Chlorine

• 1. Introduction

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  Chlorine is element number 17, a yellow-green gas with the chemical symbol Cl. In 2022 the world produced an estimated 97 million tonnes of it. Much of that gas never touches a swimming pool or a tap. It is turned into plastics and other organic chemicals inside the factories that make it. At high concentration, however, chlorine is extremely dangerous and poisonous to most living organisms. How did anyone discover that this gas was a substance in its own right? Why did leading chemists hesitate over whether it was an element at all? And how did a poison become one of the most trusted tools for cleaning water, wounds and doctors' hands?

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• 2. Overlooked For Centuries

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  Around the year 900, the authors of Arabic writings attributed to Jabir ibn Hayyan, known in Latin as Geber, were experimenting with sal ammoniac. So was the Persian physician and alchemist Abu Bakr al-Razi, who lived from about 865 to 925. Sal ammoniac is ammonium chloride. Distilled together with vitriol, a name for the hydrated sulfates of various metals, it gave off hydrogen chloride. The gaseous products seem to have been thrown away, so hydrogen chloride may have been made many times before anyone put it to chemical use.

  Mercury(II) chloride, known as corrosive sublimate, was among the first useful products. Its recipe involved heating mercury with alum and ammonium chloride, or with vitriol and common salt. That recipe appears in De aluminibus et salibus, meaning On Alums and Salts. This Arabic text from the eleventh or twelfth century was falsely credited to al-Razi. Gerard of Cremona, who lived from 1144 to 1187, translated it into Latin in the second half of the twelfth century.

  A writer known as pseudo-Geber made the next advance in De inventione veritatis, On the Discovery of Truth, written after about 1300. He found that adding ammonium chloride to nitric acid produced aqua regia, a strong solvent that can dissolve gold. Aqua regia is unstable and keeps giving off fumes that contain free chlorine gas. Nobody seems to have noticed that gas until about 1630. Then the Brabantian chemist and physician Jan Baptist van Helmont recognised it as a separate gaseous substance. A detailed study of the gas, however, had to wait until 1774 and a Swedish chemist.

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• 3. Naming An Element

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  Carl Wilhelm Scheele made chlorine in 1774 by reacting hydrochloric acid with pyrolusite, the mineral form of manganese dioxide. He noticed that the gas bleached litmus, killed insects, looked yellow-green and smelled like aqua regia. Hydrochloric acid was then called muriatic acid, and gases were called airs. So Scheele named his product dephlogisticated muriatic acid air. He is credited with the discovery, yet he failed to establish that the gas was an element.

  The chemical theory of his day held that every acid contained oxygen. The very name oxygen comes from Greek roots meaning acid and producer. Claude Berthollet and several other chemists therefore suggested that Scheele's gas combined oxygen with a still undiscovered element. They called that hypothetical element muriaticum.

  In 1809 Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to pull muriaticum out of the gas by reacting it with charcoal. They expected to release the free element along with carbon dioxide. They failed. Their report raised the possibility that the gas was itself an element, but they were not convinced.

  Sir Humphry Davy repeated the experiment in 1810 and concluded that the substance was an element and not a compound. He announced his results to the Royal Society on the 15th of November that year. He named the element chlorine after a Greek word for a pale green or green-yellow shade, in reference to its colour.

  The word halogen, meaning salt producer, was first applied to chlorine alone, in 1811, by Johann Salomo Christoph Schweigger. In 1826 Jöns Jakob Berzelius suggested using it for the whole family of fluorine, chlorine, bromine and iodine. Michael Faraday liquefied chlorine for the first time in 1823. He also showed that the so-called solid chlorine was really a chlorine hydrate. Berthollet, who had doubted that chlorine was an element, had already put the gas to work on cloth.

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• 4. From Bleach To Plastics

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  In 1785 the French chemist Claude Berthollet used chlorine gas to bleach textiles. In 1789, in his laboratory in the town of Javel, now part of Paris, he passed chlorine through a solution of sodium carbonate. The result was a weak solution of sodium hypochlorite, sold as Eau de Javel, or Javel water.

