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. 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. occurrence and production liquid chlorine analysis 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. 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. 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: 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. the salt solution (brine) is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the caustic alkali 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. 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. 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. 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). applications 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. 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. 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, and zncl2. 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. 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. 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. 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, embalming, outbreaks of epidemic disease, fever, and blackleg in cattle. disinfection 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. 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. 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, though three other reputable sources that describe that famous cholera epidemic do not mention the incident. 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.: 296 semmelweis and experiments with antisepsis ignaz semmelweis 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. 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). public sanitation liquid pool chlorine the first continuous application of chlorination to drinking u.s. water was installed in jersey city, new jersey, in 1908. 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. 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 amino acids in 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 including those in urine and sweat. 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. increasingly, monochloramine itself is being directly added to drinking water for purposes of disinfection, a process known as chloramination. 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. 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. 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. 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. middle east 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