounds

For general references to the chloride ion (Cl⁻), including references to specific chlorides, see chloride. For other chlorine compounds see chlorate (ClO₃⁻), chlorite (ClO₂⁻), hypochlorite(ClO⁻), and perchlorate(ClO₄⁻), and chloramine (NH₂Cl).

See also:

• Fluorides: chlorine monofluoride (ClF), chlorine trifluoride (ClF₃), chlorine pentafluoride (ClF₅)
• Oxides: chlorine dioxide (ClO₂), dichlorine monoxide (Cl₂O), dichlorine heptoxide (Cl₂O₇)
• Acids: hydrochloric acid (HCl), chloric acid (HClO₃), and perchloric acid (HClO₄)

See also Chlorine compounds.

[edit] Oxidation States

Oxidation state	Name	Forumula	Example compounds
– 1	chlorides	Cl –	ionic chlorides, organic chlorides, hydrochloric acid
0	chlorine	Cl2	elemental chlorine
+1	hypochlorites	ClO –	sodium hypochlorite, calcium hypochlorite
+3	chlorites	ClO2 –	sodium chlorite
+5	chlorates	ClO3 –	sodium chlorate, potassium chlorate, chloric acid
+7	perchlorates	ClO4 –	potassium perchlorate, perchloric acid, organic perchlorates, ammonium perchlorate, magnesium perchlorate

Chlorine exists in all odd numbered oxidation states from – 1 to +7, as well as the elemental state of zero. Progressing through the states, hydrochloric acid can be oxidized using manganese dioxide, or hydrogen chloride gas oxidized catalytically by air to form elemental chlorine gas. The solubility of chlorine in water is increased if the water contains dissolved alkali hydroxide. This is due to disproportionation:

Cl₂ + 2OH ^– → Cl ^– + ClO ^– + H₂O

In hot concentrated alkali solution disproportionation continues:

2ClO ^– → Cl ^– + ClO₂ ^–

ClO ^– + ClO₂ ^– → Cl ^– + ClO₃ ^–

Potassium chlorate can be crystalized from solutions formed by the above reactions. If its crystals are heated, it undergoes the final disproportionation step.

4ClO₃ ^– → Cl ^– + 3ClO₄ ^–

This same progression from chloride to perchlorate can be accomplished by electrolysis. The anode reaction progression is:^[10]

Reaction	Electrode potential
Cl– + 2OH– → ClO– + H2O + 2e–	+0.89 volts
ClO– + 2OH– → ClO2– + H2O + 2e–	+0.67 volts
ClO2– + 2OH– → ClO3– + H2O + 2e–	+0.33 volts
ClO3– + 2OH– → ClO4– + H2O + 2e–	+0.35 volts

Each step is accompanied at the cathode by

2H₂O + 2e^– → 2OH^– + H₂          −0.83 volts

[edit] Safety

Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier 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. For more information see an MSDS.

[edit] See also

• Chloride

[edit] References

1. ^ Merck Index of Chemicals and Drugs, 9th ed., monograph 2065
2. ^ WebElements.com – Chlorine. Mark Winter [The University of Sheffield and WebElements Ltd, UK]. Retrieved on March 17, 2007.
3. ^ Pauling, Linus, General Chemistry, 1970 ed., Dover publications
4. ^ Electrolytic Processes for Chlorine and Caustic Soda. Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands. Retrieved on March 17, 2007.
5. ^ The Electrolysis of Brine. Salt Manufacturers' Association. Retrieved on March 17, 2007.
6. ^ ^{a b} Kiefer, David M.. When the Industry Charged Ahead. Chemistry Chronicles. Retrieved on March 17, 2007.
7. ^ The Chlorine Industry. Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands. Retrieved on March 17, 2007.
8. ^ Chlorine. Los Alamos National Laboratory. Retrieved on March 17, 2007.
9. ^ Mahdi, Basim. "Iraq gas attack makes hundreds ill", CNN, 2007-03-17. Retrieved on March 17, 2007.
10. ^ Cotton, F. Albert and Wilkinson, Geoffrey, Advanced Inorganic Chemistry 2nd ed. John Wiley & sons, p568

[edit] External links

• Chlorine Institute - Trade association and lobby group representing the interests of the chlorine industry
• Computational Chemistry Wiki
• Chlorine Production Using Mercury, Environmental Considerations and Alternatives
• National Pollutant Inventory - Chlorine
• Links to external chemical sources

This article forms part of the series Chemical warfare
Blood agents:	Cyanogen chloride (CK) – Hydrogen cyanide (AC)
Blister agents:	Lewisite (L) – Sulfur mustard gas (HD, H, HT, HL, HQ) – Nitrogen mustard gas (HN1, HN2, HN3)
Nerve agents:	G-Agents: Tabun (GA) – Sarin (GB) – Soman (GD) – Cyclosarin (GF) – GV | V-Agents: VE – VG – VM – VX | Novichok agents
Pulmonary agents:	Chlorine – Chloropicrin (PS) – Phosgene (CG) – Diphosgene (DP)
Incapacitating agents:	Agent 15 (BZ) – KOLOKOL-1
Riot control agents:	Pepper spray (OC) – CS gas – CN gas (mace) – CR gas	v • d • e