  The Scottish chemist and industrialist Charles Tennant found a more practical route. He first made a solution of calcium hypochlorite, called chlorinated lime, and then a solid form, bleaching powder. Both released low levels of elemental chlorine and travelled more easily than sodium hypochlorite. Sodium hypochlorite stayed dilute because purifying it to remove the water made it a dangerously powerful and unstable oxidiser.

  Electricity then changed the supply. In 1884 the Chemische Fabrik Griesheim in Germany developed a chloralkali process that entered commercial production in 1888. Near the end of the nineteenth century E. S. Smith patented a way to make sodium hypochlorite by electrolysing brine. The electrolysis produced sodium hydroxide and chlorine gas, which were then mixed. The chloralkali process reached industrial scale in 1892. It now supplies most elemental chlorine, and sodium hydroxide is its most valuable product.

  The Castner-Kellner process was the first industrial method of the three that followed. It used a mercury electrode, which is toxic, to collect the sodium and prevent unwanted side reactions. The diaphragm cell uses an asbestos or polymer-fibre barrier to keep chlorine away from the sodium hydroxide and hydrogen. It avoids mercury and saves energy but yields dilute and slightly impure alkali. The membrane cell uses an ion exchange membrane. It makes very pure alkali but needs very pure brine at high concentration. Because it uses less energy, new plants almost all choose the membrane method, and older plants are being converted.

  About 15,000 chlorine-containing compounds are traded commercially. Roughly 63 percent of the elemental chlorine produced goes into organic compounds and 18 percent into inorganic chlorine compounds. The remaining 19 percent becomes bleaches and disinfection products. Between 35 and 40 percent of all chlorine becomes polyvinyl chloride, or PVC, by way of ethylene dichloride and vinyl chloride. Many of the finished products made with chlorine contain none of it.

  Cylinders of chlorine range from 450 grams to 70 kilograms, and drums hold 865 kilograms. Road tank wagons carry 15 tonnes, rail wagons carry 27 to 90 tonnes, and barges carry 600 to 1200 tonnes. Moving the gas is hard and hazardous, so production usually sits next to the users. Vinyl chloride producers such as Westlake Chemical and Formosa Plastics run integrated chloralkali plants. A separate route, the Deacon process, recovers chlorine from hydrogen chloride left over from making organochlorine compounds. It oxidises the hydrogen chloride with oxygen over a catalyst. Early catalysts were based on copper, while the Mitsui MT-Chlorine Process uses chromium and ruthenium.

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• 5. From Stench To Sanitation

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  Around 1820 the Société d'encouragement pour l'industrie nationale offered a prize to anyone who could separate the membrane of animal intestines without letting it rot. In France, intestines were processed in so-called gut factories, called boyauderies, to make musical instrument strings, goldbeater's skin and other products. The work was smelly and unhealthy. The prize went to Antoine-Germain Labarraque, a 44-year-old French chemist and pharmacist. He had found that Berthollet's Eau de Javel destroyed the smell of putrefaction and also slowed the decomposition itself.

  Labarraque's chlorides and hypochlorites of lime and of sodium soon spread beyond the gut factories. They were used on latrines, sewers, markets, abattoirs, anatomical theatres and morgues. They also worked in hospitals, prisons, infirmaries on land and at sea, stables and cattle-sheds. Since 1828 his solutions have been advocated against what was then called contagious infection, which people believed travelled in miasmas. He even recommended that doctors breathe chlorine, wash their hands in chlorinated lime and sprinkle it around patients' beds. The microbe itself would not be discovered for more than half a century.

  During the Paris cholera outbreak of 1832, large quantities of chloride of lime were used to disinfect the capital. This was chlorine gas dissolved in lime-water to form calcium hypochlorite. It removed the terrible stench from hospitals and dissecting rooms and effectively deodorised the Latin Quarter. One source claims that Dr. John Snow used chloride of lime on the water of the Broad Street pump in London in 1854. Three other sources on that epidemic do not mention it. One reference does make clear that chloride of lime was spread over the offal and filth in the surrounding streets, a common practice in mid-nineteenth-century England.

