In addition to the ionic compounds that Chlorine forms with metals, it also forms molecular compounds with non-metals such as Sulfur and Oxygen.  There are four different oxides of the element.  Hydrogen Chloride gas (from which we get Hydrochloric Acid, HCl) is an important industrial product.

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 liter of water dissolves 3.10 liters gaseous Chlorine and at 30 °C only 1.77 liters.

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.

History

Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it dephlogisticated muriatic acid 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.

1s² 2s²p⁶ 3s²p⁵

Chlorine gas, also known as bertholite, was first used as a weapon against humans in World War I by Germany on April 22, 1915 in the Second Battle of Ypres.  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 utilised by both sides as a chemical weapon.

Occurrence

In nature, chlorine is found mainly as the chloride ion (Cl^-), 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 sodium chloride dissolved in water. Along with chlorine, this chloralkali process yields hydrogen gas and sodium hydroxide, according to the chemical equation:

2 NaCl  + 2H₂O  Cl₂ + H₂ + 2 NaOH

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 bulk an apparent atomic weight of 35.5.

³⁶Cl

Trace amounts of radioactive ³⁶Cl exist in the environment, in a ratio of about 700x10^-15 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 ydrophilic 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.

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.

Mercury Cell Electrolysis

Mercury cell electrolysis was the first method used to produce chlorine on an industrial scale.  Titanium anodes are located above a liquid mercury cathode and a solution of sodium chloride is positioned between the electrodes. When an electrical current is applied, chloride is released at the titanium anodes and sodium dissolves into the mercury cathode forming an amalgam.

The amalgam can be regenerated into mercury by reacting it with water, producing hydrogen and sodium hydroxide.  These are useful byproducts. However, this method consumes vast amounts of energy and there are also concerns about mercury emissions.

Diaphragm Cell Electrolysis

In diaphragm cell electrolysis, an asbestos diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing.

This method uses less energy than the mercury cell, but the sodium hydroxide is not as easily concentrated and precipitated into a useful substance.

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

Other Methods of Production

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 catalysit 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.

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.

Applications and Uses

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 from drinking water supplies and swimming pools.  However, in most non-commercial swimming pools chlorine itself is not used, but rather the mixture sodium hypochlorite (household bleach), a mixture of sodium hydroxide and chlorine. Also, Calciumhypochlorite is used, as a cheaper alternative. Even small water supplies are now routinely chlorinated.
• Used widely in paper product production, antiseptic, dyestuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products.

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.

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.

Compounds

Besides the chloride ion (Cl^-), chlorine also exists in ionic form as: Cl⁺¹, Cl⁺⁵ and Cl⁺⁷.   Examples are: Hypochlorite (ClO^-), Chlorite (ClO₂^-), Chlorate (ClO₃^-), and Perchlorate (ClO₄^-), with ionic charges of -1, +1, +5 and +7 respectively.

• Fluorides: Chlorine Monofluoride (ClF), Chlorine Trifluoride (ClF₃), and Chlorine Pentafluoride (ClF₅)
• Oxides: Chlorine Dioxide (ClO₂), Dichlorine Monoxide (Cl₂O), and Dichlorine Heptoxide  (Cl₂O₇)
• Acids: Hydrochloric Acid (HCl),  Hypochlorous Acid (HClO), Chlorous Acid (HClO₂), Chloric Acid (HClO₃), and Perchloric Acid (HClO₄)
• Amines: Chloramine (NH₂Cl).

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

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.