  In 1847 Ignaz Semmelweis used chlorine-water to disinfect the hands of Austrian doctors. It was cheaper than chlorinated lime. He had noticed that their hands still carried the stench of decomposition from the dissection rooms into the patient examination rooms. He theorised that cadaveric particles carried decay from fresh cadavers to living patients. Soap did not remove the smell, but Labarraque's solutions did, and they proved far more effective antiseptics. Semmelweis stopped the spread of childbed fever in the maternity wards of Vienna General Hospital that same year.

  In 1916, during World War I, Henry Drysdale Dakin developed a standardised, diluted version of Labarraque's solution. It contained 0.5 percent hypochlorite and boric acid as an acidic stabiliser. Dakin gave full credit to Labarraque. Dakin's solution allowed wounds to be irrigated and treated long before the age of antibiotics. A modified version is still used, and it works against bacteria that resist multiple antibiotics.

  Jersey City, New Jersey, installed the first continuous chlorination of American drinking water in 1908. By 1918 the US Department of Treasury called for all drinking water to be disinfected with chlorine. Even small water supplies are now routinely chlorinated. In swimming pools the active killer is hypochlorous acid. Most private pools use sodium hypochlorite or solid tablets of chlorinated isocyanurates rather than chlorine itself. The familiar pool smell is not elemental chlorine. It comes from chloramine, formed when dissolved chlorine meets amines in urine and sweat. Chlorine is more than three times as effective against Escherichia coli as bromine, and more than six times as effective as iodine.

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• 6. Chlorine As Weapon

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  On the 22nd of April 1915, at the Second Battle of Ypres, the German Army used chlorine gas as a weapon for the first time. The effect on the Allied forces was devastating. Their existing gas masks were difficult to deploy and had not been widely distributed. Soldiers described a smell like a mixture of pepper and pineapple, a metallic taste, and a sting in the back of the throat and chest. The gas reacts with water in the lungs to form hydrochloric acid, which destroys living tissue and can kill.

  Fritz Haber of the Kaiser Wilhelm Institute in Berlin pioneered the weapon. He later became a Nobel laureate. He worked with the German chemical conglomerate IG Farben, which developed ways to discharge the gas against an entrenched enemy. Both sides used chlorine after its first appearance. Gas masks fitted with activated charcoal or other filters made it much less lethal than other chemical weapons. It was soon replaced by the more deadly phosgene and the more debilitating mustard gas.

  In Anbar Province, during the Iraq War in 2007, insurgents packed truck bombs with mortar shells and chlorine tanks. The attacks killed two people by explosion and sickened more than 350. Most deaths came from the force of the blasts rather than the gas, which the explosions dispersed and diluted. In some bombings over a hundred civilians were hospitalised with breathing difficulties. Iraqi authorities tightened security around elemental chlorine, which the country needs for safe drinking water.

  On the 23rd of October 2014 it was reported that the Islamic State of Iraq and the Levant had used chlorine gas in the town of Duluiyah, Iraq. Laboratory analysis of clothing and soil samples confirmed chlorine gas in a vehicle-borne bomb attack on the 23rd of January 2015. The target was Kurdish Peshmerga forces at the Highway 47 Kiske Junction near Mosul.

  Syria has used chlorine as a chemical weapon delivered from barrel bombs and rockets. In 2016 the OPCW-UN Joint Investigative Mechanism concluded that the Syrian government used it in three separate attacks. Later investigations by the OPCW's Investigation and Identification Team found the Syrian Air Force responsible for chlorine attacks in 2017 and 2018.

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• 7. Reactivity And Hazard

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  Measuring devices can detect chlorine at 0.2 parts per million, and the nose notices it at 3. Coughing and vomiting may begin at 30 parts per million and lung damage at 60. About 1000 parts per million can be fatal after a few deep breaths. The level counted as immediately dangerous to life and health is 10 parts per million. In the United States, OSHA has set the permissible exposure limit at 1 part per million. The National Institute for Occupational Safety and Health recommends 0.5 over 15 minutes. Chlorine is denser than air, so it gathers at the bottom of poorly ventilated spaces.

  Household accidents happen when hypochlorite bleach meets certain acidic drain cleaners and releases chlorine gas. Bleach mixed with ammonia, another popular laundry additive, produces toxic chloramines. Chlorine also attacks materials. Several catastrophic collapses of swimming pool ceilings came from chlorine-induced stress corrosion cracking of stainless steel suspension rods. Acetal resin and polybutene plumbing failed widely in the United States in the 1980s and 1990s. In process plants, where much of the chlorine pipework is steel, iron and chlorine can combine at high temperature in a strongly exothermic reaction and start a chlorine-iron fire.

  Chlorine trifluoride, with a melting point of -76.3 °C and a boiling point of 11.8 °C, is one of the most reactive chemical compounds known. It sets fire to hydrogen, potassium, phosphorus, sulfur, iodine and powdered iron, among other elements. It ignites water, and it ignites substances normally thought inert, such as asbestos, concrete, glass and sand. When heated, it corrodes palladium, platinum and gold, and even xenon and radon are fluorinated. Containers of nickel, copper and steel survive because a layer of unreactive metal fluoride forms on them. Its reaction with hydrazine was tried in an experimental rocket engine, but the mixture ignited with no measurable delay. Today it is mostly used in nuclear fuel processing and to clean chemical vapour deposition chambers in the semiconductor industry.

  Humphry Davy discovered chlorine dioxide in 1811, the first of the chlorine oxides. It is a yellow gas that explodes above -40 °C as a liquid and under pressure as a gas. It must therefore be made at low concentrations for bleaching wood pulp and treating water. Dichlorine heptoxide is a shock-sensitive, colourless, oily liquid. It is the only chlorine oxide that does not set organic materials on fire at room temperature. Anhydrous perchloric acid explodes on contact with most organic compounds and even oxidises silver and gold. At ultra-high pressures, such as in the cores of large planets, chlorine can form a compound with sodium, Na3Cl, that fits no traditional chemical expectations.

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• 8. Salt, Sea And Life

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  Chlorine makes up 126 parts per million of Earth's crust, which makes it the 20th most abundant element there. It is too reactive to occur as a free element, so all of it is found as chloride compounds such as table salt. Among the halogens only fluorine is more abundant in the crust. All of these deposits are dwarfed by the chloride in seawater. Higher concentrations occur in smaller bodies, such as the Great Salt Lake in Utah, the Dead Sea in Israel and underground brine wells. Chlorine has two stable isotopes, 35Cl and 37Cl, making up 76 percent and 24 percent of the natural element. Both are made in stars, in the oxygen-burning and silicon-burning processes.

  Nature itself makes organic chlorine compounds, from bacteria to humans. Dioxins have been found in the preserved ashes of lightning-ignited fires that predate any synthetic dioxins. Marine algae produce dichloromethane, chloroform and carbon tetrachloride. Most of the chloromethane in the environment comes from biological decomposition, forest fires and volcanoes. Some manufactured organochlorines are toxic, though. DDT was widely used against insects in the mid-twentieth century. It accumulates in food chains and thins the eggshells of certain bird species. The carbon-chlorine bond in chlorofluorocarbons breaks readily in the upper atmosphere to release chlorine radicals, so they have been discontinued for the harm they do to the ozone layer.

  The chloride ion is an essential nutrient, and all known species of life need it. The body uses it to make hydrochloric acid in the stomach and in cellular pump functions. The main dietary source is table salt. Blood chloride that runs too low, called hypochloremia, or too high, called hyperchloremia, is an electrolyte disturbance. Too much chloride usually produces no symptoms, but when it does they resemble those of having too much sodium. Inside the body, neutrophils oxidise chloride ions into small quantities of elemental chlorine as part of the immune response against bacteria.

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